J/ψ Polarization At Mid-rapidity In P+P Collisions At √s = 200 GeV At STAR

Experimental measurements of quarkonium cross section and polarization are two main approaches to gain insight into quarkonium production mechanism. Quarkonium polarization refers to anisotropic spatial distribution of lepton decay from quarkonium with regard to quarkonium momentum. Although many theoretical models achieve good consistency with the experimental measurement on quarkonium transverse momentum spectra, they have very different predictions on quarkonium polarization. Therefore, quarkonium polarization measurements have a strong capability to distinguish different models and portrait the quarkonium production mechanism. In this study, the main detectors in Solenoidal Tracker At RHIC (STAR), time projection chamber, time-of-flight and Barrel Electromagnetic Calorimeter are fully applied for track reconstruction, particle identification and physical quantity measurement. The raw yield of J/ψ as a function of polar and azimuthal angles is calculated from STAR datasets collected in 2012 from p+p collisions at √s = 200GeV, and the efficiency of detectors is obtained by GEANT3 simulation. A statistical estimator for J/ψ polarization parameters is constructed based on the principle of maximum likelihood estimation. Its properties are studied via Monte Carlo simulation. It is the first time that the measurements of λφ, λθφ and λinv are presented by STAR experiment. The results don’t suggest prominent J/ψ polarization. Among available theoretical models compared with the measurement, the CGC+NRQCD achieve the best agreement.