Polarized Drell-Yan studies at COMPASS

Catarina Quintans,S Lunardi,P Di Nezza,C Bucci,M Chiari,J.J Valiente-Dobon,A Dainese,C Signorini,P.G Bizzeti,R Menegazzo,A Nannini

doi:10.1051/epjconf/20146606021

Abstract

The COMPASS experiment at CERN will soon start a new series of measure- ments using a pion beam and a transversely polarized target. The study of the polarized Drell-Yan process will provide an insight of the transverse momentum dependent parton distribution functions (TMDs), which is complementary to their extraction from semi- inclusive deep inelastic scattering (SIDIS), previously measured in COMPASS. The sign change of Sivers and Boer-Mulders TMDs, when accessed from SIDIS or Drell-Yan, is predicted by theory. Its experimental observation is considered an essential test of the TMD approach. The experimental aspects of the Drell-Yan measurement in COMPASS are discussed. The set-up optimization, driven by the results of several beam tests are presented, as well as the expected event rates and statistical errors of the azimuthal asymmetries.

Highlights

The COMPASS experiment has been taking data at CERN since 2002, using a polarized muon beam impinging in a polarized target
Among the most important results obtained are the azimuthal spin asymmetries in semi-inclusive deep inelastic scattering of a muon off a transversely polarized proton. These azimuthal asymmetries relate to the convolution of a transverse-momentum dependent parton distribution function (TMD PDF) with the parton fragmentation function into a detected hadron
While in Drell-Yan an azimuthal spin asymmetry related to the Sivers transverse momentum dependent parton distribution functions (TMDs) can only arise from initial state interactions, in semiinclusive deep inelastic scattering (SIDIS) it relates to final state interactions, which results in a sign change of the TMD from one process to the other

Summary

Introduction

The COMPASS experiment has been taking data at CERN since 2002, using a polarized muon beam impinging in a polarized target. While in Drell-Yan an azimuthal spin asymmetry related to the Sivers TMD (unpolarized quark inside a transversely polarized proton) can only arise from initial state interactions, in SIDIS it relates to final state interactions, which results in a sign change of the TMD from one process to the other. This TMD is referred to as naive time-reversal odd, just as the Boer-Mulders one. The different average Q2 of the 2 measurements has to be taken into account, and the appropriate TMD evolution done

Experimental set-up

Beam tests

Findings

Summary