Longitudinal double-spin asymmetry $A_1^{\rm p}$ and spin-dependent structure function $g_1^{\rm p}$ of the proton at small values of $x$ and $Q^2$

We present a precise measurement of the proton longitudinal double-spin asymmetry A1p and the proton spin-dependent structure function g1p at photon virtualities 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 .