Discussion 08-12-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, August 12 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

• Charting emergence of N* structure and hadron mass in experiments of 22 GeV era (Victor Mokeev) - (Note: Slides were updated after the meeting to reflect the comments from the discussion)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 10; Remote participants: 37

• Goal – Measure exclusive electroproduction for various final states (πN, ππN, KY) to access nucleon resonance electrocouplings vs. Q² to learn about N* structure and emergent mechanisms that give rise to hadron mass within QCD.

• Presentation points:
  • Nucleons and excited nucleon states are the most fundamental 3-body systems in nature. If we do not understand how QCD builds each state in the N* spectrum then our understanding of the strong interaction is incomplete.
  • The experimental program is to measure differential cross sections and polarization observables for various exclusive reaction channels. Using a suitably controlled and realistic reaction model (as has been done for data from CLAS at 6 GeV), the experimental data can be fit to extract the electroexcitation amplitudes (i.e. the nucleon resonance electrocouplings) as a function of Q².
  • The Higgs mechanism accounts for only a few percent of the visible mass in the Universe. The dynamics encoded in the gluon self-interaction give rise to a running gluon mass (through a Schwinger mechanism) and, hence, a running quark mass, as a function of parton momentum. This so-called dressed quark mass function dictates N* structure and how this structure evolves vs. Q².
  • QCD-connected models that capture the dynamics of the full QCD equations of motion can (in principle) compute these electrocouplings to allow for understanding N* structure and how hadron mass arises. In other words, the electrocouplings are the bridge between experiment and theory.
  • Studies of N* structure and how the dominant part of their mass arises are complementary to similar studies in the meson sector. Both avenues of approach are essential to address the fundamental questions of the visible mass in the Universe.
  • Studies of the emergence of hadron mass through the N* electrocouplings can probe the range of quark momenta responsible for the dominant part of hadron mass in experiments at 22 GeV. These data will probe the regime from strong QCD in the infrared at low quark momenta to the (possible) onset of the perturbative QCD regime at high quark momenta.
  • A 22 GeV CEBAF facility that performs measurements of exclusive electroproduction with a large acceptance detector at luminosities of 2-5x10³⁵ cm^-2s^-1 will offer the only opportunity to explore how the dominant part of hadron mass and structure emerge through the strong interaction. Such studies cannot be done at any other existing or planned facility.

• Questions from discussion:
  • The Continuum Schwinger approach is incomplete at low Q² as it only accounts for the dressed quarks. It has been shown from the CLAS N* program that at low Q² the meson-baryon cloud that surrounds the quark core has a dramatic effect on N* structure. The impact of this meson-baryon cloud in terms of its importance vs. Q² is different for N*s of different structure.
  • Can studies of N* electrocouplings and their interpretation within QCD-connected models provide clear insights into the transition from the non-perturbative to perturbative QCD regimes?
  • Other theoretical approaches could in principle predict N* electrocouplings (including LQCD). What is possible and how can such development be advanced?
  • If 22 GeV beam energy is not sufficient to reach the “pQCD” regime, how does this impact the relevance of this N* program?
  • Are predictions of the N* electrocouplings in a pQCD framework possible?
  • The parton momentum in the M(k) dressed quark mass function needs some clarification as to its specific meaning. Does the crude approximation of k ~ sqrt(Q²)/3 adequately account for the true parton momentum range probed vs. beam energy?
  • The plot of running quark/gluon mass vs. k is a bit misleading as shown as the boundaries between sQCD and pQCD are not sharp but fuzzy and possibly not well defined.
  • There is a sense that pQCD should be much more amenable to interpretation than the non-perturbative regime. So given that the N* studies are limited (mainly) to the non-perturbative domain, how can clear/firm (i.e. model independent) statements regarding QCD and hadron structure be made?
  • It is dangerous to hold too close to any one model to make the physics case for an entire program of experimental study. How can the case for the importance of these studies of N* structure and EHM be strengthened through development of other approaches?