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

2. JLab Users’ Meeting – 8 June 2010Daria Sokhan 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. JLab Users’ Meeting – 8 June 2010Daria Sokhan 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. JLab Users’ Meeting – 8 June 2010Daria Sokhan Observed resonance spectrum PDG existence rating:**** Certain ** Evidence fair Very likely to certain. Further confirmation desirable and / or quantum number, branching fractions, etc not well determined *** * Evidence poor PDG 2004

5. JLab Users’ Meeting – 8 June 2010Daria Sokhan Missing resonances Not there in nature Or simply not yet observed N* (I = 1/2)   (I = 3/2)  Many more resonances are predicted by some models than observed Missing resonances  Too many ambiguities as insufficient experimental observables measured!

6. JLab Users’ Meeting – 8 June 2010Daria Sokhan  Real photons – well understood EM interaction, giving access to EM properties of resonances. Meson Photoproduction 4 invariant complex reaction amplitudes Experimentally, 16 single and double polarisation observables  Meson photoproduction – for pseudo-scalar mesons:  Polarised: Beam Target Recoil 12 double-polarisation observables & Beam Recoil Target  Partial Wave Analysis (PWA) fits to observables are used to extract resonance parameters (eg: angular momentum, parity), eg: SAID, MAID

7. JLab Users’ Meeting – 8 June 2010Daria Sokhan Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975) Polarisation Observables Direction of target polarisation Component of recoil polarisation measurement a) Notation: Beam polarisation Linear polarisation at angle θ to scattering plane Circular polarisation Complete measurement requires cross- section, and four double- polarisation observables! W.-T. Chiang and F. Tabakin, Phys. Rev. C 55, 2054 (1997).

8. JLab Users’ Meeting – 8 June 2010Daria Sokhan Beam Asymmetry  Beam asymmetry, , 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. JLab Users’ Meeting – 8 June 2010Daria Sokhan  EM interaction does not conserve isospin, so multipole amplitides contain isoscalar and isovector contributions of EM current: The importance of the 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! ProtonNeutron

10. JLab Users’ Meeting – 8 June 2010Daria Sokhan World Data: Σ off the Neutron  … has very few points from the neutron (most data is from proton).  … is in a limited polar-angle and energy range.  Alspector, PRL 28, 1403 (1972). Abrahamian, SJNP 32, 69 (1980). Adamyan, JPG 15, 1797 (1989). Our experiment has added 1179 new data points to the previous set of 67

11. JLab Users’ Meeting – 8 June 2010Daria Sokhan CLAS  Multi-layer onion of detectors, optisimised for charged particle detection  Very large angular coverage: Near full coverage in azimuthal angle and from 8° to 140° in scattering angle.

12. JLab Users’ Meeting – 8 June 2010Daria Sokhan 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. JLab Users’ Meeting – 8 June 2010Daria Sokhan  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%. Linearly polarised photons

14. JLab Users’ Meeting – 8 June 2010Daria Sokhan 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∙10 10 events

15. JLab Users’ Meeting – 8 June 2010Daria Sokhan Reaction Identification I  Deuterium target – quasi-free reaction with spectator proton: Cut on events with two particles, momentum-dependent  cut on proton and pion. channel:  Identify the channel:

16. JLab Users’ Meeting – 8 June 2010Daria Sokhan Reaction Identification II Cut on “missing mass” – for the spectator proton. Fiducial cuts to remove areas of bad acceptance

17. JLab Users’ Meeting – 8 June 2010Daria Sokhan  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. JLab Users’ Meeting – 8 June 2010Daria Sokhan Photon-spotting  Energy of each photon measured by the tagger.  Identify exact photon from timing coincidence – beam in 2 ns bunches.

19. JLab Users’ Meeting – 8 June 2010Daria Sokhan 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. JLab Users’ Meeting – 8 June 2010Daria Sokhan  To reduce systematics, beam polarisation plane rotated between two orthogonal directions during experiment.  extraction Fit with: Where B = PΣ

21. JLab Users’ Meeting – 8 June 2010Daria Sokhan Check of FSI < 30 MeV “missing momentum” 95 – 120 MeV “missing momentum” Quasi-free nucleon good approximation to a free nucleon: V. Vegna et al., Chin. Phys. C 33, 1249 (2009),

22. JLab Users’ Meeting – 8 June 2010Daria Sokhan Preliminary  Measurement I    from new SAID PWA fit: 2.6 SAID 09MAID 07

23. JLab Users’ Meeting – 8 June 2010Daria Sokhan Preliminary  Measurement II    from new SAID PWA fit: 2.6 SAID 09MAID 07

24. JLab Users’ Meeting – 8 June 2010Daria Sokhan Preliminary  Alspector, PRL 28, 1403 (1972). Abrahamian,SJNP 32, 69 (1980). Adamyan, JPG 15, 1797 (1989). Existing data: Only statistical error shown! Systematics: ~ 10%

25. JLab Users’ Meeting – 8 June 2010Daria Sokhan To conclude…  Beam Asymmetry  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  2, 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. JLab Users’ Meeting – 8 June 2010Daria Sokhan THANK YOU!