The spin structure of the proton at low $x$ and low $Q^2$ in two-dimensional bins from COMPASS

TL;DR

This paper presents the first two-dimensional bin analysis of the proton's spin structure functions at low x and Q^2 using COMPASS data, providing new insights into spin asymmetries in previously unexplored kinematic regions.

Contribution

It introduces a novel two-dimensional bin analysis of $A_1^p$ and $g_1^p$ at low x and Q^2, based on a significantly larger dataset from COMPASS, extending previous studies.

Findings

Measured $A_1^p$ and $g_1^p$ in new kinematic regions

Compared experimental results with theoretical models

Enhanced understanding of proton spin structure at low x and Q^2

Abstract

The longitudinal double spin asymmetries $A_1^p$ and the spin dependent structure function of the proton $g_1^p$ were extracted from COMPASS data in the region of low Bjorken scaling variable $x$ and low photon virtuality $Q^2$. The data were taken in 2007 and 2011 from scattering of polarised muons off polarised protons, resulting in a sample that is 150 times larger than the one from the previous experiment SMC that pioneered studies in this kinematic region. For the first time, $A_1^p$ and $g_1^p$ were evaluated in this region in two-dimensional bins of kinematic variables: $(x,Q^2)$, $(\nu ,Q^2)$, $(x,\nu)$ and $(Q^2,x)$. The following kinematic region was investigated: $4\times 10^{-5}<x<4\times 10^{-2}$, $0.001$~(GeV/$c$)$^2<Q^2<1$~(GeV/$c$)$^2$ and $14$~GeV$<\nu <194$~GeV. The obtained results were confronted with theoretical models.