Medium-induced jet evolution: multiple branching and thermalization (original) (raw)

Thermalization of mini-jets in a quark-gluon plasma

EPJ Web of Conferences, 2016

We complete the physical picture for the evolution of a high-energy jet propagating through a weakly-coupled quark-gluon plasma by investigating the thermalization of the soft components of the jet. We argue that the following scenario should hold: the leading particle emits a significant number of mini-jets which promptly evolve via quasidemocratic branchings and thus degrade into a myriad of soft gluons, with energies of the order of the medium temperature T. Via elastic collisions with the medium constituents, these soft gluons relax to local thermal equilibrium with the plasma over a time scale which is considerably shorter than the typical lifetime of the mini-jet. The thermalized gluons form a tail which lags behind the hard components of the jet. We support this scenario, first, via parametric arguments and, next, by studying a simplified kinetic equation, which describes the jet dynamics in longitudinal phase-space. We solve the kinetic equation using both (semi-)analytical and numerical methods. In particular, we obtain the first exact, analytic, solutions to the ultrarelativistic Fokker-Planck equation in one-dimensional phase-space. Our results confirm the physical picture aforementioned and demonstrate the quenching of the jet via multiple branching followed by the thermalization of the soft gluons in the cascades.

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

EPJ Web of Conferences, 2017

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-byevent 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].

Jet quenching in thin quark–gluon plasmas I: formalism

Nuclear Physics B, 2000

The modification and amplification of the gluon angular distribution produced along with hard jets in nuclear collisions is computed. We consider the limit of a thin quark-gluon plasma, where the number of rescatterings of the jet and gluons is small. The focus is on jet quenching associated with the formation of highly off-shell partons in hard scattering events involving nuclei. The interference between the initial hard radiation amplitude, the multiple induced Gunion-Bertsch radiation amplitudes, and gluon rescattering amplitudes leads to an angular distribution that differs considerably from both the standard DGLAP evolution and from the classical limit parton cascading. The cases of a single and double rescattering are considered in detail, and a systematic method to compute all matrix elements for the general case is developed. A simple power law scaling of the angular distribution with increasing number of rescatterings is found and used for estimates of the fractional energy loss as a function of the plasma thickness.

Jet evolution in a dense QCD medium

Nuclear Physics A, 2013

Besides the emblematic studies of the Higgs boson and the search of new physics beyond the Standard Model, another goal of the LHC experimental program is the study of the quarkgluon plasma (QGP), a phase of nuclear matter that exists at high temperature or density, and in which the quarks and gluons are deconfined. This state of matter is now re-created in the laboratory in high-energy nucleus-nucleus collisions. To probe the properties of the QGP, a very useful class of observables refers to the propagation of energetic jets. A jet is a collimated spray of hadrons generated via successive parton branchings, starting with a highly energetic and highly virtual parton (quark or gluon) produced by the collision. When such a jet is produced in the dense environment of a nucleus-nucleus collision, its interactions with the surrounding medium lead to a modification of its physical properties, phenomenon known as jet quenching. In this thesis, we develop a new theory to describe jet quenching phenomena. Using a leading, double logarithmic approximation in perturbative QCD, we compute for the first time the effects of the medium on multiple vacuum-like emissions, that is emissions triggered by the virtuality of the initial parton. We show that, due to the scatterings off the plasma, the inmedium parton showers differ from the vacuum ones in two crucial aspects: their phase-space is reduced and the first emission outside the medium can violate angular ordering. A new physical picture emerges from these observations, with notably a factorization in time between vacuumlike emissions and medium-induced parton branchings, the former constrained by the presence of the medium. This picture is Markovian, hence well suited for a Monte Carlo implementation. We develop then a Monte Carlo parton shower called JetMed which combines consistently both the vacuum-like shower and the medium-induced emissions. With this numerical tool at our disposal, we investigate the phenomenological consequences of our new picture on jet observables and especially the jet nuclear modification factor R AA , the Soft Drop z g distribution and the jet fragmentation function. Our Monte Carlo results are in good agreement with the LHC measurements. We find that the energy loss by the jet is increasing with the jet transverse momentum, due to a rise in the number of partonic sources via vacuum-like emissions. This is a key element in our description of both R AA and the z g distribution. For the latter, we identify two main nuclear effects: incoherent jet energy loss and hard medium-induced emissions. Regarding the fragmentation function, the qualitative behaviour that we find is in agreement with the experimental observations at the LHC: a pronounced nuclear enhancement at both ends of the spectrum. While the enhancement of hardfragmenting jets happens to be strongly correlated with R AA , hence controlled by jet energy loss, the enhancement of soft fragments is driven by the violation of angular ordering mechanism and the hard medium-induced emissions. We finally propose a new observable, which describes the jet fragmentation into subjets and is infrared-and-collinear safe by construction (therefore less sensitive to hadronisation effects) and we present Monte Carlo predictions for the associated nuclear modification factor. Cette thèse est le fruit de trois ans de travail, dont trois mois et demi d'écriture entre avril et juillet 2020. Je tiens à remercier ici les personnes qui ont rendu possible l'émergence de ce manuscrit. En premier lieu, il y a évidemment mes directeurs, Edmond et Gregory. Ils m'ont fait découvrir leur domaine de recherche et leur passion avec une générosité rare. Je mesure de plus en plus la chance que j'ai eu de travailler avec eux pendant ces trois années, et je pense qu'une thèse ne suffirait pas pour écrire tout ce qu'ils m'ont apporté. Je souhaite ensuite remercier Al Mueller qui est aussi à l'origine des travaux présentés dans la suite de ce document. Les discussions que nous avons pu avoir ont été riches d'enseignement. Ces moments dans ma vie de physicien balbutiant me sont précieux. Merci aux membres de l'équipe QCD de l'Institut de Physique Théorique, permanents ou seulement de passage, en particulier Jean-Paul Blaizot, François Gelis, Giuliano Giacalone, Davide Napoletano, Jean-Yves Ollitrault et Vincent Theeuwes dont la présence au laboratoire a donné à mes journées de travail cette dimension humaine essentielle. Les échanges scientifiques sur des sujets connexes au mien m'ont beaucoup aidé à me faire une idée d'ensemble du domaine dans lequel s'inscrit cette thèse. I thank the referees Carlos Salgado and Konrad Tywoniuk for accepting to read and comment this way too long manuscript. I thank also Matteo Cacciari, Leticia Cunqueiro and Samuel Wallon for accepting the invitation to be part of my jury, and for having physically been at my PhD defence in spite of the complications caused by the Covid19. I would like to thank all the researchers of the nuclear theory group at the Brookhaven National Laboratory for their hospitality during my stay in July 2019. Je voudrais en particulier remercier Yacine Mehtar-Tani pour cette invitation. Ce séjour a été très enrichissant et stimulant pour la suite.

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

Nuclear Physics A, 2017

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 [1, 2]. 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 [1]. 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 [2]. These predictions could be tested via eventby-event measurements of the di-jet asymmetry.

Jet structure in heavy ion collisions

International Journal of Modern Physics E, 2015

We review recent theoretical developments in the study of the structure of jets that are produced in ultra relativistic heavy ion collisions. The core of the review focusses on the dynamics of the parton cascade that is induced by the interactions of a fast parton crossing a quark–gluon plasma. We recall the basic mechanisms responsible for medium induced radiation, underline the rapid disappearance of coherence effects, and the ensuing probabilistic nature of the medium induced cascade. We discuss how large radiative corrections modify the classical picture of the gluon cascade, and how these can be absorbed in a renormalization of the jet quenching parameter [Formula: see text]. Then, we analyze the (wave)-turbulent transport of energy along the medium induced cascade, and point out the main characteristics of the angular structure of such a cascade. Finally, color decoherence of the in-cone jet structure is discussed. Modest contact with phenomenology is presented towards the end...

Exciting gauge unstable modes of the quark-gluon plasma by relativistic jets

Journal of Physics: Conference Series, 2008

We present a study of the properties of the collective modes of a system composed by a thermalized quark-gluon plasma traversed by a relativistic jet of partons. We find that when the jet traverses the system unstable gauge field modes are excited and grow on very short time scales. The aim is to provide a novel mechanism for the description of the jet quenching phenomenon, where the jet crossing the plasma loses energy exciting colored unstable modes. In order to simplify the analysis we employ a linear response approximation, valid for short time scales. We assume that the partons in the jet can be described with a tsunami-like distribution function, whereas we treat the quark-gluon plasma employing two different approaches. In the first approach we adopt a Vlasov approximation for the kinetic equations, in the second approach we solve a set of fluid equations. In both cases we derive the expressions of the dispersion law of the collective unstable modes and compare the results obtained.

Di-jets production in quark gluon plasmas

2014

In azimuthal di-hadron correlation two broad and narrow structures are observed, which is called “away-side jet” and “near-side jet” respectively. We can describe them as perturbations around the equilibrium baryon density by linearization of the hydrodynamic equations. It is found that the localized perturbations are able to propagate in quark gluon plasma (QGP) for long distances. The RHIC experiments indicate the existence of a hot and dense fireball of matter which behaves like a perfect fluid of quarks and gluons. Relativistic hydrodynamics is applied for describing the evolution of this fluid. Interaction between the localized perturbations on the background baryon density is able to create back-to-back di-jets structures as shown in the figure 1[1-2]. Figure 1 : the di-jets productions in a hot QGP The total energy-momentum tensor reads, ( ) ( ) Where and are the energy density and pressure respectively. The four-vector velocity is considered by and which is the Lorentz facto...

Jet formation and interference in a thin QCD medium

2015

In heavy-ion collisions, an abundant production of high-energy QCD jets allows to study how these multiparticle sprays are modified as they pass through the quark-gluon plasma. In order to shed new light on this process, we compute the inclusive two-gluon rate off a hard quark propagating through a color deconfined medium at first order in medium opacity. We explicitly impose an energy ordering of the two emitted gluons, such that the "hard" gluon can be thought of as belonging to the jet substructure while the other is a "soft" emission (which can be collinear or medium-induced). Our analysis focusses on two specific limits that clarify the modification of the additional angle- and formation time-ordering of splittings. In one limit, the formation time of the "hard" gluon is short compared to the "soft" gluon formation time, leading to a probabilistic formula for production of and subsequent radiation off a quark-gluon antenna. In the other l...