# EPS HEP 2015

22-29 July 2015
Europe/Vienna timezone

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

Jul 23, 2015, 9:30 AM
15m

### Speaker

Barbara Trzeciak (Czech Technical University in Prague)

### Description

Despite extensive studies, the J/$\psi$ production mechanism in hadron collisions is not yet exactly known. For many years, mostly J/$\psi$ differential cross-section measurements have been used to test different J/$\psi$ production models. While many models can reasonably well describe the experimental data on the J/$\psi$ cross-section in $p+p$ collisions, they have different predictions for the J/$\psi$ polarization. Therefore, measurements of the J/$\psi$ polarization may allow to discriminate among different models and provide new insight into the J/$\psi$ production mechanism. In this talk, measurements of J/$\psi$ polarization in $p+p$ collisions at $\sqrt{s}$ = 200 and 500 GeV via the dielectron decay channel at mid-rapidity with the STAR experiment will be discussed. At $\sqrt{s}$ = 200 GeV the polarization parameter, $\lambda_{\theta}$, related to the polar anisotropy was 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 $\sqrt{s}$ = 500 GeV has extended 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, $\lambda_{\phi}$, was extracted in addition to $\lambda_{\theta}$, in two reference frames: helicity and Collins-Soper. This allowed for the frame invariant parameter calculation vs $p_{T}$ in these two frames.

### Primary author

Barbara Trzeciak (Czech Technical University in Prague)