Experimental investigation of transverse spin asymmetries in muon-p SIDIS processes: Sivers asymmetries

TL;DR

This paper reports precise measurements of the Sivers asymmetry in muon-proton SIDIS processes, revealing insights into quark spin-orbit coupling and its dependence on kinematic variables, with results consistent with theoretical predictions.

Contribution

The study provides the most accurate measurements of the Sivers asymmetry in muon-proton SIDIS, confirming previous results and exploring its kinematic dependence at high energies.

Findings

Sivers asymmetry is zero for negative hadrons and positive for positive hadrons.

Smaller Sivers asymmetry for positive hadrons at x > 0.03 compared to lower energy experiments.

Asymmetry is positive in the low x region, indicating sea-quark contributions.

Abstract

The COMPASS Collaboration at CERN has measured the transverse spin azimuthal asymmetry of charged hadrons produced in semi-inclusive deep inelastic scattering using a 160 GeV positive muon beam and a transversely polarised NH_3 target. The Sivers asymmetry of the proton has been extracted in the Bjorken x range 0.003<x<0.7. The new measurements have small statistical and systematic uncertainties of a few percent and confirm with considerably better accuracy the previous COMPASS measurement. The Sivers asymmetry is found to be compatible with zero for negative hadrons and positive for positive hadrons, a clear indication of a spin-orbit coupling of quarks in a transversely polarised proton. As compared to measurements at lower energy, a smaller Sivers asymmetry for positive hadrons is found in the region x > 0.03. The asymmetry is different from zero and positive also in the low x region, where sea-quarks dominate. The kinematic dependence of the asymmetry has also been investigated and results are given for various intervals of hadron and virtual photon fractional energy. In contrast to the case of the Collins asymmetry, the results on the Sivers asymmetry suggest a strong dependence on the four-momentum transfer to the nucleon, in agreement with the most recent calculations.