Transverse Spin Effects at COMPASS

TL;DR

This paper reports on measurements of transverse spin asymmetries in semi-inclusive deep-inelastic scattering at COMPASS, providing insights into nucleon structure, including transversity and Sivers functions, through experimental data from polarized targets.

Contribution

It presents new experimental results on transverse spin effects, specifically the transversity distribution and Sivers asymmetry, from COMPASS data on polarized proton and deuteron targets.

Findings

Observation of non-zero Sivers asymmetry for the proton

First measurements of transversity distribution in semi-inclusive DIS

Enhanced understanding of parton transverse momentum and angular momentum

Abstract

Single spin asymmetries in semi-inclusive deep-inelastic scattering off transversely polarized nucleon targets have been under intense experimental investigation over the past few years. They provide new insights into QCD and the nucleon structure. For instance, they allow the determination of the third yet-unknown leading-twist quark distribution function $\Delta_{T}q(x)$, the transversity distribution. Additionally, they give insight into the parton transverse momentum distribution and angular momentum. The measurement of transverse spin effects in semi-inclusive deep-inelastic scattering is an important part of the COMPASS physics program. In the years 2002-2004 data were collected scattering a 160 GeV muon beam on a transversely polarized deuteron target. In 2007, additional data were collected on a transversely polarized proton target. New results from the analysis of the proton data will be presented. A different but not less important insight into the nucleon structure might be given by the Sivers asymmetry. This angular dependence of the cross-section arises from an intrinsic asymmetry in the parton transverse momentum distribution. The Sivers function is tightly related to the total angular momentum carried by the quarks in the nucleon. New COMPASS results for the Sivers asymmetry of the proton will be shown.