CERN Accelerating science

Quantum Chromo-Dynamics (QCD) is the field theory that describes the nuclear interactions, responsible for holding together quarks and gluons inside the atomic nuclei. Despite significant progress made in over four decades of experimental and theoretical work since QCD was established, there remain many open questions, especially regarding the fragmentation process of quarks and gluons and the behavior of QCD in the high-temperature regime. Jets are produced from hard-scattered quarks and gluons as a consequence of the confinement property of QCD, that banishes the possibility of free quarks at ordinary temperatures. However, at high energy densities, like those achieved in ultra-relativistic heavy-ion collisions, QCD predicts a transition to a phase of free quarks and gluons, called the Quark-Gluon Plasma (QGP). This thesis presents two measurements performed with the ALICE detector at the CERN Large Hadron Collider (LHC). Charm jets were tagged using D$^0$ mesons. Their cross section and fragmentation function in \pp\ collisions at the center-of-mass energy $\sqrt{s}=7$~TeV were measured and compared with theoretical predictions obtained with Monte Carlo generators based on QCD. A general agreement was found, confirming that we understand the general features of jet and heavy-flavor production in the framework of QCD. Another measurement was performed in Pb-Pb collisions at a center-of-mass energy per nucleon $\sqrt{s_{\rm NN}}=2.76$~TeV. Jets were reconstructed and their yield compared to a \pp\ reference measurement. A strong suppression was observed, which is interpreted as caused by energy loss of hard-scattered quarks and gluons in the QGP.