We analyze the photo-production of ηc,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 photo-production of ηc,b in the threshold region, at current electron facilities.
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Liu et al. (2024) studied this question.
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