Measurement of the Polarised Drell-Yan process at COMPASS

Márcia Quaresma

doi:10.1051/epjconf/20148104011

Abstract

The COMPASS experiment at CERN has been playing an important role in the studies of the spin content of the nucleon. The Semi-Inclusive Deep Inelastic Scattering (SIDIS) process gives access to the transverse momentum dependent parton distribution functions (TMDs) by the measurement of azimuthal asymmetries that have been studied in COMPASS and published in recent years. TMDs are also accessible by the transversely polarised Drell-Yan (DY) process which will be measured in COMPASS. This will be the first ever polarised DY measurement. The valence quarks region will be dominant due to the use of a negative pion beam at 190 GeV/c momentum impinging on a transversely polarised ammonia target. The QCD prediction that Sivers TMD change sign when ac- cessed by SIDIS or by DY will be checked by the COMPASS measurement. The data taking is scheduled to start in the fall of this year. After one year of data collection, a sta- tistical error below 2% in the azimuthal asymmetry related to the u quark Sivers function is expected. Details of the final experimental setup will be presented.

Highlights

The nucleon structure in QCD leading twist, taking into account the intrinsic transverse momentum of the quarks kT, is described by 8 transverse momentum dependent parton distribution functions (TMD PDFs) for each quark flavour
All the TMD PDFs are expected to be sizeable in the valence u quark region, which is dominant when a negative pion beam is used and, the QCD TMD approach is valid in the region Q (Mμμ>4 GeV/c2) pT ∼ 1 GeV/c
Each angular modulation has an amplitude that contains the convolution of two TMD PDFs

Summary

Introduction

The nucleon structure in QCD leading twist, taking into account the intrinsic transverse momentum of the quarks kT , is described by 8 transverse momentum dependent parton distribution functions (TMD PDFs) for each quark flavour Both the single polarised Drell-Yan measurement and the transversely polarised SIDIS give access to them. ATsin(2φ+φS ) relates to the beam quark Boer-Mulders h⊥1 and the target quark pretzelosity h⊥1T and ATsin(2φ−φS ) relates to the beam quark Boer-Mulders h⊥1 and the target quark transversity function h1

Experimental setup

COMPASS: projections and prior results

Event rates and statistical accuracy

Findings

Outlook