Parton energy loss mechanism in Quark-Gluon Plasma with jet quenching using LHC-ALICE data

Overview & Synopsis

Quantum chromodynamics (QCD) is the theory of the strong interaction betweenthe elementary constituents of nuclear matter, namely quarks and gluons. Normallythe latter are confined within protons and neutrons but QCD predicts also a newphase of nuclear matter, the Quark-Gluon Plasma (QGP), formed at very hightemperature and/or high density, where quarks and gluons are deconfined. Thisstate is believed to exist in the early universe and understanding its propertiesis important for studying the formation of the current universe. The creation ofQGP has been successfully achieved through high-energy heavy-ion collisions atlarge accelerators such as the RHIC and LHC. Due to the very short lifetime of theQGP and the fact that the size of the QGP medium produced in heavy-ion collisionsis on the order of a nucleus, direct observation is impossible, making its study morecomplex.The purpose of this thesis is to elucidate the mechanisms of energy loss forquarks and gluons (partons) within the Quark-Gluon Plasma (QGP) medium byobserving high-momentum partons which are created at the initial collision andpass through the QGP medium while undergoing strong interactions. The partonsthen quickly decay into numerous hadrons (jets) after their creation, making theirdirect observation impossible. However, measuring the reconstructed jets allowsaccess to the parton energy loss through the QGP medium and distinguishes amongdifferent mechanisms of parton interactions within the QGP.To date, the parton energy loss with QGP has been studied through indepen-dent measurements: the jet nuclear modification factor RAAjet, corresponding to theratio of jet yield using Pb-Pb collisions relative to p-p collisions and the azimuthalanisotropy (vjet2 ) of jet rates which results from the different path-lengths crossedby the partons in a QGP medium of elliptical shape. However, confronting inde-pendently these measurements to the parton energy loss models has not allowed toelucidate the mechanisms in play.In this thesis, the jet nuclear modification factor RAAjet and the azimuthalanisotropy (vjet2 ) have been measured under the same conditions by the LHC-ALICEexperiment corresponding to p-p and Pb-Pb collisions at a collision energy of √sNN =5.02 TeV. These new results are consistent with previous measurements and confirmthe expected jet quenching effects and the corresponding azimuthal anisotropy.To further elucidate the mechanisms of energy loss and quantify the energy lossper unit distance (ˆen[GeV/fmn]), a general simulation framework has been also de-veloped where the energy loss is parametrized as the n-power of the path-length withthe QGP. Evaluating the n-power parameter provides information about possibleenergy-loss mechanics in play of collisional, radiative, and AdS/CFT type. Con-fronting this simulation to the new measurements allow to provide new constraints2on the path-length dependency of the parton energy loss within QGP.To better understand the energy loss mechanisms involved and quantify theenergy loss per unit of distance, a new simulation was carried out in which the energyloss is parameterized as the nth power of the path-length traveled by the parton inthe QGP. The evaluation of the parameter n can allow to distinguish between thedifferent energy loss mechanisms as collisional, radiative, and AdS/CFT type. Thecomparison of this simulation with the new experimental measurements provide newconstraints on the dependence on the path length of the energy loss of the partonswithin the QGP.