COMPASS Measurements of Asymmetry Amplitudes in the Drell-Yan Process Observed from Scattering Pions off a Transversely Polarized Proton Target

In 2015 the COMPASS collaboration at CERN studied the Drell-Yanprocess with a 190 GeV/c $\pi^-$ beam on a transversely polarized$\mathrm{NH}_3$ target. From single-spin asymmetries COMPASS was able todetermine amplitudes related to the proton Sivers, transversity andprezelosity transverse momentum dependent (TMD) distributions. The mostnotable of these TMDs is the semi-universal Sivers function which is theorizedto change sign between the Drell-Yan and semi-inclusive deep inelasticscattering (SIDIS) processes. COMPASS has previously measured a non-zeroSivers amplitude from SIDIS and therefore it offers a unique opportunity,with a similar experimental setup and kinematic domain, to definitely concludeon the sign change between the Drell-Yan and SIDIS processes. Theanalysis and results of the 2015 Drell-Yan COMPASS data are presented.