Discussion 11-18-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, November 18 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 parton distributions at 22 GeV (Patrick Barry)

Comments and Questions

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Minutes/Notes (D.S. Carman)

Local participants at JLab: 7; Remote participants: 32

Goal – To study the hadronic structure of mesons to complement ongoing and planned studies of baryons as a probe of QCD.

Presentation points:

• Meson structure studies are a necessary complement to studies of baryon structure. Such efforts offer an opportunity to study emergent phenomena of QCD such as understanding how mass is generated, how quarks and gluons arrange themselves within hadrons, and notions of quark-gluon confinement.
• Meson structure studies allow for another probe of confinement scales in bound quark/gluon systems.
• Key questions in meson structure:
  • How to test universality of meson PDFs probed via the Sullivan process and in Drell-Yan reactions?
  • What is the gluonic content of pions compared with protons?
  • What is the transverse momentum structure of pions and how does it relate to the scale of confinement?
• At JLab meson (mainly pion) structure is probed through the Sullivan process where the virtual photon scatters from a pion in the meson cloud (an effective pion target). To isolate this process requires several kinematic cuts to ensure a leading scattered nucleon in DIS kinematics and a large rapidity gap between the leading nucleon and the reaction remnant (from which a pion can also detected when analyzing SIDIS).
• Accessing the pion PDF relies on a factorization of the N→N’π process from the γ*π process from which we can gain access (in principle) to the pion PDF.
• At present the data available from Sullivan-type process studies does not kinematically overlap with the data from Drell-Yan studies available from FNAL and CERN. This is a bit of an issue in the studies of the universality of pion PDFs.
• The first global QCD analysis of pion PDFs was carried out in 2018 as part of the JAM Collaboration (P. Barry et al., PRL 121, 152001 (2018)). This work shows the pion PDFs and their associated uncertainties from Drell-Yan data alone and Drell-Yan+Sullivan process data.
• The Tagged DIS (or TDIS) program at JLab is focused on a program of measurements at 11 GeV. While these studies are a valuable first step, there are questions about the reliability of isolating the Sullivan process at this energy. Studies at 22 GeV are expected to be more appropriate to eliminate contributing backgrounds and to ensure sufficient statistics in the relevant kinematic range.
• Studies to access the gluonic content of the pion require isolating the longitudinal photon contribution via a Rosenbluth-type L-T separation using multiple beam energies. Studies are already promising to access g_π using the TDIS data from the 11 GeV program, but can be extended to 22 GeV.
• Pion SIDIS via the Sullivan process is an experimental approach to access the transverse momentum structure of the pion. Already it seems apparent that the TMDs for u-quarks in the pion are different than in the proton. Studies at 22 GeV are being considered to extend the available results.

Questions/comments from discussion:

• The main aspect of the discussion was on the necessary cuts to cleanly isolate the Sullivan process to access the pion TMDs.
  • Discussion of the N→N’π leg of the Sullivan diagram – off-shell nature of the π and cuts to select kinematics as close to on-shell as possible; contamination of the N→N’π leg from other (e.g. resonant) contributions
  • Complications of Fermi motion of the nucleon in the deuteron target
  • Considerations of the p→π+n and n→π-p process in Sullivan process diagrams of the proton and the deuteron
• Extensions of the program to kaon PDFs and TMDs from a corresponding Sullivan-type process are also being considered but studies from TDIS at 11 GeV and possibilities at 22 GeV need to be fleshed out.