Dijet cross sections and parton densities in diffractive DIS at HERA

Differential dijet crosssections in dirractive deep-in elastic 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 atleast 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 part on distribution unctions previously extracted from measurement so inclusive dirractive deep-inelastic scattering.The prediction describes the dijet data well at low and intermediate z(P)(the fraction of the momentum of the dirractive exchange carried by the part on entering the hard interaction)where the gluon density is well determinedfromtheinclusivedirractive data,supporting QCD factorisation. A new set of dirractive part on distribution functions is obtained through a simultaneous fit to the dirractive inclusive and dijet crosssections.This allows for a precise determination of both the dirractive quark and gluon distributions in the range 0.05<0.9.In particular,the precision on the gluon density at high momentum fractions is improved compared to previous extractions.