Properties of the hadronic final state in diffractive deep inelastic scattering tagged with the leading proton spectrometer of ZEUS

One of the exciting results discovered at the ep collider HERA at DESY, Hamburg, is the occurrence of diffractive Deep Inelastic Scattering (DIS) events observed previously in hadron-hadron collisions. The ZEUS detector is equipped with the Leading Proton Spectrometer (LPS), which, for a restricted parameter set, allows to measure all products of the reaction ep → epX, including the final state proton, which otherwise would escape through the forward beam pipe. The nature of the diffractive exchange is studied in terms of the properties of the hadronic final state X characterized by the global event shape variables thrust and sphericity, and by the dynamic quantities of transverse momenta in and out of the event plane as a function of the energy scale (M_X) of the final state. Inclusive distributions of energy and transverse energy flow are presented as a function of pseudo-rapidity. These variables are sensitive to the parton level mechanism responsible for the diffractive exchange. The measurement of the hadronic final state is compared to results from e+e− annihilation and to predictions of various Monte Carlo models, which describe diffraction by the exchange of a hadronic object (Pomeron) and by photon dissociation into quarks and gluons, respectively. The hadronic final state exhibits a dominantly two-jet structure. This structure is also maintained by Monte Carlo models, which include gluons in the final state. The overall best description of the diffractive hadronic final state is given by models, which implements a QCD-based photon dissociative approach. The analysis is based on the 1997 data set and is restricted to the region of 4 < M_X < 35 GeV, 4 < Q2 < 150 GeV2, 0.0003 < x_IP < 0.03 and 0.03 < y < 0.7.