Jet reconstruction in Pb-Pb collisions $\sqrt{s_{\rm NN}} = 2.76$ TeV with the ALICE experiment

The first evidences of the existence of a new state of matter which is created in ultra-relativistic central heavy-ion collisions, came at the beginning of this century from measurements performed by the SPS experiments at CERN and by the RHIC experiments at BNL. This new state of matter, predicted by a number of models in the context of the Quantum Cromo-Dynamics (QCD), appears to be characterized by the underlying quark and gluon degrees of freedom and it is known as ``Quark-Gluon Plasma'' (QGP). In 2009 the Large Hadron Collider (LHC) became operational at CERN, providing pp collisions at $7$ and $8$ TeV in the center of mass and Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. While confirming the previous observations, both RHIC and LHC experiments are aiming at more complex measurements, to understand the details of the QGP, whose properties are still evanescent. Many of these observables make use of hard probes, which became accessible only at their higher collisional energy. QCD jets are produced in any kind of high-energy particle collisions; they result from the fragmentation of a hard scattered parton (quark or gluon), which means that their kinematic properties reflect, to some extent, the kinematics of the original parton. In heavy-ion collisions hard scattering processes happen in the early stages, allowing them to propagate through (and potentially be affected by) the hot and dense nuclear medium. The modification suffered by the parton, known as ``jet quenching'', can result in energy loss, widening and/or complete absorption of jets. By measuring jet-related observables in heavy-ion collisions and comparing to a pp baseline, one can get insights about the modifications induced to the parton by the medium, and thus learning about the properties of the medium itself. The reconstruction of jets in high-energy heavy-ion collisions is challenging due to the huge background coming from the underlying event. Full reconstruction of jets in the heavy-ion environment is something really new, although many results are now expected from the various experiments in the very near future. ALICE is the dedicated heavy-ion experiment at LHC. It consists of a central detector system, covering mid-rapidity ($|\eta|<0.9$) over the full azimuth, and several forward systems. The central barrel is enclosed by a large solenoidal magnet to allow track momentum reconstruction. Charged tracks are reconstructed by the Inner Tracking System (ITS), a six-layers silicon detector, and a big Time Projection Chamber (TPC). Neutral particles are reconstructed by a Pb-scintillator sampling electromagnetic calorimeter (EMCal), which covers mid-rapidity ($|\eta|<0.9$) and partial azimuth ($\Delta\phi=100^{\circ}$). The first full jet reconstruction measurement in heavy-ion collisions from the ALICE experiment is presented in this thesis. The \antikt{} algorithm, which has proven to be more suitable for the heavy-ion environment, has been employed to find jets among the particles reconstructed through the tracking detectors (charged) and the electromagnetic calorimeter (neutral). The average background momentum density has been estimated even-by-event and subtracted jet-by-jet. Region-to-region fluctuations in the background \pT{} density have been estimated through \dpT{} distributions, which represent the difference between the background in a random region and event-wise average background. In order to reduce the contamination from combinatorial jets, namely jets reconstructed out of background soft particles, a minimum leading hadron \pT{} requirement has been applied. The detector response to jet reconstruction has been simulated using pp events simulated with the PYTHIA generator and the GEANT3 transport code. From this simulation, a detector response matrix and jet efficiency have been extracted. A numerical procedure, known as unfolding, has been employed to fully correct the spectrum using the \dpT{} distribution for the background fluctuations and the detector response matrix. The fully corrected Pb-Pb jet spectrum has been compared to a pp baseline spectrum, previously measured by ALICE. The nuclear modification factor, indicated with $R_{\rm AA}$, gives a quantitative estimate of the modification suffered by the observable, the jet \pt{} in this case, due to the hot nuclear medium created in the heavy-ion collision. An $R_{\rm AA}=1$ indicates no modification, i.e. the heavy-ion collision can be described in terms of a superposition of nucleon-nucleon collisions, whereas a value different from 1 indicates suppression or enhancement due to collective effects. A strong suppression, with an $R_{\rm AA} \approx 0.25$, is observed. This observation is consistent with the single particle spectrum measurements, where a strong suppression is seen at intermediate \pt{} as well. However full jet reconstruction allows a better understanding of the energy scale of the scattered parton, which suffered energy loss in the hot medium.