Multiplicity distributions and rapidity gaps

We examine the phenomenology of particle multiplicity distributions, with special emphasis on the low multiplicities that are a background to the study of rapidity gaps. In particular, we analyze the multiplicity distribution in a rapidity interval between two jets, using the herwig QCD simulation with some necessary modifications. The distribution is not of the ‘‘negative binomial’’ form, and displays an anomalous enhancement at zero multiplicity. Some useful mathematical tools for working with multiplicity distributions are presented. It is demonstrated that ignoring particles with ${\mathit{p}}_{\mathrm{\perp }}$<0.2 GeV/c has theoretical advantages, in addition to being convenient experimentally.