Measuring centrality with slow protons in proton-nucleus collisions at 18 GeV/c

Experiment E910 has measured slow protons and deuterons from collisions of 18 GeV/c protons with Be, Cu, and Au targets at the BNL AGS. These correspond to the {open_quotes}grey tracks{close_quotes} first observed in emulsion experiments. We report on their momentum and angular distributions and investigate their use in measuring the centrality of a collision, as defined by the mean number of projectile-nucleon interactions. The relation between the measured N{sub grey} and the mean number of interactions {bar {nu}}(N{sub grey}) is studied using several simple models, one newly proposed, as well as the RQMD event generator. RQMD is shown to reproduce the N{sub grey} distribution, and exhibits a dependence of N{sub grey} on centrality that is similar to the behavior of the simple models. We find a strong linear dependence of {ovr N{sub grey}} on {nu}, with a constant of proportionality that varies with target. For the Au target, we report a relative systematic error for extracting {bar {nu}}(N{sub grey}) that lies between 10 and 20h{percent} over all N{sub grey}. {copyright} {ital 1999} {ital The American Physical Society}