Measurement of the polarizations of prompt and non-prompt J/$\psi$ and $\psi$(2S) mesons produced in pp collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper measures the polarization of prompt and non-prompt J/ψ and ψ(2S) mesons produced in proton-proton collisions at 13 TeV, revealing that non-prompt polarizations align with B meson decay predictions and prompt polarizations do not exhibit strong transverse polarization.

Contribution

It provides the first detailed polarization measurements at 13 TeV for both prompt and non-prompt charmonium states, extending previous studies and testing production models.

Findings

Non-prompt polarizations agree with B decay predictions at high pT.

Prompt polarizations exclude strong transverse polarization even at high pT.

Prompt polarizations vary significantly with pT, especially at low pT.

Abstract

The polarizations of prompt and non-prompt J$/\psi$ and $\psi$(2S) mesons are measured in proton-proton collisions at $\sqrt{s}$ = 13 TeV, using data samples collected by the CMS experiment in 2017 and 2018, corresponding to a total integrated luminosity of 103.3 fb$^{-1}$. Based on the analysis of the dimuon decay angular distributions in the helicity frame, the polar anisotropy, $\lambda_\theta$, is measured as a function of the transverse momentum, $p_\mathrm{T}$, of the charmonium states, in the 25-120 and 20-100 GeV ranges for the J$/\psi$ and $\psi$(2S), respectively. The non-prompt polarizations agree with predictions based on the hypothesis that, for $p_\mathrm{T}$ $\gtrsim$ 25 GeV, the non-prompt J$/\psi$ and $\psi$(2S) are predominantly produced in two-body B meson decays. The prompt results clearly exclude strong transverse polarizations, even for $p_\mathrm{T}$ exceeding 30 times the J$/\psi$ mass, where $\lambda_\theta$ tends to an asymptotic value around 0.3. Taken together with previous measurements, by CMS and LHCb at $\sqrt{s}$ = 7 TeV, the prompt polarizations show a significant variation with $p_\mathrm{T}$, at low $p_\mathrm{T}$.