Photoproduction of ${\eta }_{c,b}$ near threshold

We analyze the photoproduction of ${\eta }_{c,b}$ off a proton in the threshold region, in terms of C-odd gluonic correlations in the off-forward proton matrix element. Near threshold, the skewness is large, leading to a production amplitude that is dominated by four C-odd twist-3 gluon GPDs. We use the QCD instanton vacuum to estimate these C-odd contributions in the proton. The results are used to estimate the differential cross sections for coherent photoproduction of ${\eta }_{c,b}$ in the threshold region, at current electron facilities.