Estudo da coincidência elétron-jato em colisões próton-próton e próton-núcleo no experimento ALICE

Abstract

This thesis presents the first measurement of the distribution of jets that are correlated to heavy-flavour decay electron (HFe), reconstructed and identified with ALICE (A Large Ion Collider Experiment). The observables were measured in pp collisions at center of mass energy √ s = 8 TeV and in p-Pb at √ s N N = 5.02 TeV at the Large Hadron Collider (LHC). Exploring the heavy quarks created in relativistic heavy ion collisions is a powerful approach to study the new formed medium, known as Quark Gluon Plasma (QGP). This is due to the fact that they are created in the beginning of the collision, via hard scattering, and interact with the QGP throughout its whole existence. These quarks fragment into mesons that can decay (via the semi-electronic channel) into electrons (and positrons). These leptons can be used to identify the creation of a heavy quarks and allow QGP studies. The hard scattering, fragmenting and decaying processes originate several particles that are collimated in a conical region, and they can be grouped in what is called 'jet'. The reconstruction and study of jets has been used to retrieve more information about the QGP and it is also a useful probe. The selected electrons were identified by the Time Projection Chamber (TPC) and the Electromagnetic Calorimeter (EMCal). Heavy-flavour decay electrons were selected via invariant mass method. The jets were reconstructed by the Fastjet framework, with the algorithm anti − kT and R = 0.4. The HFe-jet pairs selection may allow a better comprehension of the QGP properties and its interactions with the heavy quarks. A jet p T,chjet spectrum was obtained, for pp and p-Pb, in two intervals of angular distance ∆ϕ between the jet and the electron: away and near sides. The angular distance ∆ϕ distribution was also obtained in different electron p e T intervals. The results for pp and p-Pb were compared through two observables: the first one as a function of the jet p T,chjet , and the second, as a function of electron p e T. The former consists in calculating the ratio of the jet p T,chjet spectra, in pp and in p-Pb. The latter, consists in calculating the ratio of the areas, in pp and in p-Pb, of each peak in the ∆ϕ distribution. Both were compatible with the unity, which indicates that there is no extra effect in p-Pb with respect to pp.