Elastic and proton-dissociative photoproduction at HERA
The ZEUS Collaboration, J. Breitweg et al.
Argonne National Laboratory, Argonne, IL, USA
Received: 3 December 1997
Elastic and proton-dissociative photoproduction (, ,respectively, with )has been studied in ep interactions at HERA for photon-proton centre-of-mass energies in the range 50<W<100 GeV and for |t|<0.5 GeV2, where t is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range MN2<0.1 W2. For the elastic process, the invariant mass spectrum has been investigated as a function of t. As in fixed target experiments, the resonance shape is asymmetric; this asymmetry decreases with increasing |t|, as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant production. The cross section has been studied as a function of W; a fit to the resonant part with the form Wa gives .The resonant part of the cross section is b at GeV. The t dependence of the cross section can be described by a function of the type with (stat.) +1.0-0.5 (syst.) GeV-2 and (stat.) +1.9-1.7 (syst.) GeV-4. The t dependence has also been studied as a function of W and a value of the slope of the pomeron trajectory GeV-2 has been deduced. The spin density matrix elements r0400, r041-1 and have been measured and found to be consistent with expectations based on s-channel helicity conservation. For proton-dissociative photoproduction in the mass range, the distributions of the two-pion invariant mass, W and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The t distribution has been fitted to an exponential function with a slope parameter GeV-2. The ratio of the elastic to proton-dissociative photoproduction cross section is .