Measurements of Angular Distributions of Drell-Yan Dimuons in $p+p$ and $p+d$ Interactions at 120 GeV/$c$
nucl-ex
/ Authors
K. Nagai, M. F. Hossain, A. Pun, K. Nakano, K. Liu, C. A. Aidala, J. Arrington, C. Ayuso, C. Barker, W. C. Chang
and 47 more authors
A. Chen, D. C. Christian, B. Dannowitz, J. Dove, L. El Fassi, D. F. Geesaman, R. Gilman, Y. Goto, R. S. Guo, L. Guo, T. J. Hague, R. J. Holt, D. Isenhower, E. R. Kinney, A. Klein, D. Kleinjan, C. H. Leung, P. -J. Lin, M. X. Liu, W. Lorenzon, R. E. McClellan, P. L. McGaughey, M. M. Medeiros, Y. Miyachi
/ 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.