Event-by-event picture for the medium-induced jet evolution

TL;DR

This paper provides an exact analytical description of medium-induced gluon branching in jet evolution within quark-gluon plasma, revealing large fluctuations and a KNO scaling in gluon multiplicity, with implications for LHC experiments.

Contribution

It introduces an exact solution to the stochastic branching process of jet evolution in a medium, including correlations and fluctuations, which was not previously available.

Findings

Medium-induced branchings are quasi-democratic with sizable energy fractions.

Large fluctuations in energy loss and soft gluon multiplicity are predicted.

Gluon multiplicity distribution exhibits KNO scaling.

Abstract

We discuss the evolution of an energetic jet which propagates through a dense quark-gluon plasma and radiates gluons due to its interactions with the medium. Within perturbative QCD, this evolution can be described as a stochastic branching process, that we have managed to solve exactly. We present exact, analytic, results for the gluon spectrum (the average gluon distribution) and for the higher n-point functions, which describe correlations and fluctuations. Using these results, we construct the event-by-event picture of the gluon distribution produced via medium-induced gluon branching. In contrast to what happens in a usual QCD cascade in vacuum, the medium-induced branchings are quasi-democratic, with offspring gluons carrying sizable fractions of the energy of their parent parton. We find large fluctuations in the energy loss and in the multiplicity of soft gluons. The multiplicity distribution is predicted to exhibit KNO (Koba-Nielsen-Olesen) scaling. These predictions can be tested in Pb+Pb collisions at the LHC, via event-by-event measurements of the di-jet asymmetry. Based on JHEP 1605 (2016) 008 and arXiv:1609.06104