Discussion 10-21-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, October 21 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

• Measurement of the QCD coupling α_S with JLab@22 GeV (Alexandre Deur)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 7; Remote participants: 36

Goal – To improve our knowledge of the strong coupling constant α_SS through a measurement program that will reduce the current uncertainties.

Presentation points:

• α_S is the most important quantity of QCD and is a key parameter of the Standard Model. However, it is by far the least known fundamental constant.
• There are a number of ongoing efforts to reduce the uncertainties of α_S, but there is no golden experiment to determine α_S precisely. The current strategy has been to combine independent measurements with larger uncertainties. Currently the best experimental determinations are at the Δα_S/α_S ~ 10^-2 level.
• There are good prospects of measuring α_S with JLab@22 GeV using an approach based on the Bjorken sum rule (measuring the integral over x of Γ₁^p-n vs. Q²). In this approach there is no need for an absolute cross section measurement. Only the determination of the Q² dependence is necessary with tight control over the point-to-point systematic uncertainties.
• The best measurement is one that spans the broadest range of Q² and x. The best prospect of extracting α_S is to combine data from JLab@22 GeV with the EIC. In this case it is estimated that Δα_S/α_S can be measured to the sub-percent level of 6.1x10^-3. Using the projections from the EIC measurement alone gives Δα_S/α_S ≳ 1.3%.
  • Such a combined JLab22 + EIC measurement can yield an uncertainty on α_S smaller than all of the available world data combined.
  • Measurements in different regimes of Q can be complementary. At low Q, measurements are sensitive to higher-order loop corrections. However, at higher Q, the measurements are essentially independent of these higher-order corrections and can be used to determine the value of α_S.

Questions/comments from discussion:

• Q: The biggest reduction in uncertainty on α_S seems to come from the combined JLab22+EIC data. What is the expected uncertainty from just the JLab22 measurement alone or from the JLab12+EIC measurement? Having this information available can help to better motivate the importance of the JLab22 program.
  • A: at the minimum Q2 of EIC, data from JLab12 measure only 45% of the Bjorken integral, and this quickly decreases at larger Q2. Hence, JLab12 would contribute only marginally to an α_S determination and thus cannot notably improve Δα_S/α_S. In contrast Jlab22 measures 90% of the integral, contributing dominantly.
• Q: Need a comparison of the Q² vs. x phase space for the different measurements to show overlap and differences.
  • A: This comment was made in connection with the question about the sophistication of the assessment that, with JLab22, one can reach Δα_S/α_S ~0.6%. The assessment is not as precise as that of EIC, which was done with extensive simulations and considerations. (The result of this study is published: Kutz et al. PRD 110 (2024) 7, 074004). While the estimate for JLab22 is not as sophisticated, there is also less need for such sophistication. The crucial questions are:
    • What are the actual (i.e., kinematics+detector efficiency) low-x limits and Q^{2</sup? coverage of JLab22?}
    • What are the statistics?
    • What are the experimental systematics?

The answers to (2) and (3) are straightforward: the statistics are always plenty for the Bjorken integral because data are gathered concurrently with other reactions, e.g., (semi-) exclusive ones for GPDs or transversity, which are more statistics-hungry. The systematics are already well-known because the same types of experiments were done at 6 and 12 GeV. Answering accurately question (1) is not crucial because wherever the low-x coverage will stop, EIC data will pick it up. We only need to have a reasonable answer (will JLab22 be missing 10% at low-x, or 40%?), which is straightforward. The minimum Q2 reached is irrelevant because it will clearly be below 1 GeV2 and for DIS, we consider only data at Q2 >1 GeV2. What is precisely the higher Q2 limit is also not crucial because (1) EIC data will be available and (2) the sensitivity to α_S is reduced at large Q². Thus, a detailed Monte-Carlo simulation is not required at this stage. On the other hand, it was needed for EIC, where all the answers of the above questions were not as straightforward.

• Q: Make sure tables of data show all available results not just the PDG results. It was mentioned that there is a new precision result from LHC/ATLAS.
  • A: Noted.