Abstract
The goal of high--energy heavy--ion physics is the full characterization of the quark--gluon plasma (QGP), which is a phase of strongly interacting matter where quarks and gluons are no longer confined in the nucleons and can move freely over longer distances. Such a phase probably existed shortly after the Big
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Bang, and can be produced in laboratory by heavy--ion collisions at a sufficiently large energy density. This new phase of matter is distinctly different from usual hadronic matter. Several signatures have been proposed to probe the QGP. The focus of the present work is on jet production. Jets are collimated beams of particles emitted in the collision. The case in which one jet is associated with the emission of a single direct photon ($\gamma$--jet event) is of particular interest. Photons go through the medium without interacting. For momentum conservation, the energy of the photon is equal to the energy of the initial jet. On the other side, jet particles interact with the medium, losing energy. The energy loss is an important parameter to characterize the medium density. Due to the high multiplicity in heavy--ion collisions, it is difficult to identify the case of $\gamma$--jet production. There are many sources of photons, the most dominant of which is the decay of neutral mesons (mainly $\pi^0$ and $\eta$). Analyzing the simplest case, that is p+p collisions, gives a necessary baseline measurement for $\gamma$--jet correlation studies in Au+Au collisions. This work focuses on di--hadron correlation, seen as a background study for $\gamma$--jet correlation. The properties of jets in p+p and d+Au collisions are analyzed. In this way, di--jet events are completely characterized, providing a reference frame for the study of $\gamma$--jet events. In the present analysis p+p and d+Au data, collected at the STAR experiment, have been studied. The detectors mainly used are the Barrel Electromagnetic Calorimeter (BEMC) and the Time Projection Chamber (TPC). The BEMC is a lead--scintillator sampling calorimeter located at mid--rapidity ($|\eta| has been evaluated as a function of jet particles momentum. It is possible to study the relations between the two jets of a di--jet event from the parameters describing a single jet shape. The di--jet acoplanarity is measured with an azimuthal particle correlation approach. In this thesis the results for and have been extended to higher associated particle pT compared to previous measurements from other experiments, results obtained with charged hadrons and full jet reconstruction. In the overlap region the present results agree with all the previous ones.
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