Flavour Separation of Helicity Distributions from Deep Inelastic Muon-Deuteron Scattering

TL;DR

This paper presents a leading-order analysis of helicity distributions of various quark flavors in the nucleon, using polarized deep inelastic muon-deuteron scattering data from the COMPASS experiment, revealing that strange quark polarization is compatible with zero.

Contribution

It provides the first LO extraction of helicity densities for valence, sea, and strange quarks from semi-inclusive measurements, highlighting differences from previous QCD fits.

Findings

Non-strange helicity densities agree with previous results.

Strange quark helicity distribution is consistent with zero.

Sensitivity to fragmentation functions is discussed.

Abstract

We present a LO evaluation of helicity densities of valence, \Delta u_v+\Delta d_v, non-strange sea, \Delta\bar{u}+\Delta\bar{d}, and strange quarks, \Delta s (assumed to be equal to \Delta\bar{s}). They have been obtained from the inclusive asymmetry A_{3,d} and the semi-inclusive asymmetries A^{\pi+}_{1,d}, A^{\pi-}_{1,d}, A^{K+}_{1,d}, A^{K-}_{1,d} measured in polarised deep inelastic muon-deuteron scattering. The full deuteron statistics of COMPASS (years 2002-2004 and 2006) has been used. The data cover the range Q^2 > 1 (GeV/c)^2 and 0.004<x<0.3. Both non-strange densities are found to be in a good agreement with previous measurements. The distribution of \Delta s(x) is compatible with zero in the whole measured range, in contrast to the shape of the strange quark helicity distribution obtained in most LO and NLO QCD fits. The sensitivity of the values of \Delta s(x) upon the choice of fragmentation functions used in the derivation is discussed.