Diffractive Photon Dissociation in a High Pressure Hydrogen Time Projection Chamber

We have performed an experiment at the Tagged Photon Facility of Fermilab to study the diffraction dissociation of high energy photons on hydrogen (gamma) + p (--->) X + p in the region 0.02 < (VBAR)t(VBAR) < 0.1 (GeV/c)('2), M(,x)('2)/s < 0.1. In this process, incident photons whose energies range from 70 to 140 GeV transform coherently to massive hadronic states in the mass range M(,x) < 5 GeV/c('2). We measure the inclusive differential cross section d('2)(sigma)/(dt dM(,x)('2)) (1). The behavior of this cross section, especially when compared to the corresponding cross sections for the diffraction dissociation of incident hadrons (pions, kaons, and protons), reveals some fundamental characteristics of photon hadronic interactions. We use the Recoil Technique to determine the missing mass, M(,x), and the square of the 4-momentum transfer, t. The recoil detector, TREAD, is a cylindrical time projection chamber filled with high pressure hydrogen gas which serves both as the target and as the drift medium for the ionization track created by recoil protons. The ionization drifts up to 75 cm in a high axial electric field. Concentric sense wires mounted on endplates sample different parts of the track, yielding the polar angle of the recoil. The energy of the recoil is determined by stopping the proton in scintillation counters located inside the high pressure vessel. We have found that, as in the case of hadron dissociation, the photon dissociation cross section (1) exhibits exponential t dependence, falls as 1/M(,x)('2) with M(,x)('2 )for high masses, M(,x)('2 )> 4 (GeV/c('2))('2), and obeys a Factorization Rule. If one views diffraction dissociation as mediated by Regge exchanges, this behavior is consistent with that predicted by triple-Pomeron dominance. Furthermore, the application of a Finite Mass Sum Rule to the data indicates that the photon may have a "direct" hadronic interaction in addition to that described by the Vector Dominance Model of photon hadronic interactions.