Abstract

We report on a polarization measurement of inclusive $J/\psi$ mesons in the di-electron decay channel at mid-rapidity at 2 $<p_{T}<$ 6 GeV/$c$ in $p+p$ collisions at $\sqrt{s}$ = 200 GeV. Data were taken with the STAR detector at RHIC. The $J/\psi$ polarization measurement should help to distinguish between the different models of the $J/\psi$ production mechanism since they predict different $p_{T}$ dependences of the $J/\psi$ polarization. In this analysis, $J/\psi$ polarization is studied in the helicity frame. The polarization parameter $\lambda_{\theta}$ measured at RHIC becomes smaller towards high $p_{T}$, indicating more longitudinal $J/\psi$ polarization as $p_{T}$ increases. The result is compared with predictions of presently available models. 

Target Journal: PLB

• J/psi polarization parameter as a function of J/psi pT.

• Summary

This paper reports the first STAR measurement of J/psi polarization and contributes to the evolving understanding on the J/psi production mechanisms. J/psi polarization is measured in p+p collisions at $\sqrt{s}$ = 200 GeV in the helicity frame at |y|< 1 and 2  <pT < 6 GeV/c. RHIC data indicates a trend towards longitudinal J/psi polarization as pT increases. The result is consistent, within experimental and theoretical uncertainties, with the NLO$^{+}$ CSM model.