Probing Gluons in Nuclei: the Case of ${\eta }^{\mathrm{\prime }}$

Using the recently proposed $gg-->et{a}^{\mathrm{\prime }}$ effective vertex, we investigate the production of ${\eta }^{\mathrm{\prime }}$ from gluon fusion in $pA$ collisions. We show that measuring ${\eta }^{\mathrm{\prime }}$ production cross-section at moderate $x_{{\eta }^{\mathrm{\prime }}}$ yields direct information on the small $x$ gluon distribution function of the nucleus. At RHIC, the smallest accessible $x$ turns out to be $O({10}^{-5})$ and at LHC, it is $O({10}^{-8})$. Therefore, ${\eta }^{\mathrm{\prime }}$ is an excellent probe of the Color Glass Condensate.