Angular Distribution Measurement of $p+p$ and $p+d$ Drell–Yan Processes at 120 GeV by the SeaQuest Experiment at Fermilab

In the naive Drell–Yan model, the angular distribution of dimuons shows a $\cos\theta$ modulation and no $\cos2\phi$ modulation ($\nu=0$), where $\theta$ and $\phi$ are the polar and azimuthal angles. However, $\cos2\phi$ modulation can arise from perturbative QCD effects and transverse-momentum-dependent parton distributions (TMDs). Pion-induced Drell–Yan experiments, such as NA10 and E615, observed sizable $\cos2\phi$ modulations. In contrast, the E866 experiment at Fermilab, which used an 800 GeV proton beam, reported significantly smaller modulation in proton-induced Drell–Yan and remains the only prior measurement of its angular distribution. The SeaQuest experiment at Fermilab, using a 120 GeV proton beam on liquid hydrogen and deuterium targets, has measured the angular distribution of proton-induced Drell–Yan dimuons in a different kinematic regime from E866. The results of the angular distribution analysis by SeaQuest, providing new data on the proton-induced Drell–Yan process, will be presented.