Spin Dependence of Massive Lepton Pair Production in Proton-Proton Collisions

TL;DR

This paper calculates the transverse momentum distribution of massive lepton pairs in polarized proton-proton collisions, highlighting the dominance of gluon-initiated subprocesses at high transverse momentum and proposing these measurements as a probe of the polarized gluon density.

Contribution

It provides the first detailed perturbative QCD predictions for spin-dependent lepton-pair production, emphasizing its potential to constrain the polarized gluon distribution.

Findings

Gluon-initiated subprocesses dominate at Q_T > Q/2.

Lepton-pair production offers a clean probe of polarized gluons.

Predictions are made for RHIC energies, comparable to prompt photon results.

Abstract

We calculate the transverse momentum distribution for the production of massive lepton-pairs in longitudinally polarized proton-proton reactions at collider energies within the context of perturbative quantum chromodynamics. For values of the transverse momentum Q_T greater than roughly half the pair mass Q, Q_T > Q/2, we show that the differential cross section is dominated by subprocesses initiated by incident gluons, provided that the polarized gluon density is not too small. Massive lepton-pair differential cross sections should be a good source of independent constraints on the polarized gluon density, free from the experimental and theoretical complications of photon isolation that beset studies of prompt photon production. We provide predictions for the spin-averaged and spin-dependent differential cross sections as a function of Q_T at energies relevant for the Relativistic Heavy Ion Collider (RHIC) at Brookhaven, and we compare these with predictions for real prompt photon production.