Abstract

The COMPASS experiment [COMPASS, P. Abbon et al., The COMPASS experiment at CERN, Nucl. Inst. Meth. A577, 455 (2007)] at CERN is one of the leading experiments studying the nucleon spin. These studies are being carried on since 2002, by measuring hadrons produced in deep inelastic scattering (DIS) of 160 GeV/c and 200 GeV/c polarised muons off different polarised targets (NH3 for polarised protons and 6LiD for polarised deuterons). One of the main goals is to determine how the total longitudinal spin projection of the nucleon, 1/2, is distributed among its constituents, quarks and gluons. We review here the recent results on the quark and gluon helicities obtained by COMPASS. The other major goal, whose fulfilment is needed for a complete understanding of the nucleon spin, is the determination of the transverse momentum dependent parton distributions (TMDs). Regarding this topic, the latest results on the Collins and Sivers asymmetries will be shown. The former is sensitive to the transverse spin structure of the nucleon, while the latter reflects the correlations between the quarks transverse momentum and the nucleon spin. This overview will conclude with a summary of the approved plans of COMPASS for the near future: the study of TMDs with a pioneering polarised Drell-Yan experiment and the measurement of generalised parton distributions (GPDs).

Highlights

  • In perturbative QCD, the leading twist contribution to a parton distribution function (PDF) can be written in collinear approximation as Φ(x) = [q(x) + γ5Δq(x)S L γ5ΔT q( x)S T γ1 ] (1)The quantity q(x) describes the unpolarised parton distribution as a function of the fraction of longitudinal momentum carried by the parton, Δq(x) (ΔT q(x)) represents the longitudinally polarised parton distribution function and S L (S T ) stands for the longitudinal spin of the nucleon

  • This article will conclude with a summary of the future plans concerning the study of orbital angular momentum (OAM), transverse momentum dependent parton distributions (TMDs) and generalised parton distributions (GPDs)

  • The extraction of quark helicities was performed for all light flavours from SIDIS data and from the next-to-leading order (NLO)-QCD fits to the g1 data

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Summary

Introduction

In perturbative QCD, the leading twist contribution to a parton distribution function (PDF) can be written in collinear approximation as. In COMPASS, the polarised PDFs are investigated via DIS processes for all the light quarks (u, d and s). Polarised gluon distribution function, it is only investigated from longitudinally polarised targets by studying the next-to-leading order (NLO) interaction of photongluon fusion (PGF). Measurements of transverse spin asymmetries leading to the determination of the transverse spin distributions of quarks and to the Sivers function are discussed. The latter is sensitive to the OAM of quarks and, may be of crucial importance for the determination. This article will conclude with a summary of the future plans concerning the study of OAM, TMDs and GPDs

Longitudinal Spin Structure of the Nucleon
Quark Helicities from Semi-Inclusive DIS
Gluon Polarisation
Transversity and TMDs
Outlook
Summary
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