Energy Loss of Fast Partons in a Colored Medium

Abstract

Through heavy-ion collisions it is possible to form a new state of matter called the quark gluon plasma. It is characterized by the deconfinement of quarks and guons up to distances much larger than those when they are inside a nucleus. However, the lifetime of this plasma is very short, and therefore, one have to rely on hard probes, objects with a large transverse momentum formed at a very early stage of the collision that carry information about the evolution of the produced medium. The advantage of using these probes is that they can be described by the perturbative regime of the theory of the strong interactions, the Quantum Choromodynamics, where analytical calculations from first principles are possible. By comparing the modifications of the hard probes in heavy-ion collisions with the result form other collisional systems where the energy and density are not sufficient to form the quark gluon plasma, like proton-proton collisions, one can infer the properties of the produced medium. The modifications include additional energy loss processes that are induced by the interactions with medium constituents, a process that is generically known as Jet Quenching. ! This thesis is focused on the jet quenching study to understand how the parton showering (the process by which a highly-energetic particle decreases its energy up to the hadronization scale) is modified in the presence of a medium. For that, two different approaches were followed: a more analytical one, where theoretical calculations were performed to improve the description of one of the elementary vertices of the parton branching in the presence of a medium, the gluon radiation off a quark; and a more phenomenological one, where, using a Monte Carlo protonproton event generator with medium effects, it was compared the result of several simulated events with experimental data of different jet related observables. ! The first part led to a fully consistent description of the parton branching, that contains all finite energy corrections in the approximation of small angle emissions and assuming static scattering centers (only radiative energy loss was taken into account). In particular, previous models of jet quenching were improved to take into account the emission of finite energy gluons, and where all vertex particles are allowed to have Brownian deviations in the transverse plane instead of having its motion constrained to a straight line. ! In the second part, a systematic study was conducted to understand the influence of jet reconstruction and jet quenching phenomena on different jet observables. It was compared different background subtraction methods by using a toy model to simulate the underlying event characteristic of a heavy-ion collision, in which the hard event generated by the Monte Carlo event generator was embedded in. It was possible to conclude that it is necessary an accurate description of the jet reconstruction techniques to compare models to data in order to accurately describe the medium properties. Moreover, it was observed that jet quenching approaches based on soft gluon radiation are not refuted by the experimental data, but a better description can be achieved if the new theoretical developments, like the ones that were calculated in this thesis, are implemented in the Monte Carlo codes. This would be a natural followup of this work.