Diffractive Dijets and Gap Survival Probability at HERA

by Sebastian Schaetzel (CERN)

Monday 21 Apr 2008, 16:30 → 17:30 Europe/Zurich

Council Chamber ** (CERN)

The study of diffractive proton scattering is one of the main successes of the electron-proton collider HERA. In these diffractive collisions, the proton stays intact but a hard scatter occurs which allows comparison with perturbative QCD calculations. The universality of the diffractive proton structure is investigated by comparing different hard scattering final states, and by comparing collisions initiated through the exchange of point-like photons with those induced by real photons. In this talk I will review the HERA results which can be summarised as follows: While all point-like photon scattering processes (deep-inelastic scattering, DIS) can be described using the same diffractive parton densities (factorisation), the situation is different in real photon scattering where the diffractive dijet cross section measured by H1 for E_T(jet1)>5 GeV, E_T(jet2)>4 GeV is a factor of approximately 0.5 lower than expectations based on the DIS parton densities (factorisation breaking). Strikingly, this gap survival probability factor is independent of the photon energy that manifests itself outside the jet system---direct and resolved photon processes are equally suppressed. This behaviour is unexpected, but can be understood qualitatively in the dipole picture of photon-proton scattering. I will also present latest results which indicate that the survival probability depends on the transverse energy of the jets.

Slides schaetzel_ep_20080421.pdf