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

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, ${\lambda }_{\varphi }$, 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.

arXiv comment6 pages, 3 figures, Proceedings for The European Physical Society Conference on High Energy Physics, July 22-29, 2015, Vienna, Austria