Discussion 09-09-24

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Meeting Location

The 22 GeV Open Discussions will be held here:

• Date/Time: Monday, September 9 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

• Opportunities for XYZ spectroscopy at JLab22 (Alessandro Pilloni)

Comments and Questions

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Minutes/Notes (D.S. Carman and J. Stevens)

Local participants at JLab: 12; Remote participants: 42

• This presentation laid out the motivation from a theoretical viewpoint for studying the spectroscopy of the so-called X, Y, and Z mesons that have changed the c-cbar spectroscopy landscape in recent years. It also provided some predictions based on an available VMD model to set the cross section scale for these states vs. center-of-mass energy. Note that the next discussion in the 22-GeV series on Sep. 23 will be a continuation of this topic by Sean Dobbs with the focus on experimental considerations of such a program at JLab22.

• Presentation points:
  • Evidence for a host of candidate states have been claimed in various recent experiments that has (potentially) changed the c-cbar landscape. These states are candidates for tetraquark and other exotic configurations.
  • The candidate states have mainly been seen decaying into charmonium+light meson final states (e.g. J/Ψ + π).
  • Theoretical interpretations with different production mechanisms do not, as yet, provide strong consistency.
  • The current situation opens the door for a new spectroscopy program at JLab22 based on (quasi-real) photoproduction experiments.
    • No X, Y, Z state claim has yet been free of controversy (it’s all new) – see e.g. the low-statistics measurements at COMPASS.
    • They decay final states are free from rescattering mechanisms that could mimic resonances in multi-body decays.
    • The framework for describing the production mechanism is (relatively) clean from a theory standpoint.
    • Radiative decays offer another way of discerning the nature of such states.
    • If such states can be seen in photoproduction, it is natural to then consider measurements of transition form factors as a function of Q² to probe the structure of these, possibly exotic, states.
  • Sophisticated theoretical models are not yet available, but a vector meson dominance (VMD) approach has been used to date to make predictions for the cross sections for the production of the X, Y, Z states. The VMD models have limitations and cross checks to available data for non-exotic cc ̅ processes will be important moving forward. However, the predicted cross sections are even larger than those that we have already measured for J/Ψ in Halls B and D.
  • A possible experimental program at JLab22 could focus on X and Z production, which is more favorable in JLab22 kinematics. The measurement of Y states is likely better done at the higher energies at EIC.

• Questions from discussion:
  • Are there calculations for production cross sections from a neutron (deuteron) target?
    • Yes and they can be made available.
  • To further the discussion of carrying out such a program at JLab22, the el-spectro (https://github.com/dglazier/elSpectro) event generator is being developed by Derek Glazier and has been used in Hall D for preliminary Monte Carlo studies with GlueX and EIC.
  • It is important to be able to predict the cross sections for both the resonant processes that lead to X, Y, Z production relative to the non-resonant processes in the same final state to see how well the signal can be separated from the background.
    • Adam Szczepaniak stated that there is a phenomenological model based on a double-Regge approach that may provide a reasonable estimate and there are plans to work on this.
  • How critical is an independent production mechanism compared to e+e- production?
  • Volker Burkert mentioned a study that he worked on with Derek Glazier for the Messina 22 GeV meeting that considered processes where the scattered electron was detected at very small angles (a forward-tagging facility) to be able to isolate these type of processes and access their kinematics.
  • Stay tuned for Sean Dobbs’ upcoming presentation to gain more insight into some of the ongoing considerations for designing an experimental X, Y, Z program at JLab22.