Mainz 2022 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 24: Heavy-Ion Collisions and QCD Phases V

HK 24.5: Talk

Tuesday, March 29, 2022, 17:15–17:30, HK-H1

Measurement of J/ψ polarization in proton-proton collisions at √s=13 TeV with the ALICE muon spectrometer — •Federica Zanone for the ALICE collaboration — Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Germany

ALICE is the experiment at the CERN LHC devoted to the study of quark-gluon plasma, but it is also well suited to perform other QCD studies, such as the investigation of quarkonium polarization.

Polarization is the measure of the degree to which the spin of a particle is aligned with respect to a chosen axis and reflects the process responsible for its production. At present theoretical models have difficulties in explaining both the quarkonium production cross section and polarization, so precise results from experiments are necessary to better constrain the theory approaches. The polarization of a vector meson, such as J/ψ, can be experimentally determined by measuring the angular distribution of its decay products since such distributions can be expressed in terms of the eigenvalues corresponding to the mother angular momentum eigenstates. This study focuses on the decay channel J/ψ → µ⁺µ⁻ in the pseudorapidity region −4.0<η<−2.5 and addresses the highest J/ψ statistics sample collected so far by ALICE in pp collisions. This analysis provides the measurement of J/ψ polarization parameters as a function of p_T in the helicity and Collins-Soper reference frames and investigates the range p_T<15 GeV/c, extending, for the first time, the measurement down to p_T=0. Comparisons of the results to previous measurements both in pp and PbPb collisions, as well as to theoretical models, are provided, too.