J/ψ polarization in p+p collisions at √s=200 GeV in STAR

Abstract

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