Measurement of the longitudinal spin structure of the proton by COMPASS

TL;DR

This paper reports on measurements of the proton's spin structure using polarized deep-inelastic scattering data from COMPASS, confirming the Bjorken sum-rule and providing flavor-separated quark helicity densities.

Contribution

It presents new measurements of spin asymmetries, evaluates the non-singlet structure function, and extracts individual quark helicity densities, including sea quarks, with full flavor separation.

Findings

Confirmed the Bjorken sum-rule with experimental data.

Sea quark helicity densities are consistent with zero.

Provided flavor-separated quark helicity distributions.

Abstract

The inclusive A_1,p and hadron double-spin asymmetries A_p^pi+, A_p^pi-, A_p^K+, A_p^K- measured at COMPASS (CERN SPS) in deep-inelastic scattering of a polarized muon beam off a polarized NH_3 solid target are presented. The results have been obtained with the full statistics collected in 2007 for the longitudinal target polarization. Proton asymmetries have been combined with the published deuteron ones. An evaluation of the non-singlet spin-dependent structure function g_1^NS(x,Q^2) and its first moment, which confirms the validity of the Bjorken sum-rule, is presented. A LO evaluation of polarized quark densities is also presented. The use of the proton data allows to perform a full flavor separation and to extract individual helicity densities of u, d, anti-u, anti-d and s quarks. All sea quark densities are found to be compatible with zero in the full range of the measurements.