Measurement of the Angular Distribution of Drell-Yan Production in p+Fe Interactions at 120 GeV Beam Energy

We report on progress towards a measurement of the angular distributions of Drell-Yan dimuons produced at the SeaQuest/E906 Fermilab experiment, using a 120 GeV proton beam on an Fe beam dump upstream of the dimuon spectrometer. To extract the Drell-Yan signal, a combinatoric background subtraction method was developed, after which the detector, trigger, and reconstruction efficiency is corrected using a Bayesian unfolding method. After the unfolding of the reconstructed data, we measure the angular coefficients $\lambda$, $\mu$, and $\nu$ of the Drell-Yan events in the Collins-Soper frame. Results for the angular distribution of proton-induced Drell-Yan process using an 800 GeV proton beam were reported by Fermilab experiment E-866, where the Lam-Tung relation, $1 - \lambda = 2 \nu$, was also validated; however, pion-induced Drell-Yan experiments, NA10 at CERN and E615 at Fermilab, demonstrated a clear violation of this relation. Results from the iron beam dump data at SeaQuest will provide another test of the validity of the Lam-Tung relation. In addition to the polarization of the virtual photon, the Boer-Mulders function for the Fe nucleus can also be extracted from the $\cos 2\phi$ behavior of the angular distribution.