$J/\psi$ polarization in high multiplicity $pp$ and $pA$ collisions: CGC+NRQCD approach

TL;DR

This paper predicts $J/\psi$ polarization in high multiplicity $pp$ and $pA$ collisions using the CGC+NRQCD framework, finding weak polarization that diminishes with event activity and shows little system size dependence.

Contribution

It provides the first predictions of $J/\psi$ polarization in high multiplicity small systems within the CGC+NRQCD approach, highlighting weak polarization trends.

Findings

$J/\psi$ polarization is weak and decreases with event activity.

No significant difference in polarization between $pp$ and $pA$ collisions.

Predictions cover specific rapidity, multiplicity, and energy ranges.

Abstract

Quarkonium production mechanism in high multiplicity small collision systems has recently been pursued in the color-glass-condensate (CGC) effective theory combined with non-relativistic QCD (NRQCD) factorization, allowing to study initial state interactions. Quarkonium polarization, potentially measured in future experiments, would help elucidate the quarkonium production mechanism at high multiplicities. In this paper, we provide predictions on $J/\psi$ polarization parameters in high multiplicity proton-proton ($pp$) and proton-nucleus ($pA$) collisions within the CGC+NRQCD framework. Theoretical predictions are given for $J/\psi$ rapidity $2.5< y_{J/\psi}<4$, charged-particle multiplicity pseudorapidity $|\eta_{ch} | <1$ and energies $\sqrt{S}=13\mathrm{~TeV}$ for $pp$, $\sqrt{S}=8.16\mathrm{~TeV}$ for $pA$ collisions. Considering two leptonic frame choices (Collins - Soper and helicity) we find a weak polarization of $J/\psi$ that additionally decreases with growing event activities. No significant system size dependence between $pp$ and $pA$ collisions is obtained - this could be a new constraint to initial state interactions in small collision systems.