Polarization of prompt J/psi in proton-proton collisions at RHIC

Within the framework of the nonrelativistic QCD (NRQCD) factorization approach, we compute the polarization of prompt J/psi produced at the Brookhaven's Relativistic Heavy-Ion Collider from proton-proton collisions at the center-of-momentum energy root s = 200 GeV. The perturbative contributions are computed at leading order in the strong coupling constant. The prediction reveals that the color-singlet contribution severely underestimates the PHENIX preliminary data for the differential cross section integrated over the rapidity range vertical bar y vertical bar < 0: 35 and its contribution is strongly transversely polarized, which disagrees with the PHENIX preliminary data. After including the color-octet contributions, we find that the NRQCD predictions for both the cross section and polarization over the transverse-momentum range 1.5 GeV < p(T) < 5 GeV (1.5 GeV < p(T) < 2 GeV) integrated over the rapidity range vertical bar y vertical bar < 0.35 (1.2 < vertical bar y vertical bar < 2.2) agree with the data within errors.