# First measurement of transverse-spin-dependent azimuthal asymmetries in the Drell-Yan process

## TL;DR

This paper reports the first measurement of transverse-spin-dependent azimuthal asymmetries in the pion-induced Drell-Yan process, testing fundamental QCD predictions about TMD PDFs and their universality.

## Contribution

It provides the first experimental evidence of the sign change of the Sivers function between SIDIS and Drell-Yan processes, confirming a key QCD prediction.

## Key findings

- Sivers asymmetry sign consistent with QCD predictions
- Observation of Boer-Mulders asymmetries involving transversity and pretzelosity
- Results support TMD factorization and universality in QCD

## 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/$c$, $\pi^{-}$ 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/$c^2$. 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. These DY results are obtained at a hard scale comparable to that of a recent COMPASS SIDIS measurement and hence allow unique tests of fundamental QCD universality predictions.