Contribution to the Proton Spin Puzzle from Helicity at small Bjorken $x$

TL;DR

This paper investigates the proton spin contribution from quarks and gluons at small Bjorken $x$ using helicity evolution, aiming to reduce uncertainties in global analyses by incorporating a valence quark model.

Contribution

It introduces a method to incorporate a valence quark model into small-$x$ helicity evolution to decrease free parameters in the initial conditions.

Findings

Excellent agreement between evolution and data

Large uncertainties due to many free parameters

Potential reduction of uncertainties using the valence quark model

Abstract

The contribution to proton helicity from the spins of quarks and gluons at small Bjorken $x$ can be conveniently studied through the small-$x$ helicity evolution. Recently, it has been employed in a global analysis that includes polarized inclusive and semi-inclusive deep-inelastic scattering measurements at small $x$. The analysis demonstrates an excellent agreement between the evolution and the data, but its predictions contain large uncertainties owing to the large number of free parameters in the initial conditions. The latest development aims to mitigate this issue by employing the valence quark model in the polarized proton target in order to replace most of the free parameters in the initial conditions by the physical information from the proton model.