Limiting soft particle production and QCD

Abstract

We present some basic elements of the treatment of particle multiplicities in jets from high energy collisions within perturbative QCD. Then we discuss the universal features of the inclusive particle spectrum for the limiting case of momentum p\to 0 (or p_T\to 0) as expected from soft QCD gluon bremsstrahlung. The energy independence of the invariant particle density in this limit I_0=E dN/d^3p|_{p\to 0} is predicted as well as the dependence of this quantity on the the colour factors characteristic of the underlying partonic processes. These properties are first recalled from e+e- collisions and then extended to pp and nuclear collisions according to Ref. 1. Present data support these predictions. It will be interesting to see whether new incoherent contributions show up in the new energy regime of LHC.