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

We consider the azimuthal $\cos \phi$ and $\cos 2\phi$ distributions and the Callan-Gross ratio $R=\mathrm{d}{\sigma }_L\mathrm{/}\mathrm{d}{\sigma }_T$ in heavy-quark pair electroproduction, $lN\to l^{\mathrm{\prime }}Q\bar{Q}X$, 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.