T-2, Nuclear and Particle Physics, Astrophysics and Cosmology

Studying Short-Range Nucleon-Nucleon Interactions with an EIC

Matthew Sievert

The innermost repulsive core of the nucleon-nucleon potential, essential for the stability of nuclei, remains an elusive property of first-principles QCD. High-energy elastic nucleon-nucleon scattering (at fixed center-of-mass angle) probes such a regime, but experiment reveals a surprising flavor dependence for such processes, leading to the well-established "quark counting rules." The implication that quark exchange dominates over gluon exchange in the hard process confounds the usual pQCD power counting, which suggests that gluon exchange (the "Landshoff mechanism") should be parametrically dominant over quark exchange. Still more intriguing, QCD permits two nucleons at very short distances to contain exotic configurations of quarks such as "hidden color" states, in which the 6 valence quarks are grouped into two "nucleons" which separately carry an octet color charge, but are color neutral overall. In some simple estimates these exotic states could contribute as much as 80% of the 6-quark state, yet experimentally hidden color states in nuclei have been excluded down to the few-percent level. In this talk, I will propose a new process accessible at a future electron-ion collider (EIC) which is sensitive to these very short range nucleon-nucleon interactions: the exclusive electroproduction of vector mesons off a deuteron target, in which the deuteron breaks up into a proton-neutron pair with large transverse momentum. This process is a generalization of exclusive meson production on the proton as measured at HERA, which can be described in QCD in terms of nonperturbative matrix elements known as "generalized parton distribution functions" (GPD's). For the case of a deuteron breaking up into a proton/neutron pair, the "transition GPD" depends on an additional internal momentum scale: the relative momentum of the final-state nucleons. I will argue that when this momentum scale is large, it becomes possible to extract information about the short-distance pQCD rescattering of the nucleons. Using a toy model first as a proof of principle, I will then estimate the rates for realistic nucleons to show that this process would be accessible at a future EIC. Such an analysis, which is only possible at a high-luminosity EIC, would be complementary to studies of elastic nucleon-nucleon scattering and may shed new light on the QCD content of two nucleons at very short distances.


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