Misura della polarizzazione della J/psi in collisioni p-p a all'energia nel centro di massa di 13 TeV con l'esperimento ALICE

ALICE is the experiment at the CERN LHC devoted to the study of quark gluon plasma, a deconfined state of quarks and gluons, which can be created in heavy ion collisions. However, ALICE is also well suited to perform QCD studies in proton-proton collisions. Among the topics investigated in these collisions, a relevant role is played by the study of quarkonium production, i.e. a particle composed of a heavy quark and its own anti-quark. In my thesis work I have addressed one of the main aspects of quarkonium production, i.e. the polarization, in particular of the J/ψ, a cc ̅ state. Polarization is the measure of the degree to which the spin of the particle is aligned with respect to a chosen axis. This is a property of the particle and it reflects the process responsible for its production. At present there is no theoretical model able to explain both the J/ψ production cross sections and polarization, so precise results from experiments are mandatory to find a possible theoretical explanation for these properties and eventually to further constrain them. J/ψ is a vector meson with j=1, therefore it is possible to express its quantum state as the superposition of the three angular momentum eigenstates corresponding to j_z=-1,0,1 : |ψ>=b_(-1)|1,−1>+b_0|1,0>+b_(+1)|1,+1> (where b_i is the eigenvalue corresponding to the i-th eigenstate |j,j_(z_i )>). The quantum state |ψ> is said to be transversely polarized if j_z=±1, while if j_z=0 it is said to be longitudinally polarized. Quarkonium is said to be unpolarized if the probabilities to find J/ψ in each of the three angular momentum eigenstates are the same. The polarization of a vector meson can be experimentally determined by measuring the angular distribution of its decay products: this is possible thanks to the fact that such distributions can be expressed in terms of the eigenvalues b_i. To extract this feature the pp datasets collected by the ALICE experiment at √s=13 TeV in 2016, 2017 and 2018 have been analysed. This study focuses on the J/ψ decay channel J/ψ→μ^+ μ^- and is based on data collected by the ALICE muon spectrometer in the pseudorapidity region −4.0 < η < −2.5. The polarization analysis requires to extract the number of J/ψ as a function of cosθ and φ, being θ and φ the polar and the azimuthal angles identifying the decay product emission direction in a chosen reference frame. To obtain such distributions, first, I have applied all the standard cuts at the event, single muon and dimuon level, then I have performed fits of the invariant mass spectra corresponding to different angular variables intervals. Next, I have corrected the distributions of N_(J/ψ)(cosθ) and N_(J/ψ)(φ) using the values of acceptance times detection efficiency extracted from a Monte Carlo simulation. In the end I have performed fits of these distributions using functions of the angular variables: this allows to extract the parameters carrying the information on J/ψ polarization. In my thesis work I have provided the measurement of J/ψ polarization parameters as a function of transverse momentum in two reference frames, the helicity and the Collins-Soper ones. I have performed the study in the range 0<p_T<15 GeV/c, extending, for the first time, the measurement down to p_T=0 GeV/c and with the additional advantage of the availability of an enormous statistics.