Measurement of inclusive J/$\psi$ polarization in p+p collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment

TL;DR

This study measures the polarization of J/ψ particles produced in proton-proton collisions at 200 GeV, finding most polarization parameters near zero and comparing results with theoretical models, notably favoring the Color Glass Condensate plus NRQCD approach.

Contribution

First measurement of inclusive J/ψ polarization at RHIC energies in both Helicity and Collins-Soper frames, providing data to test QCD models of quarkonium production.

Findings

Most polarization parameters are consistent with zero across transverse momentum range.

The Color Glass Condensate plus NRQCD model best describes the experimental data.

Experimental uncertainties prevent ruling out other models.

Abstract

We report on new measurements of inclusive J/$\psi$ polarization at mid-rapidity in p+p collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment at RHIC. The polarization parameters, $\lambda_\theta$, $\lambda_\phi$, and $\lambda_{\theta\phi}$, are measured as a function of transverse momentum ($p_T$) in both the Helicity and Collins-Soper (CS) reference frames within $p_T< 10$ GeV/$C$. Except for $\lambda_\theta$ in the CS frame at the highest measured $p_T$, all three polarization parameters are consistent with 0 in both reference frames without any strong $p_T$ dependence. Several model calculations are compared with data, and the one using the Color Glass Condensate effective field theory coupled with non-relativistic QCD gives the best overall description of the experimental results, even though other models cannot be ruled out due to experimental uncertainties.