ec 2 00 4 H 1 Diffractive Structure Function Measurements and QCD Fits

Measurements of diffractive structure functions in ep collisions and diffractive parton densities extracted from QCD fits are presented. At the HERA ep collider the diffractive quark structure of the proton is probed with a point-like photon (Fig. 1). The virtuality of the pho-2 β X Figure 1. Diffractive ep scattering. ton is denoted by Q 2 and sets the hard scale of the interaction. Diffraction is characterised by an elastically scattered proton which loses only a small fraction x I P of its initial beam momentum. These events are selected experimentally by detecting the proton at small scattering angles (roman pot detectors) or by requiring a large empty area in the detector between the outgoing proton and the hadronic system X produced in the interaction (rapidity gap). The squared 4-momentum t transferred at the proton vertex can be measured by tagging the proton. For the ra-pidity gap method, which accesses a much larger event sample, the cross section has to be integrated over |t| < 1 GeV 2 and in ≈ 10% of the events the proton is excited into a hadronic system of small mass < 1.6 GeV. The two methods give the same results when compared in the same kinematic range. In a picture which depicts diffraction as a two step process, the proton exchanges a diffractive object (often called the pomeron) with momentum fraction x I P and the quark struck by the pho-ton carries a fraction β of the momentum of the diffractive exchange. Additional kinematic variables are Bjorken-x x = βx I P and the inelasticity y = Q 2 /(sx) where s is the ep centre-of-mass energy squared. The cross section is proportional to the combination of two structure functions: where Y = y 2 1+(1−y) 2 is a kinematic factor resulting from the difference of the fluxes of transversely and longitudinally polarised photons from the electron. F D 2 is proportional to the diffractive γ * p cross section, whereas F D L is related only to the part induced by longitudinal photons. The factor Y is sizable at large values of y and in most of the phase space measured so far, F D L is a small correction. 2. FACTORISATION IN DIFFRACTION The diffractive structure functions have been proven to factorise into diffractive parton densities f D i of the proton convoluted with ordinary 1