Speaker
Description
The COMPASS legacy on the measurements of $g_1$ for proton and deuteron
through inclusive spin-dependent inelastic polarised-muon scattering
on longitudinally polarised NH$_3$ and $^6$LiD targets will be presented.
Results cover both deep-inelastic ($Q^2 > $ 1 (GeV/$c$)$^2$)
and non-perturbative ($Q^2 < $ 1 (GeV/$c$)$^2$) regions and a wide interval
in the Bjorken scaling variable. In the deep-inelastic set of data
the double-spin asymmetries and
spin-dependent structure functions are studied
in bins of $x$ and $Q^2$. Using those results an update of
the Bjorken sum rule is made and the QCD fit to the $g_1$
world data at the next-to-leading order
is performed.
In the nonperturbative region, studies of the spin asymmetry and
spin-dependent structure function for the proton are performed on four
2-dimensional grids of kinematic variables: $(x,Q^2), (\nu,Q^2), (x,\nu)$
and $(Q^2,x)$. The asymmetry and the structure function are found to be
positive in the whole range of $x$, $4\cdot 10^{-5} < x < 4\cdot 10^{-2}$,
for the first time displaying spin effects at such low values of $x$.