text:
\section{Introduction}
The complex structure of the nucleon is still one of the greatest challenges
for the understanding of the strong interaction in the low energy,
non-perturbative
regime. One expects that, like in nuclear structure physics, the main
properties of the interaction are reflected in the excitation spectrum of
the nucleon, but so far the correspondence between model predictions and
experimentally observed states is quite unsatisfactory. All constituent
quark models predict more states than have been observed. This problem of
`missing resonances' becomes more severe the higher the excitation energy.
However, the experimental data base is dominated by elastic scattering of
charged pions off the nucleon, which profits from large hadronic cross
sections, but is biased against states that couple only weakly to $N\pi$.
The combination of continuous wave electron accelerators with sophisticated
4$\pi$ detection systems now allows the study of photon induced reactions with
at least comparable precision as hadron induced reactions. Therefore,
photoproduction of mesons has developed into a prime tool for the
investigation of the nucleon excitation scheme
\cite{Krusche_03,Burkert_04}.

Photoproduction of light mesons like pions at high incident photon energies
involves many partial waves, so that the interpretation of the data requires
sophisticated partial wave analyses. Such programs are under way and will
largely profit from the combination of polarized photon beams with polarized
targets giving access to single and double polarization observables.
However, alternatively due to the suppression of higher partial waves,
the photoproduction of heavier mesons close to their production thresholds
may give access to resonances which contribute only weakly to other channels.
Photoproduction of $\eta$-mesons, which is completely dominated in the
threshold region by the S$_{11}$(1535) resonance, is the best studied example
for this approach
\cite{Krusche_95,Krusche_97,Ajaka_98,Bock_98,Armstrong_99,Thompson_01,Renard_02,Dugger_02,Crede_05,Nakabayashi_06,Bartholomy_07,Elsner_07,Denizli_07,Crede_09,Williams_09,Sumihama_09,McNicoll_10}.
Since the mass of the $\eta '$ ($m_{\eta '}\approx $  958 MeV \cite{PDG})
is much higher than the $\eta$-mass ($m_{\eta}\approx $  548 MeV \cite{PDG}),
resonances contributing to $\eta '$ threshold production may have masses
around 2 GeV. Of course, lower lying resonances may also contribute
due to their large widths.
Because of their iso-scalar nature, both, $\eta$ and $\eta '$ offer the
additional selectivity that only $N^{\star}$ resonances can couple to
$N\eta, N\eta '$. Excited $\Delta$-states can emit these mesons only when
decaying to other $\Delta$'s, in particular the $\Delta$(1232), and thus
contribute to the $\eta '\pi$-channel but not to single $\eta '$-production
(again such processes have been recently intensively studied for
$\eta$-production in the $\eta\pi^0$-channel
\cite{Nakabayashi_06,Ajaka_08,Gutz_08,Horn_08a,Horn_08b,Kashevarov_09,Gutz_10}).
Therefore, $\eta '$ threshold production is expected to have a large
sensitivity to $N^{\star}$ resonances at excitation energies, where the
missing resonance problem is most severe. This is illustrated in Fig.
\ref{fig:levels}, where the experimentally observed nuclear excitation scheme
is compared to model predictions.

\begin{figure}[th]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_1.eps}
}
\caption{Comparison of experimentally established nucleon resonances
\cite{PDG} (left and right) to predictions in the framework of
non-relativistic quark models \cite{CapRob} (center).
The shaded (color: green) area indicates the range accessible for $\eta '$
photoproduction in the present experiment.}
\label{fig:levels}
\end{figure}

Until recently, $\eta '$ photoproduction was not much explored, not even for
the proton. In an early attempt, Mukho\-pad\-hyay and coworkers \cite{Nimai_95}
analyzed bubble chamber data with an effective Lagrangian model and concluded
that the dominant contribution comes from the excitation of a D$_{13}$(2080)
resonance. Analyses of a more recent measurement with the SAPHIR detector
\cite{Ploetzke_98,Chiang_03} claimed contributions from different resonances
(S$_{11}$, P$_{11}$) and strong $t$-channel contributions. However, these
data are not in good agreement with three later measurements with the CLAS
detector at Jlab \cite{Dugger_06,Williams_09} and the Crystal Barrel/TAPS
setup at ELSA \cite{Crede_09}. These second generation experiments, which
profit from much better counting statistics and better control of systematic
effects due to the use of highly efficient detector systems with large and
uniform solid angle coverage, clearly supersede the previous data.
Nakayama and Haberzettl \cite{Nakayama_06} presented an
analysis of the earlier CLAS-data in the framework of an effective Lagrangian
model. They found possible contributions from S$_{11}$, P$_{11}$, P$_{13}$, and
D$_{13}$ resonances. However, also these results are far from being
unique since the available cross section data do not sufficiently constrain
them.

Additional information may be obtained by exploring the iso-spin degree of
freedom. The electromagnetic excitations of N$^{\star}$ resonances are
iso-spin dependent. Resonances which are only weakly excited
for the proton may give stronger signals for the neutron and vice versa.
Interference patterns between different resonances and between
resonances and background contributions may change due to sign changes of
the electromagnetic couplings. Again, $\eta$ photoproduction with the
S$_{11}$(1535) - D$_{13}(1520)$ interference
\cite{Krusche_95b,Weiss_03,Jaegle_08}
and the prominent excitation structure above the S$_{11}$ range, which is only
seen for the neutron \cite{Jaegle_08,Kuznetsov_07,Miyahara_07}, is a very
instructive example.

Due to the technical problems involved in the measurement of small production
cross sections off bound nucleons, photoproduction of $\eta '$-mesons off
the neutron had not been studied up to now. Here, we report the first results
for quasi-free $\eta '$-photoproduction off protons and neutrons bound in
the deuteron. The paper is organized in the following way.
Sections \ref{sec:setup} and \ref{sec:analysis} summarize the experimental setup
and the data analysis. The bases of the models the data are compared to are
discussed in section \ref{sec:models}. The results are summarized
in section \ref{sec:results}. First, the data in coincidence with recoil protons
are compared to free proton data as a check of the quasi-free production
hypothesis. Both, quasi-free proton and neutron data are then compared to
model fits. In addition to the quasi-free data, a first estimate for the cross
section of the coherent process $\gamma d\rightarrow d \eta '$ was extracted.
Final conclusions are given in section \ref{sec:conclu}.

\section{Experimental setup}
\label{sec:setup}

\begin{figure*}[th]
\resizebox{1.0\textwidth}{!}{%
  \includegraphics{fig_2.eps}
}
\caption{Overview of the experimental setup at the ELSA accelerator.
}
\label{fig:setup}
\end{figure*}

The experiment was done at the electron stretcher accelerator ELSA in Bonn
\cite{Husmann_88,Hillert_06}. For the measurements discussed here, electron
beam energies of 2.6~GeV and 3.2~GeV have been used. Real photons were produced
with the brems\-strahlung technique. Their energies were tagged via the
momentum analysis of the scattered electrons by a magnetic spectrometer, which
is schematically shown in Fig. \ref{fig:tagger} (see \cite{Mertens_08} for details).
For this experiment only the part of the focal plane covered by the scintillating
fiber detector but not the part covered by the proportional wire chamber was used.
This limited the maximum tagged photon energies to 80 \% of the electron beam
energy. The different beam settings are summarized in Tab. \ref{tab:beam}.

\begin{figure}[th]
\resizebox{0.48\textwidth}{!}{%
  \includegraphics{fig_3.eps}
}
\caption{Setup of the tagging spectrometer.
}
\label{fig:tagger}
\end{figure}

\begin{table}[h]
\begin{center}
\caption{Summary of beam times. $E_{e^-}$: electron beam energy, $E_{\gamma_t}$:
maximum energy of tagged photons, $E_{pol}$: energy of maximum linear photon beam
polarization, $\Phi_o$: energy integrated electron flux. Total life time:
beam time multiplied by acquisition life time.}
\label{tab:beam}
\begin{tabular}{|c|c|c|c|c|c|}
\hline\noalign{\smallskip}
characteristics & A & B & C & D & E\\
\hline
$E_{e^-}$ [GeV] & 2.6 & 2.6 & 3.2 & 3.2 & 3.2\\
\hline
$E_{\gamma_t}$ [GeV] & 2.0 & 2.0 & 2.5 & 2.5 & 2.5\\
\hline
$E_{pol}$ [GeV]  & 1.0  & 1.0 & unpol. & 1.2 & 1.6 \\
\hline
total life time [h]  & 138 & 18 & 189 & 25 & 25 \\
\hline
$\Phi_o$ [10$^{7}e^-$/s] & 1.75 & 1.6 & 1.6 & 2.8 & 2.8\\
\noalign{\smallskip}\hline
\end{tabular}
\end{center}
\end{table}

Due to these settings and the typical $1/E_{\gamma}$ intensity distribution
of the photon flux, the average time integrated flux at photon energies
above 2 GeV was roughly lower by a factor of two than the flux at lower energies.
The largest part of the 3.2 GeV beam time was done with a copper radiator
foil of 0.3 \% radiation lengths thickness, producing unpolarized
brems\-strahlung. For a small part of this beam time and for the running with
the 2.6 GeV electron energy a diamond radiator was used to produce a linearly
polarized photon beam via coherent bremsstrahlung (see \cite{Elsner_09}
for details about linear polarization) for the extraction of photon beam
asymmetries. However, the statistical quality of this observable was marginal
for $\eta '$ production, since for most of the beam time the setting of the
polarization peak was optimized for $\eta$-production at lower incident photon
energies (see Tab. \ref{tab:beam}). Therefore photon asymmetries have not been
analyzed for the $\eta '$-channel.

The target consisted of a vertically mounted cryostat attached to a tube
entering the Crystal Barrel detector from the upstream side. The target cell
itself was a capton cylinder (0.125 mm foil thickness) with a diameter of
3 cm and a length of 5.3 cm, filled with liquid deuterium (surface
thickness 0.26 nuclei/barn).
The reaction products emerging from the target were detected with electromagnetic
calorimeters covering almost the full solid angle; the Crystal Barrel (CB)
detector (1290 CsI crystals covered the full azimuthal angle for polar angles
between 30$^{\circ}$ and 168$^{\circ}$) \cite{Aker_92} and the TAPS detector
(528 BaF$_2$ crystals mounted as hexagonal forward wall covered polar angles down
to 4.5$^{\circ}$) \cite{Novotny_91,Gabler_94}. Plastic scintillator detectors
in front of the TAPS modules and a scintillating fiber detector \cite{Suft_05}
inside the Barrel covering the same polar-angle range were used for charged
particle identification. A schematic view of the full arrangement is shown in
Fig. \ref{fig:setup}, more details can be found in \cite{Mertens_08}, where
apart from the target an identical setup was used. The time-of-flight walls
were mounted but not used for this experiment.

\begin{figure}[hbt]
\vspace*{0.cm}
\resizebox{0.24\textwidth}{!}{%
  \includegraphics{fig_4a.eps}
}
\resizebox{0.24\textwidth}{!}{%
  \includegraphics{fig_4b.eps}
}
\caption{Front view of the TAPS forward wall. Left hand side: logical
segmentation for the LED-low trigger, right hand side: logical segmentation
for the LED-high trigger (see text).
}
\label{fig:calor}
\end{figure}

The first-level hardware trigger for the experiment was exclusively based on
signals from
the TAPS forward wall detector. The reason is that the CB was read out by
photodiodes without timing information. Measurements of reactions
off the free proton can use signals from recoil protons traversing the
Inner detector. However, in order to have an identical trigger setting for
quasi-free production off the proton and off the neutron this option was not
used. The modules of the TAPS detector were equipped with two independent
leading edge discriminators, combined in two different ways into logical
groups (see Fig. \ref{fig:calor}). For most of them (ring 12 - 5 from outer
edge to center) the lower threshold was set to $\approx$55 MeV (LED-low).
It was set to 80 MeV, 135 MeV, 270 MeV for rings 4, 3, 2, respectively.
The inner-most ring was not allowed to trigger.
The LED-high thresholds were set to 70 MeV for rings 9 - 7, rising from
105 MeV (ring 6) to 180 MeV (ring 2). Again, the inner-most ring was not
allowed to trigger and the three outer rings (block G) had no LED-high.
The first level trigger included two components: one or more
LED-low discriminators from at least two logical sections above threshold or
at least one LED high discriminator above threshold. In the second case, a
second-level trigger from the FAst Cluster Encoder (FACE) of the Crystal
Barrel, indicating at least two separated hits in the Barrel, was required
in addition. All first level triggers thus required detection of at least one
photon in TAPS. Such a trigger is only efficient for reactions with a high
multiplicity of photons like the
$\eta '\rightarrow \pi^0\pi^0\eta\rightarrow 6\gamma$
or the $\eta\rightarrow 3\pi^0\rightarrow 6\gamma$ decays.
But even then the trigger efficiency for mesons at backward angles is not
large. In principle, also events where a recoil nucleon is detected in TAPS
might activate the hardware trigger. This would, however, lead to uncontrollable
trigger efficiencies since the LED thresholds are only calibrated for photons
(recoil nucleons have different signal shapes in BaF$_2$ scintillators) and
the energy deposited by neutrons is more or less random. Therefore only events
where photons alone (identified by non-firing veto detectors and time-of-flight
versus energy) fulfilled the first level trigger conditions were accepted in
the analysis.

\section{Data Analysis}
\label{sec:analysis}
\subsection{Particle and reaction identification and extraction of cross
sections}
\label{subsec:analysis}
Photoproduction of $\eta '$-mesons was identified via the
$\eta '\rightarrow\eta\pi^0\pi^0\rightarrow 6\gamma$ decay chain, which has a
branching ratio of 8\%. Cross sections were extracted for four different
reaction types. The two most important ones are quasi-free production off the
proton $\gamma d\rightarrow (n)p\eta '$, which requires coincident detection
of an $\eta '$ and a recoil proton and quasi-free production off the neutron
$\gamma d\rightarrow n(p)\eta '$ via detection of the $\eta '$ together with
a recoil neutron. For the control of systematic uncertainties (see below) also
the inclusive reaction $\gamma d\rightarrow (np)\eta '$ with no condition
for recoil nucleons was analyzed. Finally, also an estimate of the fully
inclusive reaction $\gamma d\rightarrow X\eta '$ was obtained, where also
final states like $\eta '\pi$ contribute.

\begin{figure}[tht]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_5.eps}
}
\caption{Top to bottom: reactions $\gamma d\rightarrow (np) \eta '$,
$\gamma d\rightarrow p(n) \eta '$, $\gamma d\rightarrow n(p) \eta '$.
Left hand side: 6$\gamma$ invariant mass spectra, (Blue) dashed \
curves: background fit, (red) solid histograms: sum of background and
$\eta '$-peaks, in addition: simulated line-shapes (pink).
Center: missing mass spectra for cut on $\eta '$ invariant mass peaks,
right hand side: (black) points: missing mass spectra for events in
invariant mass peak after background subtraction, solid (red) curves:
simulated line-shapes. All spectra for incident photon energies from
threshold to 2 GeV (integrated over all beam times and the full
polar angle range).
}
\label{fig:iden}
\end{figure}

In the first step of the analysis photon and neutron
candidates (called `neutral hits') were separated from proton candidates
(called `charged hits'). This was done in the CB with the help of the
scintillating fiber detector and in TAPS with the charged particle veto
detectors. In TAPS a hit was assigned to `charged' if the veto of any cluster
module or the veto of any neighbor of the central module of the cluster had
responded (even if the neighbor module itself had no signal above threshold).
The latter condition applies to charged particles which traverse the edge of
a veto but deposit their energy in the neighbor module (due to large impact angles).
All other hits were assigned to `neutral'.
In the Barrel, a hit was assigned to `charged', if at least two layers of the
Inner-detector had recorded a hit within $\pm 10^{\circ}$ of azimuthal angle.
It was assigned to `neutral' if no layer had fired within this azimuthal angle.
Hits with one responding layer of the Inner-detector were discarded.
In the TAPS forward wall, correct identification of protons, neutrons, and
photons can be additionally controlled with a time-of-flight versus energy
analysis, while in the CB no direct separation of photons and neutrons
is possible (more details on particle identification are given in
\cite{Mertens_08} (CB) and \cite{Bloch_07} (TAPS)).

In the next step, events with at least six `neutral' hits as candidates
for the $\eta '$-decay photons were selected and assigned to four partly
overlapping classes corresponding to the above defined reaction types:
six `neutral' and one `charged' hit for the $(n)p\eta '$ final state,
seven `neutral' for the $n(p)\eta '$ final state, six or seven neutral
or six neutral and one charged for $(np)\eta '$, and at least six `neutral'
without any further condition for $X\eta '$.

The identification of the $\eta 'N$ final states was then based on
a combined invariant and missing mass analysis. The invariant mass analysis
identified the $\eta '$, the missing mass analysis excluded events where
further mesons have been produced but have escaped detection (except for
the $X\eta '$ final state where such events were included).

The invariant mass of all possible disjunct photon pairs was calculated.
Only events having at least one combination of six `neutral'
hits to two photon pairs with invariant masses between 110 and 160 MeV
(pions) and one pair between 500 and 600 MeV ($\eta$) were kept. In cases
where the photons could be combined in more than one way to fulfill this
condition, the `best' combination was chosen via a $\chi^2$ minimization:
\begin{equation}
\chi^2 = \sum_{k=1}^{3}\frac{(m_{k}(\gamma\gamma)-M_k)^2}{(\Delta
m_{k}(\gamma\gamma))^2}
\end{equation}
where for each disjunct combination of the six photons into three pairs
the invariant masses are ordered so that
$m_{1}(\gamma\gamma)\leq m_{2}(\gamma\gamma)\leq m_{3}(\gamma\gamma)$.
The $\Delta m_{k}(\gamma\gamma)$ are their uncertainties (computed
event-by-event from the detector resolution for energies and angles)
and $M_{k}$ is the $\pi^0$-mass for $k=1,2$ and the $\eta$-mass for $k=3$.
The above case applies to events with exactly six `neutral' hits, where in
total 15 different combinations are possible (events with recoil proton or
without detected recoil nucleon). For candidates for the quasi-free reaction
off the neutron (seven `neutral' hits) one must in addition loop over the
unpaired hit, since in CB photons and neutrons cannot be distinguished.
This corresponds at maximum to 105 combinations, giving rise to larger
combinatorial background. In this case, the hit which was not assigned to a
pion- or $\eta$-decay photon is accepted as neutron candidate.
In the case of the $X\eta '$ final state even higher multiplicities may
occur.

As in \cite{Mertens_08} the nominal masses of the mesons
were then used as a constraint to improve the experimental resolution by
re-calculating the measured photon energies from
\begin{equation}
E'_{1,2}=E_{1,2}\frac{m_{\pi^0, \eta}}{m_{\gamma\gamma}} \ ,
\end{equation}
where $E_{1,2}$ are the  measured photon energies, $E'_{1,2}$ the re-calculated,
$m_{\pi^0,\eta}$ are the nominal $\pi^0, \eta$ masses, and $m_{\gamma\gamma}$
the measured invariant masses.

The obtained 6-photon invariant mass distributions using the re-calculated
$\gamma$-energies are shown in Fig. \ref{fig:iden}, (left column) for the
inclusive reaction $(np)\eta '$ and in coincidence with recoil protons
$(n)p\eta '$ and neutrons $(p)n\eta '$. In all cases a clear peak is visible
at the nominal $\eta '$-mass of 958 MeV. The shape of the invariant mass peaks
has been generated via a Monte Carlo simulation with the GEANT package
\cite{Brun_86} and fitted to the data together with a background polynomial.
The peak-to-background is best for $(n)p\eta '$, intermediate for $(np)\eta '$,
and worst for $(p)n\eta '$. This is as expected from the above discussion:
events with seven `neutral' hits have a much larger chance for combinatorial
background (for example from $\eta\pi^0$, $\eta '\pi^0$, $\eta n$ final states,
when a photon is falsely assigned as neutron or vice versa)
than events with six `neutral' and one `charged' hit. This is also reflected
in the background of the missing mass spectra, which is much more pronounced
when the invariant mass background is not subtracted (compare center and right
column in Fig. \ref{fig:iden}).

For the missing mass analysis the recoil nucleons were treated as missing
particles, no matter if they were detected or not.
The missing mass $\Delta m$ of the reaction was calculated for quasi-free
production of $\eta '$ mesons off nucleons via:
\begin{equation}
\Delta m = \left|{\mbox{\bf P}_{\gamma}+\mbox{\bf P}_{N}-\mbox{\bf P}_{\eta '}}\right|
-m_{N}   \ ,
\end{equation}
where ${\mbox{\bf P}_{\gamma}}$, ${\mbox{\bf P}_{N}}$, ${\mbox{\bf P}_{\eta '}}$
are the four-momenta of the incident photon, the incident nucleon (assumed to
be at rest), and the produced $\eta '$-meson; $m_N$ is the nucleon mass.
The resulting distributions peak around zero for quasi-free $\eta '$ production
and are somewhat broadened by the momentum distribution of the bound nucleons,
which was neglected. The distributions are shown in Fig. \ref{fig:iden},
center column.
They have been constructed for an invariant mass window from 930 - 990 MeV
(see Fig. \ref{fig:iden}, left column). Since the background cannot be
completely removed by cuts on invariant mass and missing mass, in the final
step the invariant mass spectra have been analyzed (i.e. fitted by line shape
and background) for each bin of missing mass. The resulting missing mass
spectra corresponding to the invariant mass peaks are shown in
Fig. \ref{fig:iden}, right hand side. Background is much reduced and the shapes
of the missing mass peaks are quite well reproduced by the Monte Carlo
simulation.
The residual background at positive missing masses is mainly
due to the $\eta '\pi$ final state, from events where the pion escaped detection.
It becomes more important at incident photon energies above 1.6 GeV
(see also Fig. \ref{fig:total_incl}) and extends into the range of the missing
mass peak. This explains also the deviation of the simulated line shape from
the data at positive missing mass.
Therefore at energies above 1.6 GeV only events with missing mass
between -200 and 0 MeV were accepted. This reduces counting statistics by
a factor of two but guarantees negligible background contamination.
The analysis described above was done independently for each bin of incident
photon energy and $\eta '$ polar angle.

\begin{figure}[ht]
\resizebox{0.48\textwidth}{!}{%
  \includegraphics{fig_6.eps}
}
\caption{Time-of-flight versus energy for charged particles in TAPS under the
condition of invariant mass signals (cuts on invariant mass peaks, no
background subtraction) for the $\pi^0\eta$ channel (left hand side),
and the $\eta '$ channel (right hand side). The prominent band corresponds to
protons, the less prominent band to deuterons.
}
\label{fig:tofe}
\end{figure}

\begin{figure}[ht]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_7a.eps}
}
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_7b.eps}
}
\caption{Upper part: missing energy. Left hand side: deuteron missing energy,
Right hand side: $\eta '$ missing energy after cut on deuteron missing energy
between $\pm$35 MeV. Points: data, histograms: simulation of signal and background
shapes.
Bottom part: invariant mass spectra. Cut on deuteron missing energy $\pm$35 MeV,
cut on $\eta '$ missing energy: $\pm$25 MeV (left hand side),
$\pm$50MeV (right hand side). Points: data, fitted with background polynomial
plus simulated line shape.
}
\label{fig:iden_coh}
\end{figure}

Absolute cross sections were derived from the target density, the incident
photon flux, the decay branching ratio, the detection efficiency for the
$\eta '\rightarrow 6\gamma$ decay, and the detection efficiency for recoil
nucleons. The detection efficiency was determined via Monte Carlo simulations
using the GEANT3 package \cite{Brun_86}, which included all features of the
detector system and all software cuts for particle and reaction identification.
Events for the final states $(np)\eta '$, $(p)n\eta '$, and $(n)p\eta '$
were generated evenly distributed in phase-space including the effects of
nuclear Fermi motion, tracked with the GEANT package, and analyzed in the
same way as the experimental data. The tracking of recoil nucleons was done
with the GEANT-CALOR program package \cite{Zeitnitz_01}, which is optimized
for hadronic interactions from the few MeV to several GeV range, including
the interactions of low energy neutrons. The efficiency correction was then
done in the usual way as function of incident photon energy and meson cm polar
angle.

Finally, an estimate of the cross section for the coherent reaction
$\gamma d\rightarrow \eta ' d$ was extracted in the following way.
Deuterons in the TAPS detector can be identified via a time-of-flight versus
energy analysis (time-of-flight path 1.18 m) as shown in Fig. \ref{fig:tofe},
where only charged hits (identified by the veto detectors) are included.
The distribution at the right hand side of Fig. \ref{fig:tofe} is obtained for
6-photon events with an invariant mass corresponding to the $\eta '$-meson.
The left hand side of the figure shows for comparison the result for the
$\eta\pi^0$ channel, which shows a much more pronounced deuteron band since
it has a larger coherent component. Events can then be selected by a cut on
the deuteron band, which gives a quite clean data sample for the
$d\eta '$ final state.

However, due to kinematic reasons (mesons from the coherent reaction are
mostly emitted to forward angles due to the nuclear form factor) most
deuterons are emitted into the solid angle of the Barrel, where they cannot
be identified via time-of-flight. Therefore a more complicated analysis based
on reaction kinematics was necessary. The major steps are summarized in
Fig. \ref{fig:iden_coh}. For deuterons in the Crystal Barrel, first the Inner
detector was used for charged particle identification and it was required that
the deuteron candidate was co-planar with the $\eta '$-meson. Further
exploiting the two-body kinematics of the final state, the cm-energies
(photon - deuteron center of momentum system) derived from the final state
four-vectors were compared to the respective values calculated from the
incident photon energy for the deuteron (deuteron missing energy) and for
the $\eta '$ ($\eta '$ missing energy). These spectra are compared in
Fig. \ref{fig:iden_coh}, upper part to Monte Carlo simulations of the coherent
and breakup process and of residual background from $\eta\pi^0\pi^0$ phase
space contributions not related to $\eta '$ production. The corresponding
invariant mass spectra after more or less stringent cuts on missing energy
are shown in the bottom part of Fig. \ref{fig:iden_coh}. The final signal
strength was then extracted via a fit of a background polynomial and the
simulated invariant mass line shape to the data. The systematic uncertainty
of this procedure was estimated by a variation of the $\eta '$ missing energy
cut ($\pm$50 MeV, $\pm$25 MeV, -50 MeV - 0 MeV). The first cut cannot
completely suppress incoherent background, results from the most stringent,
asymmetrical cut are unfortunately limited by the statistical quality of
the data.

\subsection{Systematic uncertainties}

Due to the small cross section of $\eta '$-production, the requirement
of coincident detection of the mesons with recoil particles, and the
low trigger efficiency (only trigger signals from photons in TAPS)
counting statistics were low. Therefore statistical uncertainties
were rather large (on the order of 15 - 35 \% for the $(p)n\eta '$ final
state), which makes it difficult to investigate systematic effects
hidden in statistical fluctuations. Therefore, systematic
uncertainties were partly extracted from other reaction channels,
in particular from the $\eta\rightarrow 6\gamma$ channel.

Three different types of systematic uncertainties may effect the quasi-free
cross sections: overall uncertainties which cancel exactly in the comparison
of proton and neutron final state, uncertainties with similar effects, which
cancel to a large extend in ratios, and uncertainties related to specific
final states which do not cancel.

Into the first category fall the systematic uncertainty of the incident photon
flux, the uncertainty in target thickness (as well as effects from possible
slight misplacements of the target), and the uncertainty from the $\eta '$
decay branching ratios. An estimate of the flux uncertainty was obtained
by a comparison of $\eta$-photoproduction \cite{Jaegle_08} cross sections
obtained by a separate analysis of beam times (A) and (C)
(see Tab. \ref{tab:beam}). These two beam times used different incident electron
beam energies, so that the same photon beam energies were mapped to different
sections on the focal plane detector. Furthermore, since (C) was using an
unpolarized beam and (A) linear polarization, also the energy dependence of
the flux was different due to the coherent peak at roughly 1 GeV. Typical
deviations between the $\eta$ cross sections produced from these two beam times
are at the 5 \% level, maximum deviations around 10 \%. Since the results
from both beam times (with approximately equal statistical weight) were
averaged, we estimate the systematic flux uncertainty at 10 \%.
The overall systematic uncertainty coming from target thickness and positioning
is on the order of a few per cent. The systematic uncertainty of the branching
ratios for  $\eta '\rightarrow \eta\pi^0\pi^0\rightarrow 6\gamma$ is around
7 \%. Allowing for some cancellation, we estimate a total normalization
uncertainty of $\approx 15$ \%. A comparison of the results for
quasi-free production off the proton to free proton results for both the
$\eta$ and the $\eta '$ channel (see below) did not reveal discrepancies
beyond this level.

The second class of systematic uncertainties is dominated by the uncertainty
of the $\eta '$ identification by the missing mass and invariant mass analysis
and the simulation of the $\eta '$ detection efficiency, which are of course
related (the better the respective cuts are reflected by the simulation the
smaller the uncertainty). The simulation of the detection efficiency of photons
followed by an invariant mass analysis for meson identification is very well
under control for the CB/TAPS setup. This has been tested for example via
a comparison of the results for the $\gamma p\rightarrow p\eta$ reaction
obtained from the analysis of the $\eta\rightarrow 2\gamma$ and
$\eta\rightarrow 6\gamma$ decay channels \cite{Crede_05,Bartholomy_07,Crede_09}.
Agreement with the PDG value of the
$\Gamma_{\eta\rightarrow 3\pi^0}/\Gamma_{\eta\rightarrow 2\gamma}$ ratio
is reported in \cite{Bartholomy_07} within an uncertainty at the 2 \% level.
Since errors for the $\eta\rightarrow 2\gamma$ channel in photon detection or
invariant mass analysis enter cubed into the $\eta\rightarrow 3\pi^0$ channel,
this sets stringent limits on the uncertainty. Already a 2 \% error
in photon detection efficiency would result in an 8 \% deviation between
the two decay channels.

A further uncertainty is related to the choice of the event generator
used for the simulation. Events were generated evenly distributed in
phase-space, where the effects of Fermi motion were modeled using the deuteron
wave function in momentum space from \cite{Lacombe_81}.
Since the correction was done as function of incident photon energy
and cm polar angle of the $\eta '$, the angular distribution of the $\eta '$
mesons itself is not critical. However, deviations could arise
if for example final state interaction effects modify the correlation
between meson polar angle and kinetic energy or between meson polar angle
and the kinematic variables of the recoil nucleon.
Possible systematic effects of this kind were investigated with a different
simulation, where the detection efficiency for $\eta '$-mesons
$\epsilon(T_{\eta '},\Theta_{\eta '})$ and the
detection efficiency for recoil nucleons $\epsilon(T_{N},\Theta_{N})$, N=n,p
was quasi-factorized and parameterized in dependence of laboratory kinetic
energies $T_{\eta '}$, T$_{N}$ and polar angles $\Theta_{\eta '}$,
$\Theta_N$ of the particles. Typical efficiencies are
10~\% for $\eta '$ detection (including trigger efficiency), 95 \% for
protons, and 10 - 30~\% for neutrons (depending on energy and including
the identification of the neutron).
These kinematic observables can be directly extracted from the measured
data (the neutron kinetic energy is extracted via the over determined reaction
kinematic from the incident photon energy, the measured $\eta '$ four-vector
and the measured neutron angles). Therefore, an event-by-event efficiency
correction with the product
$\epsilon(T_{\eta '},\Theta_{\eta '})\cdot\epsilon(T_{N},\Theta_{N})$
becomes possible, which does not rely on any model assumptions about the
kinematic final state variables.
This efficiency correction does, however, not include the missing mass cut,
which depends on incident photon energy.
A correction for this effect was extracted from a phase-space simulation.
It does not much depend on details of the event generator, since it uses
only the ratio of two different analyses (with and without missing mass cut)
of the same simulation. Actually as expected from Fig. \ref{fig:iden}
the correction factor is close to 2 for a cut from -200 MeV to 0 (left half of
the peak).
The results from the two different detection efficiency simulations agreed to
better than 5 \% for all investigated reaction channels, and we assume a
systematic uncertainty in this range.

The effects from the background under the invariant mass peaks (see Fig.
\ref{fig:iden}), which is more important for the neutron channel, have not
been treated as independent systematic uncertainties, they have been included
via the peak - background separation into the statistical uncertainties.
An additional systematic uncertainty could arise from the missing mass
analysis. Variations of the accepted missing mass range show significant
influence on the extracted cross section. This, however, does not seem to
be a problem of the agreement between simulation and data, since the shapes
of the signals agree well at the left hand side of the peaks, but start to
disagree at the right hand side where background from $\eta'\pi$ final states
is expected. Therefore, for incident photon energies above 1.6 GeV only events
with $\Delta m<$0 have been accepted.
However, we assign an additional uncertainty rising from 3~\% at threshold
($\eta\pi$ background starts to contribute above 1.6 GeV) to 10~\% at the
maximum energy.
Altogether, independent on the reaction channel, we estimate an uncertainty
of 6~\% close to threshold up to 12~\% at the highest incident photon energies,
(not including effects of recoil nucleon detection).

The last class of uncertainties are those related to the detection of recoil
nucleons, which will not cancel in the comparison of neutron - proton cross
section ratios. The detection of the recoil nucleons was included in the
simulations using the GCALOR package \cite{Zeitnitz_01}, which is optimized
for this purpose. For the proton, the quality of this simulation could be
cross checked with experimental data for the $\gamma p\rightarrow p\eta$
and $\gamma p\rightarrow p\pi^0\pi^0$ reactions which have been measured
with the same setup. The proton detection efficiency was simply determined
as ratio of the number of events with detected recoil proton to the total
number of events from these reactions. The efficiencies have then also been
simulated and the simulated and measured values agree within 8 \% for slow
protons and 4 \% for fast protons.
Combining all uncertainties except the overall normalization we estimate for
the quasi-free proton channel 10~\% at threshold rising to 15~\% at 2.5 GeV.

For the neutron detection efficiency there are no direct measurements
with the combined TAPS/CB setup in Bonn. For the TAPS detector it had been
experimentally determined from the $\gamma p\rightarrow n\pi^0\pi^+$
reaction at the MAMI accelerator in Mainz \cite{Hejny_99}. The results
are consistent with the GCALOR simulation, when the conditions of the Mainz
setup are used ($T_n$=250 MeV: simulated 18.5~\%, from data 19.1 \%).
The neutron detection efficiency of the CB was measured at the LEAR ring at
CERN \cite{Schaefer_93}. Results from the present GCALOR simulation
are in good agreement with the LEAR result except for slow neutrons
($T_n<$ 75 MeV), where the efficiency is very dependent on detector thresholds
and the neutron kinetic energy. However, in any case it is necessary to
determine  `effective' neutron efficiencies which take into account the
identification of the neutrons out of at least seven neutral hits via the
invariant mass analysis discussed in Sec. \ref{subsec:analysis}. This could
only be done by simulations. The reduction of the efficiency under this
conditions compared to the situation were only neutrons are simulated is
substantial, of the order of 25~\% - 35~\%. We therefore estimate the absolute
systematic uncertainty for neutron detection at the 15 \% level. Altogether
a systematic uncertainty of 16~\% at reaction threshold up to 20~\% at
highest incident photon energies is estimated for the quasi-free neutron
channel (excluding the overall normalization uncertainty).

A further systematic uncertainty could arise from the misidentification of
recoil nucleons. While the loss of events is included in the simulated
efficiencies, misidentified protons might contaminate the neutron sample or
vice versa, where the first problem is more severe, due to the smaller
absolute detection efficiency for neutrons. The properties of the Inner
detector for proton identification have been studied in detail in
\cite{Suft_05} with simulations and data from the reaction
$\gamma p\rightarrow \pi^0 p$. The main result was that the
average efficiency for proton detection (somewhat angle dependent) is 98.9 \%
(simulation) respectively 98.4 \% (data). Also determined where the
efficiencies of all three layers of the detector (from data: 94.8 \%
(inner layer), 92.9~\% (middle layer), 88.1 \% (outer layer)). Since in this
experiment the condition for neutrons was that no layer had responded, only
about 0.04 \% of protons may be misidentified as neutrons. The probability that
a neutron activates a coincidence of two layers (condition for proton) is also
negligible. The TAPS veto detectors have on average an inefficiency for
proton detection at the 4 \% level, depending on kinetic energy. However,
for TAPS additional separation of proton and neutron hits is provided by the
time-of-flight versus energy analysis. No trace of the typical proton band
was seen for `neutral' events and the possible contamination of the neutron
sample with protons could be estimated at the 1 \% level. Cross contamination
of the recoil nucleon samples was therefore negligible.

The different systematic uncertainties are summarized in Tab. \ref{tab:systematics}.
It should be noted that the comparison of the quasi-free proton data to free
proton data, as well as the comparison of the two different neutron analyses
(see below) indicate that these estimates are pessimistic.

\begin{table}[h]
\begin{center}
\caption{Summary of systematic uncertainties for the quasi-free reactions.
$^{1)}$ photon flux, target thickness,
decay branching ratios, $^{2)}$ trigger efficiency, $\eta '$ analysis
cuts, $\eta '$ detection efficiency. When two numbers are given first corresponds
to threshold, second to $E_{\gamma}$= 2.5 GeV, and linear interpolation}
\label{tab:systematics}
\begin{tabular}{|c|c|c|}
\hline\noalign{\smallskip}
source & $\gamma d\rightarrow (n)p\eta '$  & $\gamma d\rightarrow (p)n\eta '$ \\
\hline
overall normalization$^{1)}$ & 15~\% & 15 \% \\
\hline
\hline
$\eta '$ detection$^{2)}$ & 6~\% - 12~\%  & 6~\% - 12~\%\\
\hline
recoil nucleon detection  & 8~\% - 4~\%  & 15~\%  \\
\hline
total except overall norm.  & 10~\% - 15~\% & 16~\% - 20~\%  \\
\noalign{\smallskip}\hline
\end{tabular}
\end{center}
\end{table}

For the comparison of the quasi-free $\gamma p\rightarrow \eta ' p$ and
the $\gamma n\rightarrow \eta ' n$ reactions, systematic uncertainties
except the ones from the recoil nucleon detection cancel. However, these
effects can be controlled in an independent way.
As discussed above, the cross section is constructed for $\eta '$ mesons in
coincidence with recoil protons ($\sigma_p$), for $\eta '$ mesons in
coincidence with recoil neutrons ($\sigma_n$), and for $\eta '$ mesons without
any condition for recoil nucleons ($\sigma_{np}$). Since coherent production
processes are very small (see below), the cross sections must be related by
$\sigma_{np} =\sigma_n + \sigma_p$. Therefore, the neutron cross section can
be extracted in two independent ways as $\sigma_n$ or as
$\sigma_{np}-\sigma_p$, one depending only on neutron detection efficiency,
the other depending only on proton detection efficiency. This method has been
previously tested for $\eta$-photoproduction \cite{Jaegle_08}, where excellent
agreement between the two results was found. Also for $\eta '$ the two
results are in good agreement, their weighted average $\langle\sigma_n \rangle$
is given as final result for the neutron cross section and the differences
between them (shown in Figs. \ref{fig:total},\ref{fig:coeff}) are an
independent estimate of the uncertainties introduced by the recoil particle
detection.

\section{Reaction Models}
\label{sec:models}
In the absence of any data for polarization observables,
only a preliminary interpretation of the data in the framework of reaction
models, using model dependent constraints, is possible. In 2003 Chiang et al.
\cite{Chiang_03} have developed a reggeized model for $\eta$ and $\eta '$
production ($\eta '$-MAID), which they used to analyze the then available
proton data.
They parameterized contributions from nucleon resonances in the usual way in
terms of Breit-Wigner curves with energy dependent widths. Non-resonant
Born-terms were neglected since they were expected to be small at not too
high photon energies due to the small $\eta '$-nucleon-nucleon coupling
constant $g_{\eta ' NN}$. However, they are included in the most recent
version of the model used to fit the present data. Contributions from
$t$-channel vector meson exchange are important and were incorporated via
Regge trajectories.
They found, that already a model including just one S$_{11}$ resonance
($W$=1960 MeV) together with the Regge trajectories could reproduce
the available angular distributions for $\gamma p\rightarrow p\eta '$.
Some improvement of the fit was possible by addition of a P-wave resonance,
where a P$_{11}$ or a P$_{13}$, both around $W$ = 1950 MeV, gave equally good
results. However, in the meantime, the database for the proton has been
much improved and, neither the absolute magnitude nor the extreme
forward - backward asymmetry of the early angular distributions
\cite{Ploetzke_98} have been supported by the later experiments
\cite{Dugger_06,Williams_09,Crede_09}, so that these fits needed to be updated.
For this purpose, the model was extended by addition of a D$_{13}$ resonance.
It was then fitted simultaneously to the free proton data from CLAS and ELSA,
to the present quasi-free proton data, and to the quasi-free neutron data.
The effects from Fermi-smearing, although not important, were taken into
account for the quasi-free data sets by folding the model results with the
nucleon momentum distributions.

In a different approach Nakayama and Haberzettl \cite{Nakayama_06} have
analyzed the first CLAS-data \cite{Dugger_06}. They extended their relativistic
meson-exchange model \cite{Nakayama_04} for application to the
$\gamma p\rightarrow p\eta '$ reaction by introducing contributions from
spin-3/2 resonances (the earlier version considered only spin 1/2-states)
and including energy-dependent resonance widths. In addition to the resonance
contributions nucleonic s- and u-channel diagrams and, more important,
mesonic $t$-channel contributions ($\rho$, $\omega$ exchange) are considered.
However, due to the lack of polarization observables, they find also
different solutions.
In Ref. \cite{Nakayama_06} also the $\rho$- and $\omega$-Regge trajectories
have been considered instead of the t-channel rho and omega meson exchanges
to describe the CLAS data \cite{Dugger_06}. There, it is found that the Regge
description yields similar results as the t-channel meson exchange. Therefore,
in the present work, we confine ourselves to their model with t-channel meson
exchange.
The `minimum' solution with the smallest number of resonances in addition to
the nucleonic and mesonic currents that gives an acceptable fit
includes an S$_{11}$(1958) and P$_{11}$(2104) as well as sub-threshold
P$_{13}$(1885) and D$_{13}$(1823) states (which can be considered as
non-resonant backgrounds).
In the following, we call this solution (I), whose parameter values are
summarized in Table I of \cite{Nakayama_06}. Solution (II) includes a further
D$_{13}$-state at $W$=2084 (and different parameter values for the other states
as summarized in Table II of \cite{Nakayama_06}). In further exploratory fits
(Tables III - V in \cite{Nakayama_06}), more resonances were added, but did
not improve the fit quality significantly. Therefore, in the present work we
will only discuss solution (I).
A first analysis of the present quasi-free proton and neutron data was done
in the following way. The results of the fits to the CLAS data for the free
proton (solution (I)) of \cite{Nakayama_06}) have been adopted without any
parameter change and folded with the momentum distribution of the bound proton,
using the deuteron wave function in momentum space \cite{Machleidt_89}.
They are then compared to the quasi-free proton results. For the neutron,
only the electromagnetic photon - resonance couplings of all states have been
varied, while all other resonance parameters (position, width, decay branching
ratios) have been taken from the proton fit. In a second fit, called solution
(Ia), an additional S$_{11}$ resonance was introduced because the neutron data
seems to show a broad bump at higher incident photon energies, which is not
apparent for the proton. In the following, we refer to solutions (I) and (Ia)
as the NH model.

\section{Results}
\label{sec:results}

In the following all quasi-free differential cross sections are given in the cm
(center-of-momentum) system of the incident photon and a target nucleon
{\it at rest}. Apart
from the immediate threshold region, such cross sections are only moderately
smeared out by the effects of nuclear Fermi motion and can thus be compared
almost directly to the corresponding results off free nucleons
(see \cite{Krusche_95b} for details). The angular distributions have been
fitted with Legendre polynomials
\begin{equation}
\frac{d\sigma}{d\Omega} = \frac{q_{\eta '}^{\star}}{k_{\gamma}^{\star}}
\sum_{i} A_iP_i(cos(\Theta^{\star}_{\eta '})) \ ,
\end{equation}
where the $A_i$ are expansion coefficients. The phase-space factor
${q_{\eta '}^{\star}}/{k_{\gamma}^{\star}}$ is also evaluated for the above
cm system. The total cross sections have been extracted from the leading
Legendre coefficient $A_{0}$ of these fits. For some previous free
proton results \cite{Williams_09,Crede_09}) only angular distributions but no
total cross sections or total cross sections extracted by integration of the
angular distributions have been given. For this reason, and in order to treat
all data samples in a consistent way, also for these data sets total cross
sections have been extracted in this work from the fits of the angular
distributions.

\subsection{Quasi-free proton cross section}

We first compare the results from the quasi-free $\gamma d\rightarrow (n)p\eta '$
reaction to free proton data. The angular distributions are summarized in Fig.
\ref{fig:proton}. The total quasi-free cross section off the proton is compared to
free proton results in Fig. \ref{fig:total_incl}.
This figure shows also the inclusive quasi-free cross
section ($\sigma_{np}$) of single $\eta '$ production off the deuteron
(no condition on recoil nucleon) and the fully inclusive cross section
($\sigma_x$) including contributions e.g. from $\eta '\pi$ final states.
At the highest incident photon energies roughly 50 \% of the yield comes
from such meson pairs. For the analysis of the single $\eta '$ channel
these multiple meson production reactions have been eliminated by the
condition that no further mesons have been seen in the detector and by the
kinematic constraints discussed in Sec. \ref{subsec:analysis} for events
where additional mesons have escaped detection.

For the comparison of the quasi-free and free proton cross sections,
one could fold the free cross section data with the momentum distribution
of the bound nucleons. However, simulations have shown, that for photon
energies above 1.6 GeV this effect is small compared to the uncertainty of
the data. Therefore we compare the unfolded data. Apart from a few energy
bins close to threshold, the shapes of the angular distributions of quasi-free
and free proton data are in quite good agreement (see also Fig.~\ref{fig:coeff}
for a comparison of the fitted Legendre coefficients).
For the total cross section, shown in Fig.~\ref{fig:total_incl}, the agreement
between the present quasi-free data (blue squares) and, in particular, the
recent high precision proton data from CLAS \cite{Williams_09}
(magenta stars) is excellent. The agreement with \cite{Crede_09} (open circles)
is within the systematic uncertainties. Altogether, no important nuclear
effects were observed for quasi-free $\eta '$-production off the bound proton.
Therefore, we expect that quasi-free $\eta '$-photoproduction off the bound
neutron is a reasonable approximation of the free neutron reaction.

\begin{figure}[ht]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_8.eps}
}
\caption{Total cross sections for total inclusive $\gamma d\rightarrow X\eta '$
($\sigma_{x}$), inclusive quasi-free $\gamma d \rightarrow np\eta '$ ($\sigma_{np}$) and
quasi-free proton $\gamma d \rightarrow (n)p\eta '$ ($\sigma_p$) cross sections.
The quasi-free proton cross section is compared to the free proton results from
\cite{Crede_09} (open circles) and \cite{Williams_09}
(magenta stars).
Bar histogram at the bottom: systematic uncertainty of quasi-free proton data
excluding the overall normalization uncertainty.}
\label{fig:total_incl}
\end{figure}

\subsection{Inclusive quasi-free cross section}

The most simple approach to estimate the behavior of the neutron cross section
is the measurement of the inclusive $\gamma d\rightarrow (np)\eta '$ reaction,
where only the $\eta '$ is detected, production of further mesons is excluded
by the missing mass cut, and no conditions for the detection of recoil nucleons
are  applied. Since coherent contributions are small, the result is the incoherent
sum of quasi-free proton and quasi-free neutron cross section.

The advantage of this approach is the comparably good statistical quality of
data without detection of coincident neutrons. The angular distributions for
this inclusive reaction are summarized in Fig. \ref{fig:incl} (the total cross
section $\sigma_{np}$ is included in Fig. \ref{fig:total_incl}).
The angular distributions are compared to the recent CLAS-data for the free
proton \cite{Williams_09}, scaled up by a factor of two.

\begin{figure*}[th]
\resizebox{0.99\textwidth}{!}{%
  \includegraphics{fig_9.eps}
}
\caption{Comparison of quasi-free $\eta '$ production off the bound proton
((blue) squares) to the free proton data: (black) open circles \cite{Crede_09},
(black) open crosses: \cite{Dugger_06}, (magenta) stars: \cite{Williams_09}.
The numbers given in the figure indicate the bin centers in incident
photon energy (note: first two bins below free nucleon production threshold).
Note: results from \cite{Dugger_06,Williams_09} partly not exactly
for the same energy bins as present results. Closest bins or average of
overlapping bins chosen. All uncertainties only statistical.
Lines: Solid (black): Legendre fits to data present
data, dashed (red): solution (I) NH model, dotted (blue): $\eta '$-MAID.
 }
\label{fig:proton}
\end{figure*}
\clearpage

\begin{figure*}[th]
\resizebox{0.99\textwidth}{!}{%
  \includegraphics{fig_10.eps}
}
\caption{Angular distributions for the inclusive quasi-free process $\gamma
d\rightarrow (np)\eta '$ of single $\eta '$ production (black triangles).
(Magenta) stars: free proton results from \cite{Williams_09} scaled up
by factor of two.
Note: results from \cite{Williams_09} partly not exactly
for the same energy bins as present results. Closest bins or average of
overlapping bins chosen. All uncertainties only statistical.
Full (black)  lines: Legendre fit of present data, dashed (red) lines: solution (I) of
NH model, (blue) dotted lines $\eta '$-MAID model.
}
\label{fig:incl}
\end{figure*}
\clearpage

\begin{figure}[ht]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_11.eps}
}
\caption{Total cross section for inclusive  $\sigma_{np}$ , proton $\sigma_p$ , and
neutron $\langle\sigma_n \rangle$ final state.
Bar histograms: $(\sigma_n + \sigma_p - \sigma_{np})$, curves at bottom:
$\pm$ systematic uncertainty of $\sigma_n$. Proton (neutron) data points
slightly displaced to the left (right) for better readability of the figure.
Curves: fits with reaction models model. NH model: solid: solution (I),
dotted: solution (Ia), dashed: $\eta '$-MAID.
}
\label{fig:total}
\end{figure}

Agreement between the
two data sets therefore signals regions where proton and neutron cross sections
are more or less identical. This is almost perfectly the case (sometimes with
the exception of the extreme backward angles) for incident photon energies
above 1.9 GeV. This is the region, where the angular distributions are
strongly forward peaked, which in the models is mainly attributed to the
contribution of $t$-channel processes.
At incident photon energies between 1.6 - 1.9 GeV, the inclusive cross section
is significantly smaller than twice the proton values, indicating a region,
with $\sigma_n <\sigma_p$, which could be a first hint to different resonance
contributions.
For the lowest energy bin (1475 MeV) effects of nuclear Fermi
motion become important. Close to the threshold, energy conservation
asymmetrically favors nucleon momenta anti-parallel to the incoming photon
momentum, which results in an enhancement of meson backward angles
(see \cite{Krusche_95b} for details). The data are compared to fits with the
NH (solution (I)) and MAID model.
Shown is the incoherent sum of the model results for proton and neutron,
where the neutron couplings have been fitted to the quasi-free neutron data
(see next section). Agreement with the data for the other versions of the
NH model, which are not shown, is similar to solution (I).

\subsection{Quasi-free neutron cross section}

As discussed in Sec. \ref{sec:analysis}, the quasi-free neutron cross section
can be extracted by coincident detection of the recoil neutrons or as
difference of the inclusive and quasi-free proton cross section. The two
methods give similar results, the averages of the angular distributions are
summarized in Fig. \ref{fig:neutron}. For the fitted Legendre coefficients
also the two individual data sets are shown in Fig. \ref{fig:coeff} as an
estimate of systematic uncertainties.
The total cross section is compared in Fig. \ref{fig:total} to the proton data
and to the results from the reaction models. Differences between the two
extraction methods are indicated by the bar histogram in this figure.

\begin{figure}[th]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_12.eps}
}
\caption{Angular distributions for the quasi-free $\gamma n\rightarrow n\eta '$
reaction. Only statistical uncertainties.
Solid (black) lines: Legendre fit to data. Dashed (red) lines:
solution (I) of NH model, dotted (blue) lines: $\eta '$-MAID model.
}
\label{fig:neutron}
\end{figure}

\begin{figure*}[tht]
\center{\resizebox{0.92\textwidth}{!}{%
  \includegraphics{fig_13.eps}
}}
\caption{Coefficients of the Legendre Polynomials for the fitted angular
distributions. Left hand column: inclusive reaction scaled down by factor of 2.
Center column (proton targets): quasi-free data (blue squares);
free proton data: open crosses \cite{Dugger_06} (omitted for $A_{3}$)),
open circles \cite{Crede_09}, (magenta) stars \cite{Williams_09}.
Right hand column (present quasi-free neutron data): (blue) upward triangles
from neutron coincidence, (black) downward triangles from difference of
inclusive and proton data, (red) circles from averaged data. Symbols
slightly displaced to the left (right) for upward (downward) triangles
to make the plot better readable.
In all plots solid lines: solution (I) NH model, dashed lines: $\eta '$-MAID;
for neutron: (magenta) dotted lines CLAS proton data.
}
\label{fig:coeff}
\end{figure*}

As expected from the discussion of the inclusive data, proton and neutron
angular distributions are similar in magnitude and shape for photon energies
above 1.9 GeV. However, at lower energies they are significantly different.
The high precision CLAS proton data \cite{Williams_09} show a kind of double
bump structure in the total cross section with shallow maxima around
$W$= 1975 MeV and $W$=2080 MeV (cf. Fig. \ref{fig:total_incl}), which at the
very limit of statistical significance is even reflected in the present proton
and inclusive data  (cf. Fig. \ref{fig:total}). The earlier free proton
CLAS data \cite{Dugger_06} may also contain such structures. In fact,
the model calculation of Ref. \cite{Nakayama_06}, which fits the
differential cross sections data of Ref. \cite{Dugger_06}, has predicted
such shallow bump structures in the total cross section at about the same
two energies.

\begin{table*}
\renewcommand{\arraystretch}{1.25}
\caption{Resonance parameters of the $\eta^\prime\,MAID$ model \cite{Chiang_03}.
Resonance positions $M$ and total widths $\Gamma_{tot}$ in MeV.
Resonance couplings defined by $\chi^N_J \equiv \sqrt{\beta_{N\eta'}}A^N_J$
(in units of $10^{-3}GeV^{-1/2}$), where $\beta_{N\eta'}$ denotes the
$N^*\to N\eta'$ branching ratio and, $A^N_J$, the helicity amplitude;
$N=p,n$ and $J=1/2, 3/2$. $R$ is the neutron/proton ratio of the
electromagnetic widths: $R\equiv\Gamma_{\gamma n}/\Gamma_{\gamma p}$.
Coupling constants for the background nucleon born
terms $g_{\eta 'NN}=$-0.18 and vector meson exchange
($\rho$, $\omega$),
$g_{\rho NN}=$2.0, $\kappa_{\rho NN}=$3.5,
$g_{\omega NN}=$12.0, $\kappa_{\omega NN}=$0.56 (vector coupling $g$ and
tensor/vector ratio $\kappa$), $g_{\rho\eta '\gamma}=$1.24,
$g_{\omega\eta '\gamma}=$-0.43 (for details see \cite{Chiang_03}).}
\label{tab:maid}
\begin{center}
\begin{tabular*}{\textwidth}
{@{\hspace{0.7cm}}c@{\hspace{0.7cm}}|@{\hspace{1.2cm}}c @{\hspace{1.2cm}}c
@{\hspace{1.5cm}}c @{\hspace{1.3cm}}c @{\hspace{1.3cm}}c
@{\hspace{1.3cm}}c @{\hspace{1.3cm}}c}
\hline\noalign{\smallskip}
Resonance & $M$ & $\Gamma_{tot}$ & $\chi^p_{1/2}$ &
$\chi^p_{3/2}$ & $\chi^n_{1/2}$ & $\chi^n_{3/2}$ &  $R$ \\
\noalign{\smallskip}\hline\noalign{\smallskip}
$S_{11}$  & 2004. & 286. &  19.7  &          & -14.6   &       &   -0.56   \\
$P_{11}$  & 2100. & 100. &  2.0   &          &   0.77  &       &   0.15  \\
$P_{13}$  & 1920  & 100  & -1.0   &  -4.2    &   5.0   & 0.97  &   1.4  \\
$D_{13}$  & 2150. & 230. &  12.0  &  -5.2    &  -1.4   & 0.96  &   0.02  \\
 \noalign{\smallskip}\hline
\end{tabular*}
\end{center}
\end{table*}

\begin{table*}
\renewcommand{\arraystretch}{1.25}
\caption{Resonance parameters of version (I) of the NH model \cite{Nakayama_06}.
Notation as in Tab. \ref{tab:maid}.
Background parameters: $g_{\eta 'NN}=$0.43,
$g_{\rho NN}=$3.3, $\kappa_{\rho NN}=$6.1,
$g_{\omega NN}=$10.0, $\kappa_{\omega NN}=$0, $g_{\rho\eta '\gamma}=$1.25,
$g_{\omega\eta '\gamma}=$0.44.
The spin-3/2 resonances, $P_{13}$ and $D_{13}$, are sub-threshold
resonances and, as such, they may be considered as part of the background
contribution. Further details of model
(I) may be found in Table.I of Ref.~\cite{Nakayama_06}. Version (Ia) includes
in addition a second S$_{11}$ resonance at $W$=2180 MeV and $\Gamma$=110 MeV.
}
\label{tab:nh}
\begin{center}
\begin{tabular*}{\textwidth}
{@{\hspace{0.7cm}}c@{\hspace{0.7cm}}|@{\hspace{1.2cm}}c @{\hspace{1.2cm}}c
@{\hspace{1.5cm}}c @{\hspace{1.3cm}}c @{\hspace{1.3cm}}c
@{\hspace{1.3cm}}c @{\hspace{1.3cm}}c}
\hline\noalign{\smallskip}
Resonance & $M$ & $\Gamma_{tot}$ & $\chi^p_{1/2}$ &
$\chi^p_{3/2}$ & $\chi^n_{1/2}$ & $\chi^n_{3/2}$ &  $R$ \\
\noalign{\smallskip}\hline\noalign{\smallskip}
$S_{11}$  & 1958. & 139. &  -12. &       & -17.  &       &   1.91   \\
$P_{11}$  & 2104. & 136. &  -13. &       &  -5.  &       &   0.16   \\
$P_{13}$  & 1885. &  59. &       &       &       &       &   0.02   \\
$D_{13}$  & 1823. & 450. &       &       &       &       &   1.24   \\
 \noalign{\smallskip}\hline
\end{tabular*}
\end{center}
\end{table*}

For the neutron, the first
bump is more clearly visible, while the second one is suppressed.

The fitted coefficients of the Legendre representation of the angular
distributions are summarized in Fig. \ref{fig:coeff}. For the quasi-free
neutron the results for the two different extraction methods and for the
average are shown.
Agreement of the two data sets for the even coefficients ($A_0$, $A_2$) is
mostly  within statistical uncertainties. Some discrepancies outside
statistical uncertainties are observed for $A_1$ and $A_3$. These odd
coefficients are very sensitive to the extreme forward and backward angles,
where due to small detection efficiencies uncertainties in the data are more
important. For the same reason no values are given for $A_3$ for the earlier
CLAS-data \cite{Dugger_06}.
Due to the relatively small angular coverage the fits were not sensitive to it.

The excellent agreement between free and quasi-free proton data is demonstrated
in the center column of the figure. Effects from nuclear Fermi motion are
mostly insignificant. The largest effect results again for the odd coefficients
($A_{1}$, A$_{3}$) since the asymmetric preference for nucleon momenta
anti-parallel to the photon momentum induces a false forward-backward asymmetry
in the quasi-free data. However, the effect is small. In case of the most
precise recent CLAS data, folding with Fermi motion (not shown in the figure)
improves slightly the agreement for the odd coefficients.
For a better comparison of proton and neutron data, the new CLAS proton data
are included as magenta, dotted lines into the pictures of the neutron column.
The largest difference occurs for the $A_0$ coefficient, while the results
are quite similar for the $A_1$, $A_2$, and $A_3$ coefficients.
Only close to threshold there could be some systematic deviation.

The neutron data have been fitted with the NH model solution (I) (the
other solutions give very similar results) and the $\eta '$-MAID
model. The results are shown in Figs.~\ref{fig:total}, \ref{fig:neutron},
\ref{fig:coeff}.
For both models, all resonance parameters except the electromagnetic neutron
couplings were taken over from, respectively were dominated by, the fits to
the proton data. For the NH model also a modified version with an additional
S$_{11}$ resonance was tested, which, however did not much improve the
agreement with the data. The parameters of the two models are summarized in
Tabs. \ref{tab:maid},\ref{tab:nh}. For both models agreement between fit and
data is less good as for the proton data. For the total cross section, the
MAID-fit does not well reproduce the threshold region. None of the fits
reproduces the region above 2~GeV. For the coefficients of the angular
distributions particular
$A_2$ disagrees with the data, which like for the proton rises above 2 GeV
while it is flat and very small for the fits.

In this most basic version both models include apart from the background terms
an S$_{11}$ and a P$_{11}$ resonance around $W$=2 GeV
(respectively $W$=2.1 GeV). It must be emphasized again, that in both models
these solutions are by far not unique. Nevertheless, there seems to be
agreement in so far, as both models need an S$_{11}$ resonance close to
threshold in order to fit the sharp rise of the total cross section and a
P$_{11}$ state to explain the shape of the angular distributions via an
S-P-interference term, which essentially has a linear cos($\Theta^{\star}_{\eta '}$)
behavior. Although also a P$_{13}$ state and/or the interference with
background amplitudes can give rise to such a behavior. For both states there
are candidates in the Particle Data Group Review \cite{PDG}, the
S$_{11}$(2090) and the P$_{11}$(2100) both one star states with not well
defined
positions and widths. In case of the P$_{11}$ both model analyses result in
similar positions, widths, and neutron/proton ratio of the electromagnetic
couplings, although the absolute contribution of this state is stronger
for the NH model. The S$_{11}$ has similar positions but different widths.
In case of the MAID model the proton coupling is stronger and there is a
negative sign between proton and neutron coupling, while for the NH model
the relative sign is positive and the neutron coupling is stronger.
Consequently, not even for these two `dominant' resonances agreement
is found between the two models. Obviously, further observables must be
measured to arrive at better constraints for the model analyses.

Here, one should also note, that already in the Lagrangian parameterizations
of the background terms differences occur between the two models in both, the
structure of some Lagrangians, as well as in the numerical values of coupling
constants. As an example for the vector meson currents, the coupling constant
for the $t$-channel $\omega$-exchange is positive for the NH model, while it is
negative for the MAID model (see Tabs. \ref{tab:nh},\ref{tab:maid}). On the
other hand, the corresponding coupling for the $\rho$-meson is positive in both
models. This leads to a destructive interference between this two terms in the
MAID model, while the interference is constructive for the NH model. A more
detailed analysis of the $t$-channel background terms is therefore also necessary.
This might profit from data at higher incident photon energies, where this
contribution dominates. Another example are the baryonic
background currents where the NH model uses a pure pseudo-vector coupling
at the $NN\eta'$ vertex, while MAID uses pseudo-scalar coupling, again giving
rise to a relative sign between the amplitudes of the two models.

\subsection{The coherent $\gamma d\rightarrow \eta ' d$ reaction}

Coherent photoproduction is important due to its direct connection to the
iso-scalar parts of electromagnetic transition amplitudes. However, due to the
dependence on the nuclear form factor it is strongly suppressed for
heavier mesons. The results for the total cross section obtained from the
analysis discussed in Sec. \ref{sec:analysis} are summarized in
Fig. \ref{fig:coh}. The typical size of systematic uncertainties can be
estimated by a comparison of the results from the three analyses using more
or less stringent cuts for the missing energy analysis, which is needed to
remove background from quasi-free processes. All results are on the order of
only a few nb, the values from the two more stringent cuts ($\Delta E_{\eta '}$
between $\pm$25 MeV or between -50 - 0 MeV) are in reasonable agreement, those
from the most open cut ($\pm 50$ MeV) may still include a small background
contribution.

\begin{figure}[th]
\resizebox{0.5\textwidth}{!}{%
  \includegraphics{fig_14.eps}
}
\caption{Total cross section for the coherent reaction
$\gamma d\rightarrow d\eta'$. Different symbols correspond to analyses with
different analysis cuts: closed (red) points: $\pm$25 MeV $\eta '$
missing mass, open circles: $\pm$50 MeV, open (blue) squares: -50 - 0 MeV.
Solid line: model prediction using the $\eta '$-MAID multipoles.
}
\label{fig:coh}
\end{figure}

The results from the re-fitted reggeized MAID model \cite{Chiang_03} discussed
above have been used to model the coherent reaction, using the parameters
summarized in Tab. \ref{tab:maid}. The formalism is based
on a standard impulse approximation without multiple scattering contributions,
i.e. the nuclear transition operator is taken as a sum of free single nucleon
operators sandwiched between the deuteron wave functions. The general
expression for the nuclear amplitude reads
\begin{equation}\label{10}
T_{M_fM_i\lambda_\gamma}=2\int\frac{d^3p}{(2\pi)^3}\
\psi_{M_f}^\dag\!\!\left(\vec{p}-\frac12\vec{q}\right)
t^{\lambda_\gamma}_{\gamma\eta^\prime}
\ \psi_{M_i}\!\!\left(\vec{p}-\frac12\vec{k}\right)\,,
\end{equation}
where $\vec{k}$ and $\vec{q}$ are momenta of the initial photon and a final
meson. The indices $M_f$, $M_i$, and $\lambda_\gamma$ are respectively
the $z$-projection of the final and initial deuteron spin and the photon
polarization index. For the deuteron wave function, $\psi_M(\vec{p}\,)$,
the momentum-space representation
\begin{eqnarray}\label{15}
\psi_M(\vec{p}\,) & = & \sum\limits_{L=0,2}
\sum_{M_L}(1M-M_L\, LM_L|1M)u_L(|\vec{p}\,|)\\
& & ~~~~~~~~~~~~~~~~~~~~~~~~~\times Y_{LM_L}(\hat{p})\chi_{M-M_L}
\nonumber
\end{eqnarray}
is taken, with $\chi_m$ being the triplet spin wave function.
For the spatial part $u_L(p)$ the parameterization \cite{Machleidt_87} derived
from the Bonn $NN$ potential model was used. For $\eta '$ photoproduction
details of the deuteron wave function are more important than for lighter
mesons since due to the large mass already at threshold large momentum
transfers are involved. This causes a strong dependence on the behavior of the
nuclear potential at small distances, in particular the $D$-wave component
of the wave function becomes important. The calculations predict, that for
example at photon energies around 1.5 GeV the $D$-wave contributes already
roughly 50 \% to the total cross section.

The operator for $\eta^\prime$ photoproduction on a single nucleon has the
well known spin structure in the Pauli matrix representation
\begin{equation}\label{20}
t_{\gamma\eta^\prime}^{\lambda_\gamma}=
K_{\lambda_\gamma}+\vec{L}_{\lambda_\gamma}\cdot\vec{\sigma}\,,
\end{equation}
with the spin-flip component $\vec{L}$ and the non spin-flip $K$.

The result corresponding to the parameter set from Tab. \ref{tab:maid} is
compared to the data in Fig. \ref{fig:coh}. Given the simplicity of the
impulse approximation, neglecting all two-body mechanisms as well as final
state interaction effects, the agreement is quite good, demonstrating that
the relative contribution of iso-scalar components is well represented in
the model.

The predictions for the angular dependence are very sensitive to the assumed
resonance and background contributions. For example, at low incident photon
energies, where the S$_{11}$ resonance makes a large contribution, the
spin-flip amplitude $\vec{L}$ is dominant giving rise to a forward peaking
of the cross section. At higher incident photon energies, where other
resonances and the $t$-channel background dominate, the spin-independent
part $K$ is strong. This part is proportional to sin$(\Theta^{\star}_{\eta '})$ and
thus vanishes at forward angles. Due to the statistical limitations of the
data, it was not possible to extract angular distributions. A measurement with
better statistical quality is highly desirable.

\section{Summary and Conclusions}
\label{sec:conclu}
We have reported the first measurement of $\eta '$-mesons off the deuteron.
Both, the quasi-free and the coherent reaction carry important information
about the iso-spin composition of the elementary reaction on the free nucleon.

For the proton case, it has been demonstrated with this experiment, that the
quasi-free cross section off the bound nucleon is very similar to the free
proton cross section. No significant nuclear effects e.g. from FSI processes
have been observed. At the given level of statistical precision of the data,
even effects from the momentum distribution of the bound nucleons are almost
insignificant. At low incident photon energies, they cause a small artificial
forward-backward asymmetry in the angular distributions (coefficient $A_1$
in the Legendre series). At somewhat higher energies, folding the free proton
cross section with the momentum distribution improves slightly (within
systematic uncertainties) the agreement of free and quasi-free data for the
$A_3$ coefficient of the angular distributions. Only at very low incident
photon energies, also the magnitude of the cross section is affected, but in
that range free nucleon results are also not available or not precise.
Agreement of the quasi-free data with the most recent measurement off the
free proton from the CLAS-collaboration \cite{Williams_09} was found at a
level much below the estimated systematic uncertainty of the present
experiment. Consequently, the deuteron can be regarded as very well suited
target to study the $\gamma n\rightarrow\eta ' n$ reaction.

The quasi-free reaction off the bound neutron has been studied in two different
ways with different sources of systematic uncertainty. In one approach,
the $\eta '$-mesons were detected in coincidence with the participant neutrons.
In the second approach, the cross section obtained for coincident participant
protons was subtracted from the inclusive results without condition for recoil
nucleons. Since the detection efficiency for protons and neutrons is very
different, a comparison of the two results gives a good estimate of the
systematic uncertainty for recoil particle detection. We had previously found
excellent agreement for a similar analysis of $\eta$-photoproduction
\cite{Jaegle_08}, which used the same data set. Also for the present analysis
of $\eta '$-photoproduction good agreement is found, which indicates, that
systematic effects are well under control. Only for the extreme forward and
backward angular range, some discrepancies remain, which effect mainly the
$A_{1}$ coefficient of the Legendre series for the angular distributions
(see Fig. \ref{fig:coeff}). Altogether, the quality of the present
$\gamma n\rightarrow n\eta '$ data is mostly limited by counting statistics,
not so much by the systematic effects originating from the complications of
a quasi-free reaction.

Proton and neutron cross sections behave similar at incident photon energies
above 2 GeV, where contributions from $t$-channel exchange are important.
At lower incident photon energies, in particular between 1.6 - 1.9~GeV, where
the proton cross section peaks, the behavior is different, which might point
to different resonance contributions, but could also arise from changing
interference terms between the resonances or between resonances and
non-resonant background.

The data have been compared to two different models, both with contributions
from a similar set of nucleon resonances and background terms, in
particular from $t$-channel mesonic currents. As already pointed out in
\cite{Nakayama_06} differential cross sections alone cannot uniquely determine
the contributing reaction mechanisms. Consequently, in the framework of
both models different solutions can be found. Future measurements of
polarization observables have to clarify the situation.

Finally, also a first estimate of the coherent $\gamma d\rightarrow d\eta '$
contribution at the level of at most a few nanobarn ($\sigma_{d\eta '}< 5$ nb
for all investigated photon energies is a reasonable estimate) has been
extracted. This reaction is not only important for
the iso-spin separation of the elementary production amplitudes, but aims
also at the study of the $\eta '$ nucleon interaction via FSI effects.
The extracted results are consistent with an impulse approximation,
indicating, that the iso-spin composition of the model amplitudes is at least
not unreasonable and that FSI contributions are not substantial. However, the
statistical limitation of the data is even more important at this low cross
section level, so that effects beyond impulse approximation could not be
studied.

\vspace*{1cm}
\noindent{{\bf Acknowledgments}}

{\noindent We} wish to acknowledge the outstanding support of the accelerator group and operators
of ELSA. This work was supported by Schweizerischer Nationalfonds and Deutsche
Forschungsgemeinschaft (SFB/TR-16).


text:
Faith in the Future: Seattle Union Gospel Mission
Leesa donates 293 mattresses to the historic West Coast shelter.
We're continually inspired by the uplifting work our giving partners do. It's a reminder that the world is full of heartache, yes, but it's also a place where strangers devote their lives to healing that heartache.
A few weeks back, Leesa had the opportunity to donate 293 brand-new mattresses to the Seattle Union Gospel Mission, which provides emergency and long-term shelter to the hurting and homeless of Seattle
Our Head of Social Impact, Sophie Wolfe, coordinates Leesa mattress donations through our Leesa One-Ten initiative and traveled to the West Coast to represent Leesa. The Mission's team shot these stunning photos.
Above, several guests of the Mission's Women & Children's program laugh together on a new Leesa mattress.
In its 82-year history, the faith-based Seattle Union Gospel Mission has helped thousands of people transition to stable, secure housing and rise out of poverty.
What began as a program to aid people during the Great Depression continues today, with around 1,500 people attending at least one of the daily free meals the Mission serves.
Dana, pictured above, explained how the Mission changed her life. "I was completely broken. I didn't have my loved ones, I didn't have a roof over my head, and I was caught up in addiction. I've gone through my fair share of horrible nights, and to be able to sleep comfortably and safely – it's just a world of difference." Now, Dana says she has "a year clean and nothing but hope for the future."
Michael is also a guest at the Mission. He described the importance of having a safe, comfortable place to sleep. "A good night's sleep means everything. It can totally change how your day goes."
Through our One-Ten program, Leesa donates one mattress to shelters and non-profits across the country for every 10 sold.
Learn more about the Seattle Union Gospel Mission's legacy and programs, then consider making a donation to support their work.

redpajama_set_name: RedPajamaCommonCrawl


Snowman PowerPoint is a funny snowman template for PowerPoint that you can use for Christmas celebration or snow background presentations. The slide contains a white background and a snowman image. The colors at the bottom are really attractive for celebration templates, cold background, cold slide for PowerPoint and similar.


1,389 backers pledged €55,555 to help bring this project to life.
We look forward to producing the game and getting it to backers. We will update you regularly! Also keep an eye on jollydutch.com, our Facebook and our Instagram.
Welcome to Chartered: The Golden Age, the boardgame that allows you to become a golden age merchant and kickstart the growth of Amsterdam.
How did we develop Chartered? Have a look at the (Dutch) video below.
Chartered: The Golden Age contains a double sided gameboard. One side with more room for merchants to build warehouses in the city center, usually you would use this side with four to six players. The other side is compact and situated in the harbour, where players are forced to have more interaction. The harbour side is usually used with two or three players, or for shorter four player games.
All elements in the game are premium quality from the start, so no bad quality cards or chips that need to be upgraded by unlocking stretch goals!
The main elements in Chartered: The Golden Age are the warehouses you use to build in Amsterdam. Each game includes a total of 104 stackable warehouses, in three different designs.
What do you do in the game? You build, you buy and you plan your victory. Turns are quick and the rules are short, below an overview of your typical turn followed by the 4-page rule book in different languages. Note that because of the reached stretch goal, the English, Dutch, French and German rulebooks are included in print in the final game.
Retailers, please pledge EUR 1 and contact us via KS or info@jollydutch.com to place an order.
Jolly Dutch Productions is a new game publisher, set up by Alexander and Arnold in the Netherlands. We combine our love for (board) gaming and entrepreneurial spirit to develop new games, specifically looking to bring together families and friends for a jolly evening. Having spent many months (blind) play testing, we are proud of Chartered: The Golden Age as it stands.
Henk-Jan had the challenge to turn our concepts and ideas into beautiful artwork in a matter of months.
Over the course of several years, Alexander and Arnold developed Chartered: The Golden Age to the fully designed and production-ready game it is today.
Despite it being the first game developed by Jolly Dutch Productions, Alexander and Arnold have worked with one of the largest board game producers in Europe (Ludo Fact, based in southern Germany) to ensure all obstacles are taken away and classic Kickstarter pitfalls are avoided. The designs of all elements have been thoroughly tested and checked. In the same spirit, shipping has been arranged with veteran Kickstarter project fulfilment companies.
We just need to raise the funds and place the production order! We will do everything we can to ensure on-time delivery and keep you up to date on our production and shipping processes, also informing you of any unforeseen hiccups.
You are welcomed as a fellow merchant!


text:
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Women's rights and gender equality
Conflict and arms
'Time was running out': Honduran activist's last days marked by threats
Berta Cáceres, the environmental advocate shot dead in her home in March, told friends of a hitman boasting about his plans as she 'worked frantically'
Nina Lakhani in La Esperanza
Mon 25 Apr 2016 04.08 EDT Last modified on Thu 15 Oct 2020 09.28 EDT
Berta Cáceres: an outspoken voice for the environment is silenced
In her final days, Berta Cáceres was bombarded with texts and calls warning her to give up the fight against the Agua Zarca dam, or else.
The Honduran indigenous leader told trusted friends and colleagues that some of the death threats were from a suspected sicario – or hitman – who was terrorizing community members near the dam and openly boasting of his intention to kill her.
Cáceres started making arrangements to move from her isolated bungalow on the outskirts of the city of La Esperanza to a bustling lodging house run by her organisation, the Civic Council of Popular and Indigenous Organizations of Honduras (COPINH), where she wouldn't be alone.
Garifuna indigenous people and members of the local community protest near the presidential house in Tegucigalpa during the indigenous forum 'Berta Cáceres Lives'. Photograph: Orlando Sierra/AFP/Getty Images
The day before she was murdered, Cáceres took her youngest daughter to the airport. As they hugged goodbye, she whispered a final piece of advice. "She told me: 'If something happens to me, don't be scared,'" Laura Cáceres, 23, told the Guardian.
Around lunchtime the following day, Cáceres stopped to sign some cheques at COPINH's women's centre, where she told Lilian Esperanza, a longtime friend and the group's financial coordinator, to plan for her not being around. "She wanted to change the rules so someone else could sign checks. She was worried about being murdered or imprisoned," said Esperanza. "'I keep reporting the threats, but no one pays attention,' she told me."
Less than 12 hours later, Cáceres was shot dead in her home. Her friend Gustavo Castro, coordinator of Friends of the Earth Mexico, was injured in the attack but survived by playing dead.
Despite the evidence that she had been targeted because of her campaign against the dam, police treated three of her closest colleagues – Castro and two members of COPINH – as the prime suspects.
"My daughter was systematically persecuted for years, but still, I didn't believe they would actually kill her," said Berta Flores, 83, sitting next to the candlelit altar adorned with fresh flowers and photographs.
"She worked frantically in the days before she was killed. It's as if she knew time was running out."
Cáceres was buried on 4 March on what would have been her 45th birthday.
Remembering Berta Cáceres: 'I'm a human rights fighter and I won't give up'
The murder of the celebrated activist shocked the world, despite Honduras' inglorious ranking as the deadliest country for environmental campaigners.
Cáceres, who last year won the Goldman environmental prize for her work, had reported 33 death threats linked to the campaign. Just days before her death, she accused the Honduran company behind the dam, Desarrollos Energéticos SA (Desa), of using local thugs to terrorize her and COPINH as they tried to mobilize opposition to its revised plan to dam their sacred river, the Gualcarque.
The Honduran state was complicit in this intimidation, she alleged.
Since her murder, the violence and threats against COPINH members have continued. Earlier this month, several were injured after a peaceful march including international observers was attacked by people armed with rocks and machetes. Witnesses said that the aggressors chanted "we killed the fly, only the plebs are left" as police and army officers looked on.
Among the attackers was the alleged hitman Cáceres had warned her colleagues of.
Despite international pressure on Honduras to capture the assassins – and the people who sent them – fears are mounting that COPINH members could be framed in an attempt to crush opposition to the dam and deter other communities from derailing similar projects.
Lidia Marcela Zuniga Caseres, center, daughter of Berta Cáceres, leads a march to demand justice over the murder of her mother in Tegucigalpa on 1 April. Photograph: Fernando Antonio/AP
"Blaming people close to Berta is part of the crime," said Víctor Fernández, COPINH's lawyer. "Leaders are murdered to terrorize communities, contaminate organisations and squash resistance movements. This is the pattern."
Honduras and the dirty war fuelled by the west's drive for clean energy
The current wave of repression against social and political activists in Honduras has its roots in the 2009 military-backed coup d'état. The rightwing government that replaced President Manuel Zelaya licensed hundreds of environmentally destructive mega-projects, including mines, hydroelectric dams and model cities, ignoring its obligation to seek consent from indigenous communities.
As protests mounted, violence spread: between 2010 and 2015, 109 environmental activists were murdered, reported Global Witness.
Meanwhile, concessions were granted for 50 projects, mostly dams, on Lenca territory.
The Gualcarque river is considered sacred by Cáceres's Lenca people; it also provides them food and drinking water. Although relatively small, the proposed dam would desiccate the river and render life impossible for the Rio Blanco community.
An elderly Lenca indigenous woman participates in Berta Caceres Vive - to protect their land, territories and natural resources. Photograph: Orlando Sierra/AFP/Getty Images
Cáceres filed complaints, lodged appeals and organised protests in an attempt to force a proper consultation, but the project forged ahead.
In April 2013, Cáceres orchestrated a community roadblock preventing Desa from accessing the dam site. The threats against her and other community leaders started immediately.
They were followed; shot at; harassed by police, soldiers and Desa's private security officers; and threatened with kidnap and death, according to a detailed log kept by international observers. Cáceres and her close colleagues Tomas Gómez and Aureliano Molina were forced into hiding for months until false charges of usurpation, coercion and criminal damage were dismissed.
For about two years, construction stalled. Then, late last summer, community members noticed construction was under way on the other side of the river, which is not Lenca territory. Desa had started work on a revised Agua Zarca project which would redirect the river before it reached the Rio Blanco community.
In October 2015, the community set up camp on the river bank. Over the coming weeks, they were attacked with boulders and rocks by civilians flanked by armed private security and police officers.
Around this time, the suspected hitman – who was often seen with Desa security guards – starting bragging that he would murder Caceres and other COPINH leaders, multiple sources have told the Guardian.
A woman spray-paints the phrase 'Always Alive' below a stencilled image of Berta Cáceres during the commemoration of international women's day in Tegucigalpa on 8 March. Photograph: Fernando Antonio/AP
The man was arrested in December for illegal possession of weapons, but freed the following day after the charge was downgraded, according to Fernández, COPINH's lawyer. In 2015, the man was accused of murder until a key witness changed his testimony.
Justino Paz, 64, lived in the same community – upstream from Rio Blanco - where Desa has won support by promising jobs, electricity, and a school. He said that the same man had threatened him.
"I opposed the dam because the river gives us life. He put a pistol to my head and told me to leave or die, and then burned down my house. I lost everything," said Paz.
A Desa spokesman denied any links with the suspect, saying: "He is not our employee and has no relationship with us."
Two further incidents spooked Cáceres.
On 30 November 2015, she was threatened in public by local politicians and people armed with stones, machetes and handguns. The police refused to intervene.
Then, on 20 February 2016, a COPINH group was blocked en route to the river and their vehicles vandalized. Several witnesses said they had heard Cáceres threatened by Desa employees and a local politician. Cáceres reported the incident to authorities.
Cáceres was no stranger to threats, but she was now afraid. "She wouldn't let me stay alone, and she didn't want to be alone," said her daughter Laura.
On 2 March, the day she was murdered, Cáceres and Castro, the Mexican activist, arrived home around 7.30pm. The bright green bungalow where she'd moved two months earlier is in a secluded gated community on the southern edge of town along an unpaved, unlit road. Each of the 30 or so houses is enclosed by a wire fence; the dusty complex is flanked by pine-covered mountains.
Cáceres's house is approximately 150 metres from the guarded entrance where two assassins entered around 11.30pm. Cáceres was shot four times and died almost instantly.
The first person on the scene was COPINH member Tomás Gómez, who arrived at around 2am and drove Castro to safety. Chaos ensued as investigators, COPINH members, and Cáceres' family arrived on the scene.
Auriliano Molina, with whom Cáceres had had a romantic relationship which ended last summer, arrived around 4.30am, after driving 40 miles from his home.
Over the following days, Castro, Gómez and Molina were questioned for hours.
"Berta was murdered and they're trying to criminalize me and Auriliano – the same three they tried to jail in 2013. They want COPINH to disappear," Gómez said.
Cáceres had many enemies. She had campaigned against destructive projects across the country and had been accused of obstructing development by irked politicians. But the principal source of threats against her life was linked to Agua Zarca. Yet the police pushed other theories.
Berta Caceres with her four children in a photo taken in 1999. Photograph: Courtesy Beverly Bell
The first hypothesis mooted by police was robbery. The second was a crime of passion, with Castro and Molina suggested as possible culprits. The third was an internal conflict within COPINH.
Castro – the only witness to the murder – was prevented from leaving Honduras for a month. Molina remains a prime suspect, according to a source close to the investigation not authorized to comment officially. The Honduran public ministry did not respond several requests for comment.
It was 11 days before investigators went to Desa's offices. Fernández says he told authorities about the alleged sicario the day after the murder.
A Desa spokesman said none of its employees or contractors had ever threatened Caceres or anyone from COPINH, but they could not be held responsible for third parties. The company provides only food for local people who support the dam and protest against COPINH, the spokesman added.
The clamour surrounding the official investigation continues to grow amid concerns the authorities are neither willing to solve nor capable of solving a crime so emblematic of the country's rampant repression and impunity.
The Inter-American Commission on Human Rights has offered to send a team of independent experts to investigate the murders of Cáceres and other high-profile activists. The government has declined.
Cáceres was threatened so often, for so long, that she almost grew used to it, according to her daughter. But the threats were real.
"She won the Goldman prize, she met the pope, I thought recognition would protect her," said Laura Cáceres. "Whoever was behind her murder wanted to send a message that no one is safe, that they can kill anyone. We can't accept impunity."
This article was amended on 26 April 2016 to correct a caption. The original caption referred to Lenca indigenous people; those pictured are from the Garifuna community.
Berta Cáceres


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Three-Year-Old Mountain Lion Killed By Rat Poison
Wildlife scientists warn that mountain lions could go extinct in Southern California within the next 50 years. Mountain lions populations are rapidly decreasing due to loss of habitat, lack of genetic diversity and rat poison.
National Park Service (NPS) biologists received a mortality signal from the GPS collar of a three-year-old mountain lion known as P-47. They hiked into the Santa Monica Mountains to discover what caused his death. When they came upon his body they noticed that there were no visible wounds.
Photo: Facebook/Santa Monica Mountains National Recreation Area
Santa Monica Mountains National Recreation Area posted on Facebook, "We have some sad news to report." They went on to say, "A necropsy revealed that he may have succumbed to poisoning from anticoagulant rodenticide, commonly known as rat poison. Testing on a sample of his liver showed that he had been exposed to not just one, but SIX different anticoagulant compounds! Internal hemorrhaging was also found in his head and lungs."
The young lion likely ingested the poisons when he ate other animals. "Researchers believe mountain lions are exposed through secondary or tertiary poisoning, meaning that they eat an animal that ate the poisonous bait, such as a ground squirrel, or an animal that ate an animal that consumed the poisonous bait, such as a coyote."
Photos: Facebook/Santa Monica Mountains National Recreation Area
P-47 tied for the record of largest male, last weighing in at 150 pounds. Biologist hoped that he would breed and increase the genetic diversity of the species.
Rat poison has become a real problem and is impacting many other species. "Biologists have documented the presence of anticoagulant rodenticide compounds in 21 out of 22 local mountain lions that they have been tested, including in a three-month-old kitten."
NPS study found coyotes and bobcats were impacted by rat poison with 83% of coyotes and 92% of bobcats exposed to anticoagulant poisons. It was the second leading cause of death for coyotes in the study.
Biologists urge people to stop using rat poison which inadvertently poisons wildlife. They suggest alternatives that will not harm other species. Please share this with your friends and family and urge them to stop using rat poison.
Watch the video below for alternatives to rat poison.