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 λ θ 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.