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

Miguel A Escobedo,Edmond Iancu

doi:10.1016/j.nuclphysbps.2017.05.019

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 [1, 2].

Highlights

One of the observables in which the formation of a collective medium in heavy-ion collisions manifests in a very clear way is the dijet asymmetry, the energy difference between two approximately back-to-back jets [3, 4]
The two energetic jets are initially created in a hard process, due to momentum conservation the two jets will have back-to-back directions and approximately the same energy
The fact that the formation of the two jets does not happen at the center of the fireball translates in an asymmetry in the effective length of the medium seen by each jet that at the same time translates into an asymmetry in the energy loss

Summary

Introduction

One of the observables in which the formation of a collective medium in heavy-ion collisions manifests in a very clear way is the dijet asymmetry, the energy difference between two approximately back-to-back jets [3, 4]. Imaging the case in which the hard process happens at the center of the collision and the medium length seen by each jet is exactly the same Even in this case, because energy loss is a stochastic process, it can happen that by chance one of the jets loses more energy than the other, and if fluctuations are large this will be quite common. In real world there would be fluctuations both in the effective length and in the energy loss and there is no trivial way to disentangle them Another issue we want to discuss in this proceedings is what are the properties of gluons produced by the energy loss mechanism.

Jet quenching formalism

The gluon spectrum and the average energy loss

The 2-point function and the fluctuations of the energy loss

The n-point functions and KNO scaling

Conclusions