Discussion 07-29-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, July 29 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

• Pion and Kaon form factors with JLab at 22 GeV (Garth Huber)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 13; Remote participants: 44

• Goal – Measure π⁺ and K⁺ charge form factors F_π and F_K. Data to increasingly higher Q² beyond those from the old 6 GeV and current 12 GeV JLab programs, to a potential 22 GeV program, allow for improved separation between the soft and hard contributions to provide (cleaner) access to information governing hadron mass generation.

• Presentation points:
  • The π⁺ form factor is the best hope of observing experimentally the QCD transition from soft to hard physics as it should occur at lower Q² than for the proton (as the virtual photon momentum sharing is only between two quarks instead of three).
  • Study of the K⁺ form factor allows for probe of meson structure when s quark is substituted for a d quark.
  • Experiments to measure e’π⁺ and e’K⁺ final states rely on scattering from the meson cloud surrounding the inner quark core.
  • At small momentum transfer (-t), the pion/kaon pole term is expected to dominate the longitudinal cross section σ_L, which is directly proportional to the pion/kaon charge form factor F_π(Q²,t)/F_K(Q²,t).
  • There are drawbacks to this approach to accessing the form factors that include isolating σ_L and the assumption that the pion pole term dominates σ_L that requires theory input.
  • The initial program is planned for Hall C detecting the scattered electron in the HMS and the recoiling meson in the SHMS.
  • The experimental program is being considered in 2 phases. Phase 1 assumes the current HMS and SHMS but is limited to 18 GeV electrons (and Q² < 12 GeV²) due to the momentum limitations of the spectrometers. Phase 2 assumes an upgrade of the HMS to detect higher momentum mesons to allow for 22 GeV electrons (and allow for Q² up to 15 GeV²). It is essential to have good momentum resolution and PID in the spectrometers to reduce systematic uncertainties.
  • The program relies absolute cross section measurements at different beam energies to perform Rosenbluth σ_L / σ_T separations. The key to reduce the experimental uncertainties is to take data at a given (Q², W, t) over the broadest possible beam energy range to allow for the largest possible spread in the virtual photon polarization parameter ε.
  • Planned measurements of F_π and F_K at the EIC have different systematics as they cannot separate σ_L and σ_T as all high energy measurements essentially take place in kinematics with ε=1.

• Questions from discussion:
  • The assumption that σ_L is dominated by the pion pole term needs more consideration as the ratio of t-channel contributions to contact-term contributions is/could be very kinematics dependent. It is essential that all diagrams are included in the reaction model to reduce theoretical uncertainties on the extraction of the form factors.
  • Can F_π and F_K be reliably extracted from the unseparated cross sections as will be done in the EIC program to test/limit theoretical uncertainties?
  • The Phase 1 program at 18 GeV overlaps in Q² with the 12 GeV program. This should be exploited in the planning to study systematic uncertainties.
  • Could the experiment be designed in such a manner to get to low enough momentum transfer -t to overlap the pion pole?
  • New LQCD calculations for π and K electroproduction are becoming available that could be relevant to reduce theoretical uncertainties on the extraction of F_π and F_K and should be considered.
  • What is meant by the transition from soft to hard QCD exactly? How can this be interpreted for the form factors vs. Q²?
  • The overlap of the planned 12 GeV and 22 GeV JLab π and K form factor measurements with EIC should be careful considered in terms of the theoretical uncertainties in the form factor extraction as the t-coverage in the different measurements is quite different. Need to elaborate on and extend comparisons/contrasts between the JLab and EIC programs.
  • What is the impact of radiative corrections on the systematic uncertainties in the measured cross sections vs. kinematics?
  • Explore experiment parameters in terms of beam energy choices and spectrometer settings to maximize range of ε points in the Rosenbluth separation. This is a key point to reduce statistical uncertainties on the σ_L vs. σ_T separation.