Low-mass lepton pair production at large transverse momentum

TL;DR

This paper develops a perturbative QCD framework to analyze low-mass lepton pair production at high transverse momentum, introducing universal fragmentation functions and applying them to RHIC collision data.

Contribution

It introduces a systematic factorization approach for low-mass lepton pairs at large transverse momentum using universal fragmentation functions and models their evolution.

Findings

The transverse momentum distribution can be factorized into universal functions and calculable hard parts.

The model for lepton pair fragmentation functions is evolved to RHIC scales.

Sensitivity of the distribution to gluon density is discussed.

Abstract

We study the transverse momentum distribution of low-mass lepton pairs produced in hadronic scattering, using the perturbative QCD factorization approach. We argue that the distribution at large transverse momentum, $Q_T \gg Q$, with the pair's invariant mass $Q$ as low as $Q \sim \Lambda_{\mathrm{QCD}}$, can be systematically factorized into universal parton-to-lepton pair fragmentation functions, parton distributions, and perturbatively calculable partonic hard parts evaluated at a short distance scale $\sim {\cal O}(1/Q_T)$. We introduce a model for the input lepton pair fragmentation functions at a scale $\mu_0\sim 1$ GeV, which are then evolved perturbatively to scales relevant at RHIC. Using the evolved fragmentation functions, we calculate the transverse momentum distributions in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions at RHIC. We also discuss the sensitivity of the transverse momentum distribution of low-mass lepton pairs to the gluon distribution.