Study of J/psi polarization in proton-proton collisions with the ALICE detector at the LHC

The main purpose of the ALICE experiment is the study and characterization of the Quark Gluon Plasma (QGP), a state of nuclear matter in which quarks and gluons are deconfined. Quarkonia (bound states of a heayvy quark Q and its anti-quark Q) constitute one of the most interesting probes of the QGP. Besides this motivation, the study of quarkonium production is very interesting since it can contribute to our understanding of Quantum Chromodynamics, the theory of strong interactions. The formation of quarkonium states in hadronic collisions is not yet completely understood. The two main theoretical approaches to describe the production of quarkonium states, the Color Singlet Model and the Non-Relativistic QCD framework (NRQCD), have historically presented problems to simultaneously describe the production cross section and polarization of such states. On the experimental side, quarkonium polarization measurements have not always been complete and consistent between them. So, neither from the theoretical nor from the experimental point of view the situation was clear.Improved methods for the measurement of quarkonium polarization have been recently proposed, highlighting the necessity to perform the measurements of all polarization parameters with respect to different reference axes. In this context, new measurements could help to improve and set new constraints to the calculations. ALICE has measured the J/ψ polarization in pp collisions at √8= 7 TeV. The higher statistics of the 8 TeV data with respect to the 7 TeV data allows to extend the pT range of the measurements. This thesis presents a complete measurement of J/ψ polarization, i.e. the three polarization parameters, in two polarization frames : the Collins-Soper and Helicity frames. The results show no significant J/ψ polarization in the kinematic domain studied: 2.5 < y < 4.0 and 2 < pT < 15 GeV/c. The measurement of a frame invariant parameter λ, was also performed to ensure that no bias was present in the analysis procedure. The comparison with different theoretical predictions shows that there is not yet a satisfactory description of quarkonium production. None of the present theoretical approaches is able to describe both, the cross section and polarization measurements