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

• Received 5 November 2001

DOI:https://doi.org/10.1103/PhysRevC.65.065201