1. Michael Dugger* Arizona State University
S = 0 pseudoscalar meson photoproduction from the proton:
γ p → π0 p, γ p → π+ n γ p → η p, γ p → η' p
Michael Dugger*
Arizona State University
*Work at ASU is supported by the U.S. National Science Foundation 1

2. Outline Motivations – Theoretical & Practical Experimental Facilities
New and Existing Data
Brief Look At Models
Conclusions/What’s Next
*Work at ASU is supported by the U.S. National Science Foundation 2

3. Motivations Theoretical Practical
*Work at ASU is supported by the U.S. National Science Foundation 3

4. Motivations (Theoretical)
Pions are the lightest mesons and are copiously produced in the strong interaction.
The η and η' mesons have isospin ½ and limit one step excited states of the proton to also be isospin ½. The η and η‘ act as isospin filters to the resonance spectrum. This might be useful for find “missing” resonances
The η and η' have strange content but no net strangeness: May be useful in determining differing strange quark content of the proton
The η' is the only isosinglet. This can be used to indirectly probe gluonic coupling to the proton
*Work at ASU is supported by the U.S. National Science Foundation 4

5. Motivations (Practical)
Photoproduction:
Electromagnetic interactions are well understood
Real photons are particularly simple: Only 2 polarization states.
Non-strangeness reactions:
The outgoing proton is easy to identify and has relatively little contamination
*Work at ASU is supported by the U.S. National Science Foundation 5

6. Experimental Facilities:
SAPHIR (Spectrometer Arrangement for Photon induced Reactions. ELSA)
CLAS (CEBAF Large Acceptance Spectrometer. JLab)
CB-ELSA (Crystal Barrel at ELSA)
GRAAL (at the ESRF in Grenoble)
*Work at ASU is supported by the U.S. National Science Foundation 6

7. SAPHIR at Bonn Electron Stretcher
Photon energies up to 3.0 GeV
Good for charged particles
Small acceptance
Drift chamber
Time of flight
Electrocalorimeter
Magnet
*Work at ASU is supported by the U.S. National Science Foundation 7

8. CLAS Photon energies up to 5.7 GeV Good for charged particles
Large acceptance
*Work at ASU is supported by the U.S. National Science Foundation 8

9. CB-ELSA TAPS Crystal Barrel Photon energies up to 3.2 GeV
Good for neutral particles
Large acceptance
TAPS
Crystal Barrel
*Work at ASU is supported by the U.S. National Science Foundation 9

10. GRAAL Photon energies up to 1.1 GeV Good for neutral particles
Large acceptance
Variable
collimator
Fixed
collimator
Plastic
scintillator
barrel
Cleaning
magnet
Target
Cylindrical
wire chambers
BGO
calorimeter
Vacuum system
Shielding wall
Scintillator and lead
sandwich
*Work at ASU is supported by the U.S. National Science Foundation 10

11. New γ p → π0 p Results New results from GRAAL: dσ/dΩ and Σ
New results from CLAS: dσ/dΩ
*Work at ASU is supported by the U.S. National Science Foundation 11

12. New GRAAL γ p →π0 p Results
SAID
BONN-PNPI
(Anisivich, Klempt)
dσ/dΩ from 555 to 1021 MeV
MAID
*Work at ASU is supported by the U.S. National Science Foundation 12

13. New GRAAL γ p →π0 p Results
SAID
BONN-PNPI
(Anisivich, Klempt)
dσ/dΩ from 1036 to 1496 MeV
MAID
*Work at ASU is supported by the U.S. National Science Foundation 13

14. New GRAAL γ p →π0 p Results
SAID
BONN-PNPI
(Anisivich, Klempt)
Σ from 1036 to 1496 MeV
MAID
*Work at ASU is supported by the U.S. National Science Foundation 14

15. Preliminary CLAS γ p →π0 p Results
dσ/dΩ from to GeV
SAID (Blue line)
Mainz (Blue points)
CB-ELSA (Red points)
CLAS (Black points)
*Work at ASU is supported by the U.S. National Science Foundation 15

16. Preliminary CLAS γ p →π0 p Results
dσ/dΩ from to GeV
SAID (Blue lines)
CLAS (Black points)
CB-ELSA (Red points)
*Work at ASU is supported by the U.S. National Science Foundation 16

17. γ p →π0 p Polarization Observables
Beam polarization
Target asymmetry
Recoil polarization
Double polarization
*Work at ASU is supported by the U.S. National Science Foundation 17

18. Status: γ p →π0 p Database
World data set becoming quite thorough
Cross Section
New beam asymmetry and dσ/dΩ measurements from GRAAL
New dσ/dΩ from CLAS
CLAS experiment (g8b) just finished with data taking for beam asymmetry
CLAS double polarization (target & beam) slated for 2007
*Work at ASU is supported by the U.S. National Science Foundation 18

19. New γ p → π+ n Results New results from CLAS: dσ/dΩ
*Work at ASU is supported by the U.S. National Science Foundation 19

20. Preliminary CLAS γ p →π+ n Results
Differential cross sections
Eγ from to GeV
SAID (Blue lines)
CLAS (Black points)
*Work at ASU is supported by the U.S. National Science Foundation 20

21. Preliminary CLAS γ p →π+ n Results
Differential Cross sections
Eγ from to GeV
SAID (Blue lines)
CLAS (Black points)
*Work at ASU is supported by the U.S. National Science Foundation 21

22. γ p →π+ n Polarization Observables
Beam polarization
Recoil polarization
Target asymmetry
Target asymmetry
Double polarization
*Work at ASU is supported by the U.S. National Science Foundation 22

23. γ p →π+ n
World database has good coverage for beam polarization, recoil polarization, and target asymmetry between 20° and 120° and up to Eγ = 2300 MeV
Cross Section
Double polarization data for G and H between 40° and 100° and up to Eγ = 1900 MeV
New CLAS data to cover dσ/dΩ up to Eγ = 2275 MeV
Cross section
*Work at ASU is supported by the U.S. National Science Foundation 23

24. γ p →η p More polarization measurements coming
Data becoming quite thorough
More polarization measurements coming
More dσ/dΩ measurements with absolute photon flux determination coming
dσ/dΩ
Beam polarization
Target polarization
*Work at ASU is supported by the U.S. National Science Foundation 24

25. New CLAS γ p →η' p Results SAPHIR (Blue points) CLAS (Black points)
SAPHIR used indirect method for photon flux
SAPHIR used a branch to determine η' and had only ~1% acceptance
CLAS η' results were obtained in similar manner as the CLAS π0 results
*Work at ASU is supported by the U.S. National Science Foundation 25

26. New CLAS γ p →η' p Results Solid line: Nakayama and Haberzettl
Dashed line: Model inspired by A. Sibertsev
Solid line → Nakayama, Haberzettl:
Relativistic meson-exchange model
s-channel j=1/2 and j=3/2 resonances
t-channel ρ, ω exchange
Nucleon exchange in s- and u-channels
Contact current to make gauge-invariant
Dashed line → Sibertsev/Dugger:
Relativistic meson-exchange model
s-channel j=1/2 resonances
t-channel ρ, ω exchange
Nucleon exchange in s- and u-channels
*Work at ASU is supported by the U.S. National Science Foundation 26

27. Some π and η Photoproduction Modelsγ
MAID:
Isobar resonances
Born terms
t-channel ρ, ω exchange
Newly Reggeized for higher energy data γ
Anisivich, Klempt:
Resonances are Breit-Wigner except for two channel K matrix for S11(1535) and S11(1650)
Reggeized t- and u-channel
Coupled channels (dσ/dΩ and Σ: π+ n, π0 p, η p, K Λ,K Σ)
Li, Saghi:
Chiral constituent quark model
SU(6) x O(3) symmetry broken by gluon exchange γ 27

28. difficulty in determining width for the S11(1535)?
Red: π+ n and π0 p
Blue: η p
LS: Li, Saghi
AK: Anisovich, Klempt
S11(1650) causing
difficulty in determining width for the S11(1535)?
Missing resonance D15(2070) found?
Exotic at 1800 MeV? 28

29. Conclusions/What’s Next
π0, π+, η, and η' photoproduction off the proton
World database greatly enhanced during past several years
Evidence that a missing D15(2070) resonance may have been found
Need for more polarization observables and coupled channel analyses to constrain the theoretical models
More data coming, including double polarization observables from CLAS in 2007
*Work at ASU is supported by the U.S. National Science Foundation 29

30. Acknowledgements NSF ASU Group Members J. Ball P. Collins
E. Pasyuk
B. Ritchie
CLAS Collaboration
Theoretical curves for η'
Kanzo Nakayama
Helmut Haberzettl
New data from GRAAL
Annalisa D’Angelo
Dominique Rebreyand
Carlo Schaerf
*Work at ASU is supported by the U.S. National Science Foundation 30