Measurement of the Angular Distributions of Drell-Yan Dimuons

The angular differential cross section for the Drell-Yan (DY) process can be parametrized by $\frac{d\sigma}{d\Omega}\propto 1+ \lambda \cos^2 \theta +\mu \sin 2\theta \cos\phi +\frac{\nu}{2}\sin^2\theta \cos 2\phi$, where $\lambda$, $\mu$, and $\nu$ are the angular distribution parameters vs $p_T$. $\theta$ and $\phi$ denote the polar and azimuthal angles, respectively for the positive lepton produced. The Lam-Tung relation, $1-\lambda = 2 \nu$, was validated by Fermilab E-866 for proton induced Drell-Yan scattering; However pion induced DY shows a much stronger cos2$\theta$ angular dependence and a violation of the Lam-Tung relation. In pion induced DY the antiquark is a valance quark, whereas in proton induced DY (in a forward acceptance spectrometer) it is a sea quark, so E-866 probed the antiquark sea of the nucleon. The SeaQuest experiment, also using proton induced DY, will improve on the measurement of the angular dependencies at a lower energy (120 GeV), taking advantage lower backgrounds and an increase in Drell-Yan cross section at lower energies. The Boer-Mulders correlates the quark correlates between the quark transverse spin and momentum. Improved data from SeaQuest will help determine the Boer-Mulders function. Funding for this work was provided in part by the U.S. DOE Office of Science.