Discussion 10-07-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, October 7 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

• Super-fast quarks as a probe of the origin of the EMC effect (John R. Arrington)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 9; Remote participants: 46

• This presentation focused on a 22 GeV program to explore the origin of the so-called “EMC effect” through electron scattering measurements of nuclear Parton Distribution Functions (PDFs) in the x > 1 regime from a variety of nuclear targets relative to deuterium.

Presentation points:

• The EMC effect is the observation of a suppression of the ratio of various nuclear cross sections relative to deuterium as a function of increasing fractional parton momentum x in the range from ~0.35 to 0.7. The ratio of σ_A/σ_D in this regime is linear and the slope is essentially independent of mass number A. This observation amounts to a strong suppression of nuclear PDFs in the valence regime.
• The origin of the suppression is not fully understood (even after many years of study and consideration). Conventional binding/smearing can account for some of the suppression, but not enough to explain the data. Different models with very different dynamical effects (some with quite “exotic” explanations) predict this suppression vs. x, but they cannot all be correct. So other observables are needed to provide insight into the dynamical origin of the EMC effect.
• This proposed 22 GeV program represents an extension of an ongoing JLab program from the 6-GeV and 12-GeV eras. A number of insights have been gleaned from the JLab DIS program to date:
  • Measurements in light nuclei led to examining more detailed nuclear structure.
  • Demonstrated non-trivial correlations between EMC effects and short-range correlations (SRCs) that are often explained in terms of local-density or high-virtuality effects.
  • New studies of the A and N/Z dependence, flavor dependence, and spin dependence of the suppression can provide sensitive tests of various models (and hopefully rule some of them out).
  • “Tagged” DIS measurements are more recent and provide new information, albeit with limitations and model dependence of their interpretation.
• Studies of the EMC effect at x>1, i.e. with super-fast quark (SFQ) distributions, provide entirely new tests that can allow for a clean(er) interpretation if the beam energy is sufficiently high.
  • Note in a simple convolutional model super-fast quarks are associated with high-x quarks in high momentum nucleons.
• One potential explanation for the EMC effect was due to the presence of non-hadronic components within the nucleus, i.e. six-quark bags. This can be tested in the SFQ domain and where six-quark bag models predict a dramatic enhancement in the structure function. The six-quark bag is just a simple model used to look at the impact quantitatively, but any model allowing for more direct momentum exchange between high-x quarks in the overlapping nucleons will give a similar prediction.
• Because the SFQ distributions are generated by high-momentum nucleons, one also expects an enhancement of nuclear effects from models including off-shell effects, which leads to a suppression of the SFQ distribution.
• At 22 GeV DIS kinematics can reach Q² from 20-40 GeV2 for x up to 1.4. MC simulations using the Hall C HMS/SHMS configuration have been completed to provide the count rate estimates included in the JLab 22 GeV White Paper.
• Theory input needed:
  • Calculations of SFQ distributions for the deuteron based on “conventional” dynamical effects.
  • Evaluation of models of the EMC effect in a consistent fashion.
  • Examine A-dependence and Q² dependence of suppression.
  • Define the scaling regime.

Questions/comments from discussion:

• What is the accuracy of theory predictions of scaling vs. Q². How high of a beam energy is considered “high enough”?
• Resonant contributions likely do not decrease relative to non-resonant contributions for increasing Q². It is important not to make assumptions on the scaling (or deviations from scaling) in the different kinematic domains.
• Measurements of SFQs at 6 GeV, 11 GeV, and 22 GeV all cover essentially the same x region up to ~1.4. However, the key aspect of the 22 GeV data is the Q² coverage. At higher Q², the ratio of DIS/QE becomes more favorable, and it is this ratio that allows for the cleaner interpretation of the dynamics and the scaling data. This should be made clearer in the arguments.
• What can we learn if six-quark bag contributions to the nuclear wavefunction are shown to be important (or non-negligible) or non-existent?