Parton Energy Loss and Modified Beam Quark Distribution Functions in Drell-Yan Process in p+A Collisions

TL;DR

This paper investigates how initial multiple parton scattering and energy loss affect Drell-Yan processes in proton-nucleus collisions, showing that energy loss has a minor effect at high energies but becomes significant at lower energies and large momentum fractions.

Contribution

It introduces a nuclear modification of beam quark distribution functions incorporating parton energy loss, providing a quantitative framework for understanding Drell-Yan suppression in p+A collisions.

Findings

Energy loss effects are negligible at 800 GeV in E866 kinematics.

Parton shadowing explains most observed suppression at high energies.

Significant suppression predicted at 120 GeV due to initial state energy loss.

Abstract

Within the framework of generalized collinear factorization in perturbative QCD (pQCD), we study the effect of initial multiple parton scattering and induced parton energy loss in Drell-Yan (DY) process in proton-nucleus collisions. We express the contribution from multiple parton scattering and induced gluon radiation to the DY dilepton spectra in terms of nuclear modified effective beam quark distribution functions. The modification depends on the quark transport parameter in nuclear medium. This is similar to the final-state multiple parton scattering in deeply inelastic scattering (DIS) off large nuclei and leads to the suppression of the Drell-Yan cross section in $p+A$ relative to $p+p$ collisions. With the value of quark transport parameter determined from the nuclear modification of single-inclusive DIS hadron spectra as measured by the HERMES experiment, we calculate DY spectra in $p+A$ collisions and find the nuclear suppression due to beam parton energy loss negligible at the Fermilab energy $E_{\rm lab}$=800 GeV in the kinematic region as covered by the E866 experiment. Most of the observed nuclear suppression of DY spectra in E866 experiment can be described well by parton shadowing in target nuclei as given by the EPS08 parameterization. The effect of beam parton energy loss, however, becomes significant for DY lepton pairs with large beam parton momentum fraction $x^{\prime}$ or small target parton momentum fraction $x$. We also predict the DY cross section in $p+A$ collisions at lower beam proton energy $E_{\rm lab}$=120 GeV and show significant suppression due to initial state parton energy loss at moderately large $x^{\prime}$ where the effect of parton shadowing is very small.