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

TL;DR

This paper calculates the polarization of prompt J/psi particles in proton-proton collisions at RHIC using NRQCD, finding that including color-octet contributions aligns predictions with experimental data.

Contribution

It provides a detailed NRQCD-based prediction of J/psi polarization and cross sections at RHIC energies, incorporating both color-singlet and color-octet effects.

Findings

Color-singlet contribution underestimates data and predicts transverse polarization.

Including color-octet contributions improves agreement with experimental cross sections.

Predictions match PHENIX data within errors when both contributions are considered.

Abstract

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 sqrt{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 |y|<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 |y|<0.35 (1.2 < |y| < 2.2) agree with the data within errors.