Accessing sea quark's angular momentum through polarized target Drell-Yan single-spin asymmetry measurements

A Letter-Of-Intent (P-1039) has been submitted to the Fermilab's Program Advisory Committee in May 2013, for a measurement of transversely polarized proton target (NH$_3$) single-spin asymmetry (SSA) in Drell-Yan reaction with a 120 GeV/c unpolarized proton beam using a similar setup as in the ongoing unpolarized target experiment (E906). The goal of this LOI is to clearly pin down the $\bar{u}$-quark Sivers distribution in the $x$ range of 0.1-0.3, where a large sea flavor asymmetry ($\bar{d}/\bar{u}$) has been observed. A non-vanishing quark Sivers distribution arises from the imaginary piece of amplitudes interference between quark angular momentum $L=0$, and $L \neq 0$ wave functions. Existing semi-inclusive DIS Sivers-type SSA data from HERMES, COMPASS and JLab-Hall A, while sensitive to valence quarks' Sivers distributions, do not provide much constrains on sea quarks' Sivers distributions. In the case that $\bar{u}$-quark carries zero angular momentum, one expects $\bar{u}$-quark's Sivers distribution to vanish, therefore observing a zero target SSA in Drell-Yan reaction in P-1039.