JLab and J-PARC measurements for J / ψ production at threshold

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Abstract
New threshold measurements for γ p → p J / ψ → p ( μ + μ − ) by 007 and γ * p → p J / ψ → p ( e + e − ) by CLAS12 allow us to extend the previous phenomenological determination of the J / ψ -proton scattering length, α J / ψ p , using GlueX threshold data for γ p → p J / ψ → p ( e + e − ) . The agreement between all three J / ψ datasets shows no indication of systematic differences between methodologies. Furthermore, perturbative QCD predictions support the phenomenological determination of heavy vector meson-nucleon scattering lengths. The role of the nucleon form factor, F N , is discussed, and a possible correction to the phenomenological scattering length for heavy vector meson photoproduction is calculated using the pole form of F N . Upcoming J-PARC threshold measurements of the reaction π − p → n J / ψ → n ( l + l − ) will help to evaluate the possible role of heavy pentaquark, P c , states in low-energy J / ψ production and the effects caused by the nucleon form factors.
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Funding Information
  • U.S. Department of Energy
  • Office of Science
  • Nuclear Physics (DE–SC0016583)
  • National Research Foundation of Korea
  • Ministry of Science and ICT, South Korea (RS–2024–00436392)

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