Photoproduction of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>η</mml:mi><mml:mrow><mml:mi>c</mml:mi><mml:mo>,</mml:mo><mml:mi>b</mml:mi></mml:mrow></mml:msub></mml:math> near threshold

We analyze the photoproduction of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msub><a:mi>η</a:mi><a:mrow><a:mi>c</a:mi><a:mo>,</a:mo><a:mi>b</a:mi></a:mrow></a:msub></a:math> 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 <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:msub><c:mi>η</c:mi><c:mrow><c:mi>c</c:mi><c:mo>,</c:mo><c:mi>b</c:mi></c:mrow></c:msub></c:math> in the threshold region, at current electron facilities. Published by the American Physical Society 2024