Discussion 12-02-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, December 2 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

• Impact of High-x JLab Data on Extraction of Transversity PDFs and Tensor Charges (Christopher Cocuzza)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 4; Remote participants: 31

Goal – To gain further insight into hadron structure from information on transversity PDFs and tensor charges from high-x JLab data at 12 GeV and 22 GeV.

Presentation points:

• Approaches to access transversity PDFs:
  • Global analysis in framework of collinear factorization relying on dihadron fragmentation functions
  • Global analysis including TMDs + collinear twist-3 observables
  • Lattice QCD
• Global QCD analyses use input streams from data (e.g. transverse single-spin asymmetries) and encoded dynamics. Using data sampling techniques, parameters from best fit can be extracted. The variation in the parameters from the approach over many samples provides the parameter uncertainties.
  • This approach is a powerful technique to understand how constraints from new data affect the TMDs.
• The tensor charge is another transversity distribution that is attractive to probe low-energy physics not accounted for in the Standard Model.
  • The disagreement of the tensor charge as determined from the different approaches to access transversity has given rise to a “transverse spin puzzle”.
• The extraction of the tensor charge from the available experimental data and LQCD inherently relies on extrapolations to both low-x and high-x where data do not exist.
  • The available data from BaBar, Belle, BES-III, COMPASS, HERMES, and STAR exists in the range from x=0.05 to x=0.6.
  • Data from JLab at 12 GeV and 22 GeV can provide essential data at high-x that provides critical data to allow controlled extrapolations to reduce uncertainties on the extracted TMDs. This is critical to resolving the “transverse spin puzzle”.
• Recent work has focused on how new transversity observables from JLab at 12 GeV (CLAS12 and SOLID) and from JLab at 22 GeV (CLAS22) can provide for improved constraints on transversity distributions (e.g. δu, δd, xh₁^u_v, xh₁^d_v)
• Data from SOLID on the neutron would provide the strongest experimental constraints on the down quark.
• Data from Hall B with CLAS12 and CLAS22 could (potentially) solve the transverse spin puzzle for δu.

Questions/comments from discussion:

• The theoretical framework and its predictions are based on a number of assumptions and simplifications. What is the impact of these assumptions and simplifications on the stability of the predictions?
• What is the effect of the parameterization bias in the global fits?
• Projections for SOLID at 22 GeV have not been included. What impact would these data have on the transversity distributions in reducing the uncertainties?
• It should be expected that at higher Q², the contributions from the “soft” physics contributions would be reduced and hence better under control. This aspect was not considered in detail and should be expanded upon to improve the case for the 22 GeV upgrade.
• The case for why measuring the tensor charge (and other transversity observables) to higher precision should be further developed to make the case for what uncertainties are required/necessary to have a substantive impact on hadron structure approaches and physics beyond the Standard Model.
• The notion of resolving the transverse spin puzzle (i.e. achieving consistency between global fits of data and LQCD) should be developed further in making the case for the 22 GeV upgrade.