April 24, 2017
The COMPASS collaboration at CERN submitted their 2015 measurement for publication in Physical Review Letters. The Sivers amplitude from the 2015 COMPASS spin-dependent Drell-Yan data provides the first hint of a sign change of the Sivers function measured in Drell-Yan (DY) as compared to measured in Semi-Inclusive Deep-Inelastic scattering (SIDIS). This sign change is a fundamental prediction of Quantum Chromo Dynamics, or QCD, the underlying theory of the strong interaction between quarks in the proton. The sign change is a milestone of the Nuclear Science Advisory Committee (NSAC). The Sivers function is one of the proton's Transverse-Momentum Dependent parton distributions (TMDs) and describes the correlation between the transverse spin of the parent proton and the transverse momentum of the probed quark. COMPASS has previously measured the Sivers amplitude in SIDIS and is therefore in the unique situation to conclude on the sign change without involving uncertainties related to TMD evolution.
The UIUC COMPASS group has significantly contributed to the success of the measurement in terms of instrumentation, coordination and data analysis. The Illinois authors are: Vincent Andrieux, Ihn-Jea Choi, Francesca Giordano, Robert Heitz, Jaakko Koivuniemi, Yakov Kulinich, Alain Magnon, Naomi Makins, Marco Meyer, Jen-Chieh Peng, Matthias Grosse Perdekamp, and Caroline Riedl. The arXiv version of the paper is available here: https://arxiv.org/abs/1704.00488. A second COMPASS run of spin-dependent DY in 2018 will enhance the statistical significance of the measurement.