  Differential dijet cross sections in diffractive deep-inelastic scattering
are measured with the H1 detector at HERA using an integrated luminosity of
51.5 pb-1. The selected events are of the type ep --> eXY, where the system X
contains at least two jets and is well separated in rapidity from the low mass
proton dissociation system Y. The dijet data are compared with QCD predictions
at next-to-leading order based on diffractive parton distribution functions
previously extracted from measurements of inclusive diffractive deep-inelastic
scattering. The prediction describes the dijet data well at low and
intermediate zpom (the fraction of the momentum of the diffractive exchange
carried by the parton entering the hard interaction) where the gluon density is
well determined from the inclusive diffractive data, supporting QCD
factorisation. A new set of diffractive parton distribution functions is
obtained through a simultaneous fit to the diffractive inclusive and dijet
cross sections. This allows for a precise determination of both the diffractive
quark and gluon distributions in the range 0.05<zpom<0.9. In particular, the
precision on the gluon density at high momentum fractions is improved compared
to previous extractions.

High Energy Physics - Experiment