Longitudinal double-spin asymmetry A1p and spin-dependent structure function g1p of the proton at small values of x and Q2

Abstract

We present a precise measurement of the proton longitudinal double-spin asymmetry A1p and the proton spin-dependent structure function g1p at photonvirtualities 0.006(GeV/c)2<Q2<1(GeV/c)2 in the Bjorken x range of 4×10−5<x<4×10−2. The results are based on data collected by the COMPASS Collaboration at CERN using muon beam energies of 160GeV and 200GeV. The statistical precision is more than tenfold better than that of the previous measurement in this region. In the whole range of x, the measured values of A1p and g1p are found to be positive. It is for the first time that spin effects are found at such low values of x.