QCD initial-state radiation at small χ

For hadron-hadron and hadron-lepton collisions the range of applicability of perturbative QCD has recently been extended to the semi-hard phase-space region of small χ, with χ ≅ Q2/s. In this region the multi-parton distributions have a factorized structure and can be described as a branching process. This branching is very different from the one used in the present Monte Carlo generators based on the Altarelli-Parisi equation. Indeed the new branching for small χ leads to the Lipatov equation for the structure function. The main properties of the emission in the small χ regions are presented. (author)