Quark Helicity Distributions from Longitudinal Spin Asymmetries in Muon-Proton and Muon-Deuteron Scattering

We present a leading order evaluation of the helicity distributions for the three lightest quarks and anti-quarks flavours, derived from the double-spin asymmetries measured by the COMPASS collaboration. The data, obtained by scattering a 160 GeV muon beam off longitudinally polarised NH$_{3}$ and $^{6}$LiD targets, cover the Bjorken variable $x$ range 0.004 $< x <$ 0.7. The resulting values of the sea quark distributions are all 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 anti-strange helicity distributions, both compatible with zero. The integrated value of the polarised flavour sea asymmetry is found to be slightly positive, about 1.5 standard deviations away from zero.