Measurement of Transverse-Spin-Dependent Asymmetries in the Drell-Yan Process by Compass
Marcia Quaresma1*
1Physics department, Academia Sinica, Taipei, Taiwan
* Presenter:Marcia Quaresma,
The first pion induced polarised Drell-Yan measurement took place in the COMPASS experiment at CERN, in 2015. A 190 GeV/c negative pion beam and a transversely polarised ammonia target were used. The first results were published in the Physical Review Letters in September 2017 with the title "First Measurement of Transverse-Spin-Dependent Azimuthal Asymmetries in the Drell-Yan Process". The measured asymmetries, related with Transverse Momentum Dependent (TMD) Parton Distribution Functions (PDFs), will be presented. A special focus will be given to the Sivers asymmetry, which is related with the orbital angular momentum of the partons inside the nucleon. The Sivers function sign has the peculiarity to be process dependent, which constitutes a fundamental QCD prediction and may be proven using the COMPASS data. The 2018 COMPASS data-taking will be again dedicated to the measurement of the polarised Drell-Yan measurement, with essentially the same conditions as in 2015, but with improved beam particle identification detectors, giving the possibility to distinguish between pions, kaons and anti-protons.

Keywords: Drell-Yan, TMD PDFs, Sivers function, Orbital angular momentum, COMPASS experiment