Adependence of highly inelasticp-nucleus collisions

The nuclear ($A$) dependence of the event structure in the central and forward (projectile fragmentation) regions has been studied for highly inelastic proton-nucleus ($pA$) collisions at an incident momentum of 800 GeV/c. Events with high values of transverse energy ($E_t$) were selected using the large-acceptance Fermilab multiparticle spectrometer. The multiplicity distributions and energy flows in different regions of phase space are presented for six nuclear targets: H, Be, C, Al, Cu, and Pb. The event structure for highly inelastic $pA$ collisions is strongly dependent on the transverse energy observed in the central region. However at a fixed $E_t$, there is only a weak nuclear dependence of event structure in the central and forward fragmentation regions. This observation is consistent with a thermodynamical description of inelastic $pA$ interactions and suggests that the observed $E_t$ is a good measure of the inelasticity of the reaction. The nuclear dependence of event structure on the particle density and the energy in the forward fragmentation region is also discussed. In contrast with $E_t$ as a variable, the particle density and the forward missing energy provide measures of event inelasticity that are $A$ dependent.