Jet evolution in a dense QCD medium (original) (raw)

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.