Discussion 09-09-24

Speakers and participants, please review the guidance provided on the main page. This agenda page is editable by anyone that has a Jefferson Lab computing account. Feel free to log in and post comments, questions, or answers to questions in the section below.

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

• (Add content here)

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.
    • The 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 c-cbar 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.