A Study of the Dynamics of Di ractive Photoproduction at HERA

A measurement is presented using the H detector of the di erential cross section d dM X for di ractive photoproduction processes in which the proton vertex is elastic and the nal state photon system has mass MX GeV The average p centre of mass energy is GeV Using the forward components of the detector events are selected on the basis of a large gap in the pseudorapidity distribution of nal state hadrons adjacent to the leading proton By using a reconstruction method that is insensitive to any non containment of parts of the nal state in the photon direction the cross section is evaluated for values of MX down to those consistent with the exclusive production of the lowest lying vector meson states and The total cross section for the quasi elastic production of one of these states or for the non resonant photon dissociation to is found to be stat sys b The total cross section for photon dissociation to the continuum of higher mass states in the measured kinematic range is stat sys b In the high mass region the di erential cross section is tted to the triple Regge form d dM X M X n and a value n stat sys is obtained By demanding activity in the pseudorapidity range and that the largest rapidity gap in the main part of H is bounded by the forward edge in acceptance a limited measurement of the di erential cross section d dM X has been made for di ractive processes in which the proton dissociates to any state with mass mp MY GeV The cross section for such processes that are quasi elastic at the photon vertex is found to be stat sys b An investigation of the double dissociative di ractive process in which both MX and MY are large suggests that its cross section is similar to that of single photon dissociation The acceptance of the H Forward Muon Trigger in its con guration is evaluated from simulations For tracks passing through the rst three double layers of drift cells the acceptance is found to be for single muons generated in the range of polar angle o o with momentum p  GeV For tracks that also traverse the iron toroidal magnet of the system and the three subsequent double layers the acceptance is for muons in a similar range of polar angle and with p  GeV