Discussion 07-01-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, July 1 at 12:30 PM Jefferson Lab Local Time
• Physical Location: CEBAF Center F224/5
• Virtual Location: Zoom Meeting Number 161 111 8017 (The password is the two-digit number that appears before "GeV" in the first sentence of this section.

Agenda

• Organizational Updates (Matt)
• Threshold J/ψ production and proton gravitational form factors (Lubomir Pentchev)

Comments and Questions

• Measuring the differential cross section of charmonium production from threshold to O(~100GeV) establishes if there is a common production mechanism across the entire energy range. If this the case one can use the high energy phenomenology and QCD arguments to evaluate the gluon form factors of the nucleon. This would be a discovery of a new QCD phenomenon, since what is known so far from e.g. from light vector mesons is that threshold and high energy production are driven by very different mechanisms (e.g. production of N^*’s vs diffraction). (From Adam S.)
• Is there a concise explanation why the two-gluon exchange process probes only the gluon structure of the proton and not the quarks? One would naively think that gluons couple to quarks and gluons. Is there a model-dependent assumption here?

Minutes/Notes (D.S. Carman)

Local participants at JLab: 10 Remote participants: 50

• Goal – Use threshold charmonium production to access the gluonic structure of the proton

• Presentation points:
  • Gluonic contribution to the mechanical properties of the proton are expected to be equally important to that from the u+d+s quarks.
  • Quark masses and kinematic energy of quarks + gluons not enough to explain the proton mass – the gluon condensate (or anomalous contribution) to the proton mass is significant.
  • Extraction of gluonic form factors from JLab J/Ψ data (GlueX + Hall C) cannot distinguish between various model approaches, e.g. GPD-based calculations and holographic QCD approaches. Statistics in the range of Eγ from 9.3-10.8 GeV are limited.
  • Study of χ_c, χ', and J/Ψ states provide complementary information to understand the reaction mechanism near threshold.

• Questions from discussion:
  • Need to more articulately define the goal of the program and the assumptions made that are necessary to interpret the data (model independent parts vs. model dependent parts).
  • What is the applicability of the main J/Ψ production diagram? How model-dependent is the factorization assumption? How can assumptions be tested experimentally?
  • The different model approaches need to be carefully presented to clarify what physics they contain and what limiting assumptions are made.
  • Different notations used (mass terms, mechanical property terms) can be confusing to non-experts.
  • Comparisons of different programs/facilities need to be laid out in detail: GlueX (at 17 GeV, 22 GeV), GlueX phase-III upgrade, SOLID in Hall A, CLAS12, CLAS22, EIC. These comparisons should be quantitative and define the unique aspects of each program (energy range, polarization, flux, Q² range, acceptance, etc). Pros/cons (or strengths/weaknesses) of the different programs/facilities should be compared and contrasted.
  • What impact do open charm channels have on interpreting the J/Ψ+p final state? Can contributions from channels like D^(*)Λ_c be disentangled from t-channel J/Ψ production? What about non-resonant production mechanisms? How do these contributing channels affect comparisons with theory?