Proton’s gluon GPDs at large skewness and gravitational form factors from near threshold heavy quarkonium photoproduction
Yuxun Guo, Xiangdong Ji, Feng Yuan
Abstract
We study the exclusive near threshold photoproduction of heavy quarkonium in the framework of the generalized parton distribution (GPD) factorization, taking the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>J</a:mi><a:mo>/</a:mo><a:mi>ψ</a:mi></a:math> production as an example. Because of the threshold kinematics, the Compton-like amplitudes are related to gluon GPDs at large skewness <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi>ξ</c:mi></c:math>, distinct from the common kinematics in asymptotic high energy where the skewness is typically small. We discuss the nature of large-<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mi>ξ</e:mi></e:math> expansion of these amplitudes in terms of the moments of gluon GPDs in the large-<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mi>ξ</g:mi></g:math> limit. Based on that, we propose several ways to extract the first few moments of the gluon GPDs from these amplitudes, with the leading ones corresponding to the gluonic gravitational or energy-momentum tensor form factors (GFFs). We apply these methods to analyze the recent near threshold <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mi>J</i:mi><i:mo>/</i:mo><i:mi>ψ</i:mi></i:math> production measurements by the <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:mi>J</k:mi><k:mo>/</k:mo><k:mi>ψ</k:mi></k:mrow></k:math> 007 experiment and GlueX Collaboration and find that the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mi>ξ</m:mi></m:math> scaling of the measured differential cross sections is consistent with the asymptotic behavior. However, the current data are not accurate enough yet for a complete determination of the gluonic GFFs, and therefore we consider some prospects for better extractions in the future. Published by the American Physical Society 2024