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Dan Watts (Edinburgh), Igor Strakovsky (GWU) and the CLAS Collaboration

New Measurement of Beam Asymmetry from Pion Photoproduction on the Neutron using CLAS. Daria Sokhan. IPN Orsay, France. Dan Watts (Edinburgh), Igor Strakovsky (GWU) and the CLAS Collaboration. JLab Users’ Meeting – 9 June 2010. Forecast…. Nucleon resonance spectrum

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Dan Watts (Edinburgh), Igor Strakovsky (GWU) and the CLAS Collaboration

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  1. New Measurement of Beam Asymmetry from Pion Photoproduction on the Neutron using CLAS Daria Sokhan IPN Orsay, France Dan Watts (Edinburgh), Igor Strakovsky (GWU) and the CLAS Collaboration JLab Users’ Meeting – 9 June 2010

  2. Forecast… • Nucleon resonance spectrum • Pseudo-scalar Meson Photoproduction • Polarisation observables –Beam Asymmetry • The attraction of pions and importance of neutrons • CLAS and details of the g13 experiment • Highlights of analysis • Preliminary Beam Asymmetry in 1.6 < W < 2.3 GeV

  3. Nucleon excitation spectrum • Little known - predictions primarily from calculations on the Lattice and phenomenological models (eg: constituent quark-model, di-quark model, quark – flux tube model…) • Clear indication of resonances Free proton photoabsorbtion cross-sections from various meson channels (PDG 2002)

  4. Observed resonance spectrum PDG 2004 PDG existence rating: ****Certain **Evidence fair *Evidence poor Very likely to certain. Further confirmation desirable and / or quantum number, branching fractions, etc not well determined ***

  5. Missing resonances ? ? Or simply not yet observed Not there in nature Missing resonances • N* (I = 1/2) • D (I = 3/2) • Many more resonances are predicted by some models than observed • Too many ambiguities as insufficient experimental observables measured!

  6. Beam & 12 double-polarisation observables Target Recoil Meson Photoproduction • Real photons – well understood EM interaction, giving access to EM properties of resonances. • Meson photoproduction – for pseudo-scalar mesons: • 4 invariant complex reaction amplitudes • Experimentally, 16 single and double polarisation observables Polarised: Beam Target Recoil S T R • Partial Wave Analysis (PWA) fits to observables are used to extract resonance parameters (eg: angular momentum, parity), eg: SAID, MAID

  7. a) Notation: Beam polarisation Linear polarisation at angle θ to scattering plane Circular polarisation Direction of target polarisation Component of recoil polarisation measurement Polarisation Observables Complete measurement requires cross-section, S, T, R and four double-polarisation observables! W.-T. Chiang and F. Tabakin, Phys. Rev. C 55, 2054 (1997). Barker, A. Donnachie, J. Storrow, Nucl. Phys. B95, 347 (1975)

  8. Beam Asymmetry • Beam asymmetry, S, from linearly polarised photons – crucial observable to constrain PWA. • Many wide, overlapping resonances expected to couple to the pion channel. • Current experiment: • Advantages of pion photoproduction: sensitivity to many resonances, large cross-section, easy detection.

  9. The importance of the Neutron • EM interaction does not conserve isospin, so multipole amplitides contain isoscalar and isovector contributions of EM current: Proton Neutron • Proton data alone does not allow separation of the isoscalar, A(0), and isovector, A(1), components. • Need data on both proton and neutron!

  10. World Data: Σ off the Neutron • … has very fewpoints from the neutron (most data is from proton). • … is in a limitedpolar-angle and energy range. • Alspector, PRL28, 1403 (1972). • Abrahamian, SJNP32, 69 (1980). • Adamyan, JPG15, 1797 (1989). Our experiment has added 1179 new data points to the previous set of 67

  11. CLAS • Multi-layer onion of detectors, optisimised for charged particle detection • Very large angular coverage: Nearfull coverage in azimuthal angle and from 8° to 140° in scattering angle.

  12. The Photon Beam • Produced via bremsstrahlung of electron beam in a radiator • Energies up to 6 GeV • Photons “tagged” in the Tagger→ culprit photon causing reaction can be identified and its energy measured.

  13. Linearly polarised photons • Coherent bremsstrahlung from unpolarised electrons passing through a highly ordered crystalline radiator, typically 20 – 50 µm diamond. • Crystal orientation chosen to produce a “coherent” peak of polarised photons at the required energy. • Polarisation up to > 90%.

  14. The g13b Experiment • Experimental run: March – June 2007 • Electron energies: 3.3 – 5.2 GeV • Linearly polarised photons produced via coherent bremsstrahlung • Six photon energy settings in range: 1.1 - 2.3 GeV, with two orthogonal polarisation orientations. • Target: liquid Deuterium • Single charged particle trigger. Total of 3∙1010 events

  15. Reaction Identification I • Deuterium target – quasi-free reaction with spectator proton: • Identify the channel: • Cut on events with two particles, momentum-dependent b cut on proton and pion.

  16. Reaction Identification II • Fiducial cuts to remove areas of bad acceptance • Cut on “missing mass” – for the spectator proton.

  17. Cut on low “missing momentum” below 0.12 GeV where quasi-free contribution dominates. • Cut on proton and pion back-to-back in CMS: coplanarity.

  18. Photon-spotting • Energy of each photon measured by the tagger. • Identify exact photon from timing coincidence – beam in 2 ns bunches.

  19. Extracting the Asymmetry • Reaction axes: in the Centre of Momentum System (CMS) • φ: angle of beam polarisation plane in CMS w.r.t. reaction plane. • Asymmetry from cos(2φ) fit to the φ-distribution of pions:

  20. Sextraction • To reduce systematics, beam polarisation plane rotated between two orthogonal directions during experiment. Fit with: Where B = PΣ

  21. Check of FSI • Quasi-free nucleon good approximation to a free nucleon: V. Vegna et al., Chin. Phys. C 33, 1249 (2009), < 30 MeV “missing momentum” 95 – 120 MeV “missing momentum”

  22. Preliminary SMeasurement I SAID 09 MAID 07 c2from new SAID PWA fit: 2.6

  23. Preliminary SMeasurement II SAID 09 MAID 07 c2from new SAID PWA fit: 2.6

  24. Preliminary S Only statistical error shown! Systematics: ~ 10% Existing data: • Alspector, PRL 28, 1403 (1972). • Abrahamian,SJNP 32, 69 (1980). • Adamyan, JPG 15, 1797 (1989).

  25. To conclude… • Beam AsymmetryS measured in range 1.6 < W < 2.3 GeV, for the channel • A sizeable asymmetry changing both with scattering angle and energy can be observed. • Data included in a new SAID PWA analysis – good c2, significant changes from previous SAID PWA observed • Greatly expanded the sparse world data set on the neutron with > 1000 additional points • Will aid in constraining amplitudes of PWAs, en route to a “complete measurement” of polarisation observables. • Shed new light on the nucleon excitation spectrum!

  26. THANK YOU!

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