Collimation of average multiplicity in QCD jets

Revert field The collimation of average multiplicity inside quark and gluon jets is investigated in perturbative QCD in the modified leading logarithmic approximation (MLLA). The role of higher order corrections accounting for energy conservation and the running of the coupling constant leads to smaller multiplicity collimation as compared to leading logarithmic approximation (LLA) results. The collimation of jets produced in heavy-ion collisions has also been explored by using medium-modified splitting functions enhanced in the infrared sector. As compared to elementary collisions. the angular distribution of the jet multiplicity is found to broaden in QCD media at all energy scales.We would like to thank M.-A. Sanchis Lozano for useful comments on the manuscript. F.A. thanks the hospitality of CERN PHTH department where part of this work has been performed. R.P.R. is grateful for the hospitality and nice stay at Max Planck Institut für Theoretische Physik in Munich and thanks W. Ochs for enlightening discussions. He also thanks the warm welcome at Universitat de València-CSIC and IFIC, where this work was completed, and acknowledges support from the Generalitat valenciana under grant PROMETEO/2008/004.Peer reviewe