“Measurement of the polarizations of prompt and nonprompt J/ψ and ψ(2S) mesons produced in pp collisions at √s = 13 TeV

Quantum Chromodynamics (QCD) is the sector of the Standard Model responsible for describing interactions between quarks and gluons. Because of the unique nature, within the Standard Model, of its coupling constant, calculations in QCD remain a challenge. Quarkonia, bound states of a heavy quark and its respective antiquark, are the simplest case study for hadron formation processes, and so are the ideal probes for a better understanding of QCD.

In this thesis, we study one of the fundamental properties of quarkonium states, their polarization, characterized experimentally by the angular distribution of their decay products. Using data samples collected by the CMS experiment in 2017 and 2018, corresponding to a total integrated luminosity of 103.3 fb−1 , we measure the polarizations of prompt and non-prompt J/ψ and ψ(2S) mesons produced in proton-proton collisions at √ s = 13 TeV, using the dimuon decay channel. The polar anisotropy parameter in the helicity frame, λ HX ϑ , is measured as a function of the transverse momentum, pT, of the two charmonium states, in the 25–120 GeV range for the J/ψ and in the 20–100 GeV range for the ψ(2S).

The measurements are compared with predictions. The non-prompt polarizations agree with predictions based on the hypothesis that, above a pT of around 25 GeV, these states are predominantly produced in two-body B meson decays. The prompt results definitively exclude the theory expectation of strong transverse polarizations (λϑ =+1) for the pT range under study, which exceeds 30 times the J/ψ mass, as λϑ tends to an asymptotic value around 0.3 over this range. Taken together with previous LHC measurements for √ s = 7 TeV, compatible in the overlap pT region, the prompt λϑ shows a significant variation with pT, at low pT.