Exploring the Partonic Structure of Hadrons through the Drell-Yan Process

TL;DR

This review surveys how the Drell-Yan process probes the partonic structure of hadrons, emphasizing unpolarized sea-quark distributions, nuclear modifications, and access to transversity and quark helicities with polarized beams. It covers the formalism of the Drell-Yan cross section, the role of factorization and higher-order corrections, and key experimental findings such as the pronounced dbar vs ubar asymmetry in the nucleon and the absence of large nuclear effects. It also discusses angular distributions, the Lam-Tung relation, and the role of transverse momentum dependent distributions (Sivers and Boer-Mulders) in unpolarized and polarized Drell-Yan, outlining upcoming measurements (E906, J-PARC, PAX, RHIC-spin) that will tighten constraints on h1, f1T⊥, and Delta distributions.

Abstract

The Drell-Yan process is a standard tool for probing the partonic structure of hadrons. Since the process proceeds through a quark-antiquark annihilation, Drell-Yan scattering possesses a unique ability to selectively probe sea distributions. This review examines the application of Drell-Yan scattering to elucidating the flavor asymmetry of the nucleon's sea and nuclear modifications to the sea quark distributions in unpolarized scattering. Polarized beams and targets add an exciting new dimension to Drell-Yan scattering. In particular, the two initial-state hadrons give Drell-Yan sensitivity to chirally-odd transversity distributions.

Figures (9)

• Figure 1: The dilepton cross section as measured by Christenson et al.PhysRevD.8.2016, showing a rapid decrease as a function of dilepton mass. The excess of events in the $3 < M_{\gamma^*} < 4$ region is from the dilepton decay of the $J/\psi$.
• Figure 2: Feynman diagram for the leading order Drell-Yan process.
• Figure 3: The $\cos\theta$ dependence of the proton-proton Drell-Yan cross section as measured by the Fermilab E866/NuSea experiment. The curve shows the result of a fit of the data to $A\left( 1 + \lambda \cos^2 \theta \right)$E866_angle.
• Figure 4: Feynman diagram for the terms of next-to-leading order in $\alpha_s$ for the Drell-Yan process: (a) and (b) QCD Compton; (d) and (e) gluon production and (f) vertex correction Kubar:1980zv.
• Figure 5: Drell-Yan absolute cross sections measured by Fermilab E866/NuSea (proton-proton and proton deuterium) Webb:2003bjWebb:2003ps, E772 (proton-deuterium) PhysRevD.50.3038PhysRevD.60.119903 and E605 (proton-copper) PhysRevD.43.2815 compared with NLO cross section calculations based on the CTEQ6 Pumplin:2002vw and MRST Martin:2006qz parton distributions. There is an overall 6.5% normalization uncertainty on the E866/NuSea data.