Jet evolution in a dense medium: event-by-event fluctuations and multi-particle correlations

Miguel A Escobedo,Edmond Iancu

doi:10.1016/j.nuclphysa.2017.05.031

Abstract

We study the gluon distribution produced via successive medium-induced branchings by an energetic jet propagating through a weakly-coupled quark-gluon plasma. We show that under suitable approximations, the jet evolution is a Markovian stochastic process, which is exactly solvable. For this process, we construct exact analytic solutions for all the n-point correlation functions describing the gluon distribution in the space of energy [M. A. Escobedo, E. Iancu, Event-by-event fluctuations in the medium-induced jet evolution, JHEP 05 (2016) 008. arXiv:arXiv:1601.03629, doi:http://dx.doi.org/10.1007/JHEP05(2016)008, M. A. Escobedo, E. Iancu, Multi-particle correlations and KNO scaling in the medium-induced jet evolution, JHEP 12 (2016) 104. arXiv:arXiv:1609.06104, doi:http://dx.doi.org/10.1007/JHEP12(2016)104]. Using these results, we study the event-by-event distribution of the energy lost by the jet at large angles and of the multiplicities of the soft particles which carry this energy. We find that the event-by-event fluctuations are huge: the standard deviation in the energy loss is parametrically as large as its mean value [M. A. Escobedo, E. Iancu, Event-by-event fluctuations in the medium-induced jet evolution, JHEP 05 (2016) 008. arXiv:arXiv:1601.03629, doi:http://dx.doi.org/10.1007/JHEP05(2016)008]. This has important consequences for the phenomenology of di-jet asymmetry in Pb+Pb collisions at the LHC: it implies that the fluctuations in the branching process can contribute to the measured asymmetry on an equal footing with the geometry of the di-jet event (i.e. as the difference between the in-medium path lengths of the two jets). We compute the higher moments of the multiplicity distribution and identify a remarkable regularity known as Koba-Nielsen-Olesen (KNO) scaling [M. A. Escobedo, E. Iancu, Multi-particle correlations and KNO scaling in the medium-induced jet evolution, JHEP 12 (2016) 104. arXiv:arXiv:1609.06104, doi:http://dx.doi.org/10.1007/JHEP12(2016)104]]. These predictions could be tested via event-by-event measurements of the di-jet asymmetry.

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 jets will lose energy inside of the medium, the amount of energy loss will depend on the size of the medium that they traverse
The difference is that in a medium fluctuations and correlations are much more important. In this proceedings we have reviewed the computation of the fluctuations of 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]. The point of the collision region in which the hard process takes place is not always the center, a deviation from this point will have as a consequence that the effective size of the medium seen by each jet will be different. The two jets will lose energy inside of the medium, the amount of energy loss will depend on the size of the medium that they traverse. This might not be the whole story. A similar result was obtained recently by a Monte Carlo computation in [7] Another issue we want to discuss in this proceedings is what are the event-by-event 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