Discussion 08-26-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, August 26 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

• Studies of partonic distributions and interactions using SIDIS with JLab22 (Harut Avakian)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 17; Remote participants: 43

• Goal – Study the non-perturbative QCD dynamics in 3D space of semi-inclusive deep-inelastic scattering (SIDIS) reactions. The measurements are, by nature, multi-dimensional in the relevant kinematic variables (e.g. x, Q², z, p_T, φ) and this enables a probe of the transverse structure of hadrons encoded in the 3D partonic distribution functions (TMDs and GPDs) with controlled systematics in the kinematics dominated by the valence quarks (i.e. x > 0. 1).

• Presentation points:
  • The current status of the SIDIS program at JLab was reviewed stressing that the JLab program is complementary to that planned program at the EIC, mainly in terms of the kinematic coverage of the programs, providing a critical input in the valence region.
  • The main focus of the program are measurements of cross sections, multiplicities, and spin asymmetries of pions and kaons in a multi-dimensional grid. The program includes measurements with polarized beam, longitudinally polarized proton/deuteron targets, and transversely polarized proton targets.
  • Theory computations of multi-dimensional SIDIS cross sections are considered very difficult and so far limited to small k_T/Q. The assumption that at higher energies theory computations will be more reliable may not be fully justified.
  • Current theoretical models of SIDIS rely on the applicability of factorization to separate the hard and soft/non-perturbative parts of the process. So far factorization has been demonstrated only for transverse virtual photons. Factorization allows for the description of the SIDIS process using distribution functions (TMDs, PDFs) and fragmentation functions (TMD FF).
  • The main issues in comparison of theory computations with experimental data are associated with the contribution of 1) longitudinal virtual photons and 2) diffractive vector meson contributions to the observables.
  • Where is the evidence that the contributions responsible for the differences in theory vs. data are a “morass” and possibly due to the breakdown of factorization only relevant in "few GeV" kinematics?
    • Longitudinal photons – At low energies it is expected that factorization does not hold (or does not hold so well). But, it seems that F_UU,L (i.e. σ_L) is dominant at high energies. In JLab kinematics, σ_L and σ_T can be “separated” or “controlled” kinematically. So JLab studies of the impact of longitudinal photons are critical for interpretation of polarized SIDIS, including EIC data.
    • Diffractive vector mesons - These contributions violate the factorization picture of SIDIS based on the dominance of the leading-twist contribution, with the diffractive ρ being the most important, and can confuse the interpretation. Ignoring that can significantly bias quark contributions. However, JLab provides the possibility of understanding how the ρ contributes and it can, in fact, be cut out in the analysis quite easily. This is crucial for interpretation in terms of TMDs and SIDIS data in general, and for EIC in particular.
    • Another consideration is the interpretation of processes in kinematics associated with the forward (current) and backward (target) fragmentation regions. But contributions to SIDIS in these different kinematic regions nominally associated with x_F > 0 and x_F < 0 can be separated more precisely using beam SSAs.
  • JLab22 will serve as a bridge between the JLab12 and EIC programs, providing measurements of relevant observables that are critical for interpretation of polarized SIDIS experiments at large x.

• Questions/answers from discussion:
  • It is important to frame the discussion in terms of the complementarity of the JLab and EIC programs. Pitting the physics case in terms of “us” vs. “them” can be antagonistic. It is much better to focus on the different kinematic ranges and what we can learn about non-perturbative strong interaction dynamics in the separate programs and how they can help to serve each other. It seems that a natural point of complementarity is that JLab at lower energy is more sensitive to quark TMDs and EIC at higher energy is more sensitive to gluon TMDs. A key aspect of the JLab SIDIS program at 22 GeV is the high-luminosity and high resolutions of measurements that cannot be matched at EIC.
    • Answer: Apparently some confusion has been generated. Making the case that JLab measurements are relevant for interpretation of the EIC data in the valence region shouldn’t cause any controversy.
  • Studying SIDIS at JLab as a function of beam energy, 6, 8, 12, 15, 18, 22 GeV can be used to test scaling violations vs. distance scale. This can tell us more clearly where the partonic degrees of freedom are relevant and can tell us about TMD evolution. This point is not well developed in the 22 GeV White Paper and could be worth revisiting.
    • Answer: Measurements at different beam energies will certainly be part of the program, also needed for longitudinal photon contributions, for scaling violations indeed will be great to quantify.
  • How is the interpretation of TMDs affected by other intermediate channels besides the ρ, e.g. N* and Delta;, …?
    • Answer: Other channels are part of the JLab physics program. One of the main focuses of the upgrade program is the search of the complementarity that make contributions from diffractive ρs, expected to be significant at any polarized SIDIS measurements, a higher priority.