J/$\psi$ polarization measurements in p+p collisions at $\sqrt{s}$ = 200 and 500 GeV with the STAR experiment

TL;DR

This paper reports measurements of J/psi polarization in proton-proton collisions at 200 and 500 GeV, analyzing angular anisotropies to compare with theoretical models and extend previous polarization data.

Contribution

It provides new polarization measurements at 500 GeV over a wider transverse momentum range and compares polarization parameters across different reference frames.

Findings

Polarization parameter $ ext{lambda}_ heta$ varies with $p_T$ and matches some model predictions.

First measurement of $ ext{lambda}_ heta$ and $ ext{lambda}_ extphi$ at 500 GeV.

Frame-invariant polarization parameters are calculated for the first time in this context.

Abstract

In these proceedings, measurements of J/psi polarization in p+p collisions at 200 and 500 GeV via the dielectron decay channel at mid-rapidity with the STAR experiment are discussed. At 200 GeV the polarization parameter, $\lambda_{\theta}$, related to the polar anisotropy is obtained in the helicity frame as a function of transverse momentum, 2 < $p_{T}$ < 6 GeV/c, and compared to different model predictions. A new J/psi polarization measurement at 500 GeV extends the previous analysis to a wide transverse momentum range of 5 < $p_{T}$ < 16 GeV/c. Also, the polarization parameter related to the azimuthal anisotropy, $\lambda_{\phi}$, is extracted in addition to $\lambda_{\theta}$, in two reference frames: the helicity and Collins-Soper frames. This allows for the frame invariant parameter calculation vs $p_{T}$ in these two frames.