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Michael Dugger* Arizona State University

S = 0 pseudoscalar meson photoproduction from the proton:. γ p → π 0 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. Outline. Motivations – Theoretical & Practical

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Michael Dugger* Arizona State University

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  1. S = 0 pseudoscalar meson photoproduction from the proton: γ p → π0 p, γ p →π+ nγ 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 Electrocalorimeter Time of flight 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 • 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/dΩfrom 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/dΩfrom 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) • Σfrom 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/dΩfrom 0.675 to 1.425 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/dΩfrom 1.475 to 2.125 GeV • SAID (Blue lines) • CLAS (Blackpoints) • 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 0.625 to 1.625 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 1.675 to 2.275 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 • 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 Models γ • 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. 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

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