Event Structure of Proton-Nucleus Collisions with High Transverse Energy.

The event structure of proton-nucleus (pA ) interactions has been studied as a function of the inelasticity of the reaction. The data were taken with the Fermilab Multiparticle Spectrometer. The secondary particles produced in the collisions of 800 GeV/c protons on hydrogen, beryllium, carbon and lead targets were detected by highly segmented calorimeters covering the central and forward fragmentation regions. Events with high values of transverse energy (E_{t}) were selected with full azimuth "GLOBAL" triggers. Calorimeter module responses were organized into clusters thereby providing characteristics of event structure such as multiplicities, average transverse momenta (< p_ {t}>) and leading particle effects. The dependences of these quantities on E _{t}, the multiplicity density ({dnover dy}), and the energy of the beam jet have been examined. The event structure of pA interactions strongly depends on E_{t}. However at a fixed E_{t}, there is weak nuclear dependence of the average event structure in both the central and forward fragmentation regions. This observation is consistent with a thermodynamical description of pA interactions and suggests that E _{t} is a good variable to measure the inelasticity of the reaction. The dependence of < p_{t} > on {dnover dy} has been compared for pp and pPb events. At a fixed value of {dnover dy}, < p _{t} > is higher for pPb. A non-statistical event by event fluctuation in < p _{t} > has been observed. This fluctuation decreases with increasing nuclear weight, in agreement with previous results for light nuclear targets. The dependence of the event characteristics in the central region on the properties of the beam jet has been examined for minimum bias and high E_ {t} events. At a fixed value of the beam jet energy, the energy in the central rapidity region increases with nuclear weight for minimum bias events. At a fixed value of E_{t}, the average event characteristics in the central region show no dependence on the beam jet energy. The fraction of E_ {t} carried by the leading particle in the central region increases with the collimation of the beam jet for pp events. This effect is not observed for pPb events.