# 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.

## Key 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.