Quark helicity distributions from longitudinal spin asymmetries in muon-proton and muon-deuteron scattering

TL;DR

This paper reports measurements of double-spin asymmetries in semi-inclusive deep-inelastic muon scattering, leading to the extraction of quark helicity distributions, showing small sea quark polarizations and no significant strange-antistrange difference.

Contribution

First leading order extraction of light quark and antiquark helicity distributions from COMPASS semi-inclusive data covering a broad x range.

Findings

Sea quark helicity distributions are small and x-independent.

Strange and antistrange helicity distributions are compatible with zero.

The light-quark sea asymmetry is slightly positive, about 1.5 sigma from zero.

Abstract

Double-spin asymmetries for production of charged pions and kaons in semi-inclusive deep-inelastic muon scattering have been measured by the COMPASS experiment at CERN. The data, obtained by scattering a 160 GeV muon beam off a longitudinally polarised NH_3 target, cover a range of the Bjorken variable x between 0.004 and 0.7. A leading order evaluation of the helicity distributions for the three lightest quarks and antiquark flavours derived from these asymmetries and from our previous deuteron data is presented. The resulting values of the sea quark distributions are small and do not show any sizable dependence on x in the range of the measurements. No significant difference is observed between the strange and antistrange helicity distributions, both compatible with zero. The integrated value of the flavour asymmetry of the helicity distribution of the light-quark sea, \Delta u-bar - \Delta d-bar, is found to be slightly positive, about 1.5 standard deviations away from zero.