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 $Q^2$

TL;DR

This paper reports a highly precise measurement of the proton's spin asymmetry and structure function at very low x and Q^2, revealing positive spin effects in a previously unexplored kinematic region.

Contribution

It provides the first evidence of spin effects at very low x values, with significantly improved statistical precision over previous measurements.

Findings

Measured $A_1^{ m p}$ and $g_1^{ m p}$ are positive across the entire x range.

First observation of spin effects at such low x values.

Results are based on high-precision data from the COMPASS experiment.

Abstract

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 $x$.