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

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

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

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

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

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

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

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

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

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

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

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

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

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

Preliminary CLAS γ p →π0 p Results dσ/dΩ from 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

Preliminary CLAS γ p →π0 p Results dσ/dΩ from 1.475 to 2.125 GeV SAID (Blue lines) CLAS (Black points) CB-ELSA (Red points) *Work at ASU is supported by the U.S. National Science Foundation 16

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

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

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

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

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

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

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

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

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

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

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

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

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

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