1. The Program at Jefferson Lab
Spectrum of Mesons
Quark model, lattice calculations
The GlueX experiment
Early physics
Asymmetries, SDMEs, Charm at threshold
Summary
Elton S. Smith, Jefferson Lab
APCTP Workshop on “The Nature of Hadron Mass and Quark-Gluon Confinement from JLab Experiments in the 12-GeV Era”
Pohang, Korea, July 1-4, 2018

2. q q QCD Spectroscopy S2 S1 L
Quark model is amazingly successful at describing the hadron spectrum.
Yet most of the hadron mass is not due to quarks q q
Search for glue using non-qq degrees of freedom S1 S2 L
S = S1 + S2
J = L + S
P = - (-1)L
C = (-1)L+S
JPC = 0– – – 1+ – 2++ …
Allowed combinations

3. q q QCD Spectroscopy S2 S1 L
Quark model is amazingly successful at describing the hadron spectrum.
Yet most of the hadron mass is not due to quarks q q
Search for glue using non-qq degrees of freedom S1 S2 L
S = S1 + S2
J = L + S
P = - (-1)L
C = (-1)L+S
JPC = 0– – – 1+ – 2++ …
Allowed combinations
JPC = 0– – 0+ – 1– – …
Not-allowed: exotic

4. Tetraquarks and Pentaquarks (Heavy Quarks)
BESIII PRL 110 (2013)
BELLE PRL 110 (2013)

5. Tetraquarks and Pentaquarks (Heavy Quarks)
BESIII PRL 110 (2013)
BELLE PRL 110 (2013)
LHCb PRL 115 (2015)

6. COMPASS: Exotic p1, h1, h’1?
Asymmetry in cosq is a result of interference between even and odd partial waves
Exotic 1-+ larger in h’p- than hp-
No resonance parameters are presented for the exotic L odd- waves
COMPASS PLB 740 (2015) 303

7. LQCD Meson spectrum for light quarks
Dudek PRD 83 (2011)
Mp~400 MeV
Dudek PRD 84 (2011)

8. LQCD Meson spectrum for light quarks
Dudek PRD 83 (2011)
Mp~400 MeV
Dudek PRD 84 (2011)
0-+, 1--, 2-+, 1-+
Supermultiplet
States with non-trivial gluonic fields

9. Models for hybrid mesons
Guo PRD 78 (2011)
Mass (MeV)
0−+
1−−
2−+
1−+
0+−
2+−
0−+
1−−
2−+
1−+
0+−
2+−
Constituent Gluon
JgPC=1+−
Lattice QCD
Compare to approach and spectrum in
Xu et al., arXiv:

10. Photoproduction g  ,, X Possible decay modes g e N
Very little photoproduction data in this energy range
Approximately the 70% of total cross section in the energy region Eg ~ 7-12 GeV has multiple neutrals and is completely unexplored
Polarized photons may help disentangle different production mechanisms N g e X
Possible decay modes
g  ,,

11. Jefferson Lab site

12. Photon beam and experimental area

13. Linearly Polarized Photon Beam
Measured Spectrum
(over PS acceptance)
Calculated Spectrum
(12 GeV beam) 4
nominal
tagging
interval
GlueX PRC95 (2017)

14. Pb-glass detector (Fcal) Superconducting 2 T solenoid
Hall D – GlueX detector
Pb-glass detector (Fcal)
Time-
of-flight
(tof)
Hermetic detection
of charged and
neutral particles in
solenoid magnet
Barrel
Calorimeter
(Bcal)
Target
(LH2)
Flux (peak) ~ 2x107 g/s
18,000 FADCs
4,000 pipeline TDCs
50 KHz L1 trigger
600 MB/s to tape
Tagger Spectrometer
(Upstream)
Tracking
Cathode strips
Drift chambers
Straw tubes
Superconducting 2 T solenoid

15. Viewer single-event Tracks, calorimeter showers reconstructed
Side view
Top view

16. Program to study meson spectrum
Collect Data
Determine Observables
Understand Production
Mechanisms
Determine Mass-independent Amplitudes
Identify known mesons
Search for exotics

17. Program to study meson spectrum
Collect Data
Determine Observables
Understand Production
Mechanisms
Determine Mass-independent Amplitudes
Identify known mesons
Theoretical Models
(e.g. JPAC….)
Search for exotics
Amplitude Analysis

18. Program to study meson spectrum
Collect Data
Current Efforts
Asymmetries
SDMEs
Cross sections
………
Opportunistic
Physics
Determine Observables
Understand Production
Mechanisms
Determine Mass-independent Amplitudes
Identify known mesons
Theoretical Models
(e.g. JPAC….)
Search for exotics
Amplitude Analysis

19. p0 and h azimuthal asymmetry
JPAC PRD 92 (2015) p0 h

20. p0 and h asymmetries GlueX PRC95 (2017) 042201
p0 and h azimuthal asymmetries measured for 0 < -t < 1.5 GeV2
Measurements are being compared to model calculations to understand particle exchange mechanisms
First asymmetry measurements for h at this energy

21. Pseudo-scaler asymmetries
Production is consistent with vector exchange dominance
Expect similar mechanism for exotics

22. gp -> p- D++ Asymmetry
See Yu, Session 2

23. Spin density matrix elements (SDMEs)
See McGinley, JLab User’s Meeting 2018
SDMEs measure the transfer polarization from the photon to the vector meson V
JPAC PRD 97 (2018)
F is the angle between the photon polarization and the production plane
Helicity Conservation:
Standard Decomposition

24. gp -> fp Consistent with s-channel helicity conservation
Production mechanism dominated by Pomeron exchange
A. Barnes, Ph.D. UConn, 2017

25. gp -> w p Expect contributions from pseudo- scalar exchange
Consistent with previous SLAC results
Provides insight into the exchange mechanisms
M. Staib, Ph.D. CMU, 2017

26. Opportunities in spectroscopy: 4g systems?
Previous photoproduction data with multiple neutrals is very limited
Systems decaying with L=odd to hp have exotic JPC.

27. Opportunities in spectroscopy: 4g systems
expanded scale ?
Previous photoproduction data with multiple neutrals is very limited
Systems decaying with L=odd to hp have exotic JPC.

28. Charm production near threshold
See Pentchev, JLab User’s Meeting 2018
Brodsky et.al. PLB 498 (2001) 23
GlueX Energy Range

29. Preliminary J/y results
Preliminary cross sections Eg~8-12 GeV

30. Preliminary J/y results
Preliminary cross sections Eg~8-12 GeV

31. Preliminary J/y results
Pc(4450)
No indication of LHCB pentaquark
Preliminary cross sections Eg~8-12 GeV

32. Summary
We have mounted the GlueX experiment in Hall D at Jefferson Lab devoted to the study of hybrid mesons. Their existence and properties will yield fundamental information regarding the force that confines quarks.
On the road toward finding exotic hybrids, we are
Measuring beam-spin asymmetries for several particles to understand production mechanisms
Measuring spin density matrices for vector mesons
Working on amplitude analysis of some simple systems such as hp and h’p (exotic JPC for L=odd)
Pursuing opportunistic physics such as J/y production at threshold
Upcoming experiments:
Primakoff h and rare h decays
High Intensity GlueX (with DIRC)
Charged Pion Polarizability
Also see Somov, Session 4

33. Decay modes for exotics