The neutron electric dipole moment (nEDM)
is a measure for the distribution of positive
and negative charge inside the neutron. A
finite electric dipole moment can only exist
if the centers of the negative and positive
charge distribution inside the particle do
not coincide. So far, no neutron EDM has been
found. The current best upper limit amounts
to |dn| < 3.0×10−26 e⋅cm.
== Theory ==
A 
permanent electric dipole moment of a fundamental
particle violates both parity (P) and time
reversal symmetry (T). These violations can
be understood by examining the neutron's magnetic
dipole moment and hypothetical electric dipole
moment. Under time reversal, the magnetic
dipole moment changes its direction, whereas
the electric dipole moment stays unchanged.
Under parity, the electric dipole moment changes
its direction but not the magnetic dipole
moment. As the resulting system under P and
T is not symmetric with respect to the initial
system, these symmetries are violated in the
case of the existence of an EDM. Having also
CPT symmetry, the combined symmetry CP is
violated as well.
=== Standard Model prediction ===
As it is depicted above, in order to generate
a finite nEDM one needs processes that violate
CP symmetry. CP violation has been observed
in weak interactions and is included in the
Standard Model of particle physics via the
CP-violating phase in the CKM matrix. However,
the amount of CP violation is very small and
therefore also the contribution to the nEDM:
|dn| ~ 10−31 e⋅cm.
=== Matter–antimatter asymmetry ===
From the asymmetry between matter and antimatter
in the universe, one suspects that there must
be a sizeable amount of CP-violation. Measuring
a neutron electric dipole moment at a much
higher level than predicted by the Standard
Model would therefore directly confirm this
suspicion and improve our understanding of
CP-violating processes.
=== Strong CP problem ===
As the neutron is built up of quarks, it is
also susceptible to CP violation stemming
from strong interactions. Quantum chromodynamics
– the theoretical description of the strong
force – naturally includes a term which
breaks CP-symmetry. The strength of this term
is characterized by the angle θ. The current
limit on the nEDM constrains this angle to
be less than 10−10 rad. This fine-tuning
of the θ-angle, which is naturally expected
to be of order 1, is the strong CP problem.
=== SUSY CP problem ===
Supersymmetric extensions to the Standard
Model, such as the Minimal Supersymmetric
Standard Model, generally lead to a large
CP-violation. Typical predictions for the
neutron EDM arising from the theory range
between 10−25 e⋅cm and 10−28 e⋅cm.
As in the case of the strong interaction,
the limit on the neutron EDM is already constraining
the CP violating phases. The fine-tuning is,
however, not as severe yet.
== Experimental technique ==
In order to extract the neutron EDM, one measures
the Larmor precession of the neutron spin
in the presence of parallel and antiparallel
magnetic and electric fields. The precession
frequency for each of the two cases is given
by
h
ν
=
2
μ
n
B
±
2
d
n
E
{\displaystyle h\nu =2\mu _{\text{n}}B\pm
2d_{\text{n}}E}
,the addition or subtraction of the frequencies
stemming from the precession of the magnetic
moment around the magnetic field and the precession
of the electric dipole moment around the electric
field. From the difference of those two frequencies
one readily obtains a measure of the neutron
EDM:
d
n
=
h
Δ
ν
4
E
{\displaystyle d_{\text{n}}={\frac {h\,\Delta
\nu }{4E}}}
The biggest challenge of the experiment (and
at the same time the source of the biggest
systematic false effects) is to ensure that
the magnetic field does not change during
these two measurements.
== History ==
The first experiments searching for the electric
dipole moment of the neutron used beams of
thermal (and later cold) neutrons to conduct
the measurement. It started with the experiment
by Smith, Purcell and Ramsey in 1951 (and
published in 1957) obtaining a limit of |dn|
< 5×10−20 e⋅cm. Beams of neutrons were
used until 1977 for nEDM experiments. At this
point, systematic effects related to the high
velocities of the neutrons in the beam became
insurmountable. The final limit obtained with
a neutron beam amounts to |dn| < 3×10−24
e⋅cm.After that, experiments with ultracold
neutrons took over. It started in 1980 with
an experiment at the Leningrad Nuclear Physics
Institute obtaining a limit of |dn| < 1.6×10−24
e⋅cm. This experiment and especially the
experiment starting in 1984 at the Institut
Laue-Langevin pushed the limit down by another
two orders of magnitude yielding the above
quoted best upper limit in 2006, revised in
2015.
During these 50 years of experiments, six
orders of magnitude have been covered thereby
putting stringent constraints on theoretical
models.
== Current experiments ==
Currently, there are at least seven experiments
aiming at improving the current limit (or
measuring for the first time) on the neutron
EDM with a sensitivity down to 10−28 e⋅cm
over the next 10 years, thereby covering the
range of prediction coming from supersymmetric
extensions to the Standard Model.
nEDM experiment at Oak Ridge National Laboratory
nEDM experiment running (n2EDM under construction)
at the UCN source at the Paul Scherrer Institute
UCN nEDM experiment under construction at
TRIUMF
Cryogenic neutron EDM experiment being set
up at the Institut Laue-Langevin
nEDM experiment being envisaged at the Spallation
Neutron Source
nEDM experiment being built at the Institut
Laue-Langevin
nEDM experiment being built at the Forschungsreaktor
München II
== See also ==
Electron electric dipole moment
