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

TL;DR

This study measures the angular distributions of Drell-Yan muons in proton-proton and proton-deuterium collisions at 120 GeV/c, revealing significant deviations from pQCD predictions and indicating nonperturbative effects.

Contribution

First measurement of Drell-Yan muon angular distributions at this energy showing unexpected $ ext{cos} 2 ext{phi}$ modulations and deviations from theoretical models.

Findings

Pronounced $ ext{cos} 2 ext{phi}$ modulation observed.

Significant deviations from pQCD predictions.

Evidence of nonperturbative QCD contributions.

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 ($\theta$) and azimuthal ($\phi$) angles in the Collins-Soper frame are measured within the kinematic range of $4.5 < m_{\mu\mu} < 10\ \mathrm{GeV}/c^2$, $0.19 < p_T < 2.24\ \mathrm{GeV}/c$, 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 $\cos 2\phi$ 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.