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 $A_1^{rm p}$ and the proton spin-dependent structure function $g_1^{rm p}$ at photon virtualities $0.006~({rm GeV}/c)^2<Q^2 < 1~({rm GeV}/c)^2$ in the Bjorken $x$ range of $4 times 10^{-5} < x < 4 times 10^{-2}$. The results are based on data collected by the COMPASS Collaboration at CERN using muon beam energies of $160~{rm GeV}$ and $200~{rm GeV}$. 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 $A_1^{rm p}$ and $g_1^{rm p}$ are found to be positive. It is for the first time that spin effects are found at such low values of $x$.