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Michael Dugger*

Arizona State University

*Work at ASU is supported by the U.S. National Science Foundation

S = 0 pseudoscalar meson photoproduction from the proton:

γ p → π0 p, γ p → π+ nγ p → η p, γ p → η' p

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2*Work at ASU is supported by the U.S. National Science Foundation

Outline

• Motivations – Theoretical & Practical

• Experimental Facilities

• New and Existing Data

• Brief Look At Models

• Conclusions/What’s Next

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

Motivations

• Theoretical

• Practical

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4*Work at ASU is supported by the U.S. National Science Foundation

Motivations (Theoretical)

• 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' is the only isosinglet. This can be used to indirectly probe gluonic coupling to the proton

• The η and η' have strange content but no net strangeness: May be useful in determining differing strange quark content of the proton

• Pions are the lightest mesons and are copiously produced in the strong interaction.

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5*Work at ASU is supported by the U.S. National Science Foundation

Motivations (Practical)

• Electromagnetic interactions are well understood

• Real photons are particularly simple: Only 2 polarization states.

Photoproduction:

Non-strangeness reactions:

• The outgoing proton is easy to identify and has relatively little contamination

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6*Work at ASU is supported by the U.S. National Science Foundation

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)

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7*Work at ASU is supported by the U.S. National Science Foundation

SAPHIR at Bonn Electron Stretcher

Drift chamber

Time of flight Magnet Electrocalorimeter

• Photon energies up to 3.0 GeV• Good for charged particles• Small acceptance

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8*Work at ASU is supported by the U.S. National Science Foundation

CLAS

• Photon energies up to 5.7 GeV• Good for charged particles• Large acceptance

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9*Work at ASU is supported by the U.S. National Science Foundation

CB-ELSA

• Photon energies up to 3.2 GeV• Good for neutral particles• Large acceptance

Crystal BarrelTAPS

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10*Work at ASU is supported by the U.S. National Science Foundation

GRAAL

• Photon energies up to 1.1 GeV• Good for neutral particles• Large acceptance

Plastic scintillatorbarrel

Cylindrical wire chambers

BGO calorimeter

Target

Shielding wall

Vacuum system

Cleaning magnet

Fixedcollimator

Variablecollimator

Scintillator and leadsandwich

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11*Work at ASU is supported by the U.S. National Science Foundation

New γ p → π0 p Results

• New results from GRAAL: dσ/dΩ and Σ

• New results from CLAS: dσ/dΩ

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12*Work at ASU is supported by the U.S. National Science Foundation

New GRAAL γ p →π0 p Results• dσ/dΩ from 555 to 1021 MeV

• SAID• MAID

• BONN-PNPI(Anisivich, Klempt)

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13*Work at ASU is supported by the U.S. National Science Foundation

New GRAAL γ p →π0 p Results• dσ/dΩ from 1036 to 1496 MeV • MAID

• SAID • BONN-PNPI(Anisivich, Klempt)

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14*Work at ASU is supported by the U.S. National Science Foundation

New GRAAL γ p →π0 p Results• Σ from 1036 to 1496 MeV

• SAID• MAID

• BONN-PNPI(Anisivich, Klempt)

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15*Work at ASU is supported by the U.S. National Science Foundation

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)

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16*Work at ASU is supported by the U.S. National Science Foundation

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)

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17*Work at ASU is supported by the U.S. National Science Foundation

γ p →π0 p Polarization Observables

Recoil polarization

Beam polarization Target asymmetry

Double polarization

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18*Work at ASU is supported by the U.S. National Science Foundation

Status: γ p →π0 p Database• World data set becoming quite thorough

• 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

Cross Section

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19*Work at ASU is supported by the U.S. National Science Foundation

New γ p → π+ n Results

• New results from CLAS: dσ/dΩ

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20*Work at ASU is supported by the U.S. National Science Foundation

Preliminary CLAS γ p →π+ n Results • Differential cross sections• Eγ from 0.625 to 1.625 GeV

• SAID (Blue lines)

• CLAS (Black points)

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21*Work at ASU is supported by the U.S. National Science Foundation

Preliminary CLAS γ p →π+ n Results • Differential Cross sections• Eγ from 1.675 to 2.275 GeV

• SAID (Blue lines)• CLAS (Black points)

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22*Work at ASU is supported by the U.S. National Science Foundation

γ p →π+ n Polarization ObservablesBeam polarization

Double polarizationTarget asymmetryTarget asymmetry

Recoil polarization

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*Work at ASU is supported by the U.S. National Science Foundation

γ p →π+ n

Cross section

• World database has good coverage for beam polarization, recoil polarization, and target asymmetry between 20° and 120° and up to Eγ = 2300 MeV

• New CLAS data to cover dσ/dΩ up to Eγ = 2275 MeV

Cross Section• Double polarization data for G and H between 40° and 100° and up to Eγ = 1900 MeV

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24*Work at ASU is supported by the U.S. National Science Foundation

γ p →η p

dσ/dΩ Target polarizationBeam polarization

• Data becoming quite thorough

• More polarization measurements coming

• More dσ/dΩ measurements with absolute photon flux determination coming

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25*Work at ASU is supported by the U.S. National Science Foundation

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

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26*Work at ASU is supported by the U.S. National Science Foundation

New CLAS γ p →η' p Results • Solid line: Nakayama and Haberzettl• Dashed line: Model inspired by A. Sibertsev

• Solid line → Nakayama, Haberzettl:1. Relativistic meson-exchange model2. s-channel j=1/2 and j=3/2 resonances3. t-channel ρ, ω exchange4. Nucleon exchange in s- and u-channels5. Contact current to make gauge-invariant

• Dashed line → Sibertsev/Dugger:1. Relativistic meson-exchange model2. s-channel j=1/2 resonances3. t-channel ρ, ω exchange4. Nucleon exchange in s- and u-channels

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γ

γ

γ

• Anisivich, Klempt:1. Resonances are Breit-Wigner except for two channel K matrix for S11(1535) and S11(1650)2. Reggeized t- and u-channel3. Coupled channels (dσ/dΩ and Σ: π+ n, π0 p, η p, K Λ,K Σ)

• Li, Saghi:1. Chiral constituent quark model2. SU(6) x O(3) symmetry broken by gluon exchange

• MAID:1. Isobar resonances2. Born terms3. t-channel ρ, ω exchange4. Newly Reggeized for higher energy data

Some π and η Photoproduction Models

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MAID 2002 LS 2002

State Mass π+ n, π0 p η p η p η p

N* P11 1440 ♦ ♦N* D13 1520 ♦ ♦ ♦ ♦N* S11 1535 Width=191 Width=162N* S11 1650 ♦ ♦ ♦ ♦N* D15 1675 ♦ ♦ ♦ ♦N* F15 1680 ♦ ♦ ♦ ♦N* D13 1700 ♦ ♦ ♦ ♦N* P11 1710 ♦ ♦N* P13 1720 ♦ ♦ ♦ ♦N* P11 1840N* D13 1875N* P13 1900 ♦N* F15 2000 ♦ ♦ ♦N* D15 2070 ♦ ♦N* D13 2170N* P13? 2200 ♦

Exotic ? 1800 ♦

Δ P33 1232 ♦Δ S11 1620 ♦Δ D33 1700 ♦Δ F35 1905 ♦Δ P33 1920 ♦Δ D33 1940 ♦Δ F37 1950 ♦

AK 2005

Width =210

• S11(1650) causingdifficulty in determining width for the S11(1535)?

•Missing resonance D15(2070) found?

• Exotic at 1800 MeV?

• Red: π+ n and π0 p• Blue: η p

• LS: Li, Saghi

• AK: Anisovich, Klempt

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29*Work at ASU is supported by the U.S. National Science Foundation

Conclusions/What’s Nextπ0, 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

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30*Work at ASU is supported by the U.S. National Science Foundation

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

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