Inelastic Distributions and Color Structure in Perturbative QCD

Abstract The tree-like evolution of a QCD jet in a planar gauge is studied, with particular emphasis on the soft parton region of the phase space. We calculate (in a gauge-invariant way) multiplicities and multiplicity distributions, and find a Q 2 dependence which is more than logarithmic but less than a power. We also compute the resulting KNO scaling function, thus confirming the existence of long-range correlations. A general method for selecting semi-inclusive distributions is given, and applied to the study of colorless clusters. The mass of such clusters is of order of the confinement scale Q 0 , and their distributions are simply related to the ones of partons of off-shell mass Q 0 ( α ( Q 0 2 )⪅ 1). We also give the general structure of the tree evolution at the “exclusive” level, and we find p ⊥ spectra which are damped in all relative transverse momenta. Nevertheless, the integrated exclusive probabilities sum up correctly to one in a non-trivial way.