Measurements of Angular Distributions of Drell-Yan Dimuons in $p+p$ and $p+d$ Interactions at 120 GeV/$c$

Abstract

We present experimental results on the angular distributions of Drell-Yan muons produced by a 120 GeV/$c$ proton beam interacting with liquid hydrogen and deuterium targets. The dimuon angular distributions in both polar ($\hat{I}¸$) and azimuthal ($\ddot{I}•$) angles in the Collins-Soper frame are measured within the kinematic range of $4.5<m_{\hat{I}¼\hat{I}¼}<10\mathrm{GeV}/c^2$, $0.19<p_T<2.24\mathrm{GeV}/c$, and $0<x_F<0.95$. Unlike the results of a previous proton-induced Drell-Yan experiment at a higher energy, the data reveal a pronounced $\cos 2\ddot{I}•$ modulation in the angular distributions. Comparison with perturbative QCD (pQCD) predictions shows statistically significant deviations, with p-values of 3.5\% for the $p+p$ and 1.5\% for the $p+d$ Drell-Yan processes. These results suggest the presence of nonperturbative QCD contributions.

6 pages, 3 figures