Abstract
The first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan (DY) process is reported. We use the CERN SPS 190 GeV/c π^{-} beam and a transversely polarized ammonia target. Three azimuthal asymmetries giving access to different transverse-momentum-dependent (TMD) parton distribution functions (PDFs) are extracted using dimuon events with invariant mass between 4.3 GeV/c^{2} and 8.5 GeV/c^{2}. Within the experimental uncertainties, the observed sign of the Sivers asymmetry is found to be consistent with the fundamental prediction of quantum chromodynamics (QCD) that the Sivers TMD PDFs extracted from DY have a sign opposite to the one extracted from semi-inclusive deep-inelastic scattering (SIDIS) data. We present two other asymmetries originating from the pion Boer-Mulders TMD PDFs convoluted with either the nucleon transversity or pretzelosity TMD PDFs. A recent COMPASS SIDIS measurement was obtained at a hard scale comparable to that of these DY results. This opens the way for possible tests of fundamental QCD universality predictions.
Highlights
The first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced DrellYan (DY) process is reported
The observed sign of the Sivers asymmetry is found to be consistent with the fundamental prediction of quantum chromodynamics (QCD) that the Sivers TMD parton distribution functions (PDFs) extracted from DY have a sign opposite to the one extracted from semiinclusive deep-inelastic scattering (SIDIS) data
We present two other asymmetries originating from the pion Boer-Mulders TMD PDFs convoluted with either the nucleon transversity or pretzelosity TMD PDFs
Summary
The COMPASS experiment at CERN [23,24] has the unique capability to explore the transverse-spin structure of the nucleon in a similar kinematic region by two alternative experimental approaches, i.e., SIDIS and DY, using mostly the same setup This offers the opportunity of minimizing uncertainties of TMD evolution in the comparison of the Sivers TMD PDFs when extracted from these two measurements to test the opposite-sign prediction by QCD. When the polarizations of the produced leptons are summed over, the general expression for the cross section of pion-nucleon DY lepton-pair production off a transversely polarized nucleon comprises five transverse spindependent asymmetries (TSAs), including the Sivers TSA Those three TSAs that can be described by contributions from only twist-2 TMD PDFs will be addressed in this Letter. The TSAs AwT in Eq (1) are defined as amplitudes of a given azimuthal modulation w 1⁄4 wðφS; φCSÞ, divided by the spin and azimuth-independent part of the DY cross section and the corresponding depolarization factor
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