Boer–Mulders function of the pion and pretzelosity distribution of the proton in the polarized pion-proton Drell–Yan process at COMPASS

We present a phenomenological analysis of the q T -weighted transverse spin dependent azimuthal asymmetry recently measured by the COMPASS Collaboration in polarized pion-proton Drell–Yan process. In the kinematical regimes explored by experiments, we consider the leading-twist contributions from the Boer–Mulders distribution functions h 1 ⊥ ( x , k T 2 ) of both the pion and the proton, the transversity distribution h 1 ( x , k T 2 ) and the pretzelosity distribution h 1 T ⊥ ( x , k T 2 ) of the proton, with the unpolarized transverse-momentum-dependent distribution function f 1 ( x , k T 2 ) of the proton being also involved in the calculation. By comparing the data reported by the COMPASS Collaboration with theoretical calculated results, we find that the COMPASS measurements represent the first experimental evidence of the Boer–Mulders effect in polarized Drell–Yan process. We also test the universality between proton and pion Boer–Mulders functions.