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

Figures (3)

• Figure 1: Angle definitions of the Drell-Yan process in the Collins-Soper frame. It is a rest frame of the virtual photon. The hadron plane is defined by the momenta of two interacting hadrons ($h_1$ and $h_2$). The $z$ axis is taken on the hadron plane so that the angle to $h_1$ is equal to that to $h_2$. The lepton plane is defined by the momenta of two final-state leptons ($\mu^+$ and $\mu^-$) and the $z$ axis.
• Figure 2: Parameters of the angular distributions $\mu$ (top) and $\nu$ (bottom) obtained from LH$_2$ target events (the red points and systematic uncertainty bands). The black triangle points show the E866 results E866pp and the blue lines show the pQCD calculations PhysRevD.99.014032
• Figure 3: Parameters of the angular distributions $\mu$ (top) and $\nu$ (bottom) obtained from LD$_2$ target events (the red points and systematic uncertainty bands). The black triangle points show the E866 results E866pd, the blue lines show the pQCD calculations PhysRevD.99.014032, the green triangle points show the NA10 results NA10, and open circle points show the E615 results E615_2.