Longitudinal double spin asymmetry $A_1^p$ and spin-dependent structure function $g_1^p$ of the proton at low $x$ and low $Q^2$ from COMPASS

TL;DR

The COMPASS experiment measured the proton's spin asymmetry and structure function at very low x and Q^2, revealing non-zero spin effects in a previously unexplored region, with a significantly larger data set than prior studies.

Contribution

This work provides the first observation of spin effects at extremely low x and Q^2, using a much larger data sample than previous experiments, enhancing understanding of proton spin structure.

Findings

Non-zero, positive asymmetries observed at low x and Q^2.

First detection of spin effects in this low x region.

Data set is two orders of magnitude larger than previous measurements.

Abstract

The COMPASS experiment at CERN has collected a large sample of events of inelastic scattering of longitudinally polarised muons off longitudinally polarised protons in the non-perturbative region (four-momentum transfer squared $Q^2<1$ GeV$^2$/$c^2$), with a Bjorken scaling variable in the range $4\times 10^{-5}<x<4\times 10^{-2}$. The data set is two orders of magnitude larger than the similar sample collected by the SMC experiment. These data complement our data for polarised deuterons. They allow the accurate determination of the longitudinal double spin asymmetry $A_1^p$ and of the spin-dependent structure function $g_1^p$ of the proton in the region of low $x$ and low $Q^2$. The preliminary results of the analysis of these data yield non zero and positive asymmetries and of the structure function $g_1^p$. This is the first time that spin effects are observed at such low $x$.