Professor Brian Foster OBE, FRS

Abstract:

This paper presents the first analysis of diffractive photon dissociation events in deep inelastic positron-proton scattering at HERA in which the proton in the final state is detected and its momentum measured. The events are selected by requiring a scattered proton in the ZEUS leading proton spectrometer (LPS) with xL > 0.97, where xL is the fraction of the incoming proton beam momentum carried by the scattered proton. The use of the LPS significantly reduces the contamination from events with diffractive dissociation of the proton into low mass states and allows a direct measurement of t, the square of the four-momentum exchanged at the proton vertex. The dependence of the cross section on t is measured in the interval 0.073 < \t\ < 0.4 GeV² and is found to be described by an exponential shape with the slope parameter b = 7.2 ± 1.1(stat.)^+0.7 -0.9(syst.) GeV^-2. The diffractive structure function F^D(4) 2 is presented as a function of xIP, ≃ 1 - xL and β, the momentum fraction of the struck quark with respect to xIP, and averaged over the t interval 0.073 < \t\ < 0.4 GeV² and the photon virtuality range 5 < Q² < 20 GeV². In the kinematic range 4 x 10^-4 < xIP < 0.03 and 0.015 < β < 0.5, the xIP dependence of F^D(4) 2 is fitted with a form (1/xIP)^a , yielding a = 1.00 ± 0.09 (stat.)^+0.11 -0.05(syst.). Upon integration over t, the structure function F^D(3) 2 is determined in a kinematic range extending to higher xIP and lower β compared to our previous analysis; the results are discussed within the framework of Regge theory.

Abstract:

Photon diffractive dissociation, γp → Xp, has been studied at HERA with the ZEUS detector using ep interactions where the virtuality Q2 of the exchanged photon is smaller than 0.02 GeV2. The squared four-momentum t exchanged at the proton vertex was determined in the range 0.073 < \t\ < 0.40 GeV2 by measuring the scattered proton in the ZEUS Leading Proton Spectrometer. In the photonproton centre-of-mass energy interval 176 < W < 225 GeV and for masses of the dissociated photon system 4 < Mx < 32 GeV, the t distribution has an exponential shape, dN/d\t\ ∝ exp (-b\t\), with a slope parameter b = 6.8 ± 0.9 (stat.) +1.2-1.1 (syst.) GeV-2.