Probing the linearly polarized gluons in unpolarized proton with heavy-quark pair production

TL;DR

This paper investigates how azimuthal asymmetries and the Callan-Gross ratio in heavy-quark pair production can reveal the presence and characteristics of linearly polarized gluons inside unpolarized protons, offering insights into proton spin structure.

Contribution

It demonstrates that azimuthal asymmetries and the Callan-Gross ratio are highly sensitive to the gluonic Boer-Mulders function, providing a new method to probe gluon polarization in nucleons.

Findings

Azimuthal asymmetries can reach values up to 1 depending on gluon polarization.

The Callan-Gross ratio is significantly affected by the linearly polarized gluons.

Future collider measurements can clarify the gluon contribution to proton spin.

Abstract

We consider the azimuthal $\cos \varphi$ and $\cos 2\varphi$ distributions and the Callan-Gross ratio $R={\rm d}\sigma_L/{\rm d}\sigma_T$ in heavy-quark pair electroproduction, $lN\rightarrow l^{\prime}Q\bar{Q}X$, as probes of linearly polarized gluons in unpolarized nucleons. Our analysis shows that the azimuthal asymmetries and Callan-Gross ratio are predicted to be large and very sensitive to the contribution of the gluonic counterpart of the Boer-Mulders function, $h_{1}^{\perp g}$, describing the linear polarization of gluons inside unpolarized nucleon. In particular, the maximum values of the azimuthal distributions vary from 0 to 1 depending on $h_{1}^{\perp g}$. We conclude that future measurements of these quantities at the proposed EIC and LHeC colliders could clarify in details the proton spin decomposition puzzle.