New Measurement of Beam Asymmetry in Pion Photoproduction on Neutron

1. New Measurement of Beam Asymmetry from Pion Photoproduction on the Neutron using CLAS Daria Sokhan On behalf of the CLAS collaboration and Jefferson Laboratory: NSTAR 2009, Beijing, China – 20 April 2009

2. Outline… • Pseudo-scalar Meson Photoproduction • Polarisation observables –Beam Asymmetry • The attraction of pions and importance of neutrons • JLab and details of the g13 experiment • Highlights of analysis • Early peek at raw Beam Asymmetry in 1.67 < W < 2.5 GeV • Status of analysis and what’s to come

3. Meson Photoproduction • Major aim of electromagnetic beam facilities: to better establish the resonance spectrum of the nucleon and explore poorly established resonances • Photons give access to EM properties of resonances • Photoproduction of pseudo scalar mesons described in terms of 4 complex reaction amplitudes • Experimentally, yields 16 observables - each a different combination of amplitudes: and 3 single polarisation observables: 12 Double-polarisation observables from polarised combinations of beam-recoil, beam-target and target-recoil. & • Beam Asymmetry, Σ • Target asymmetry, T • Recoil polarisation, P

4. 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 and required Experiments I. Barker, A. Donnachie, J. Storrow, Nucl. Phys. B95, 347 (1975)

5. Beam Asymmetry • Beam asymmetry, Σ, 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.

6. 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!

7. World Data: Σ off the Neutron • Alspector, PRL28, 1403 (1972). • Abrahamian, SJNP32, 69 (1980). • Adamyan, JPG15, 1797 (1989). • … has very fewpoints from the neutron (most data is from proton). • … is in a limitedpolar-angle and energy range.

8. The Jefferson Laboratory CLAS: • Multi-layer onion of detectors for charged and neutral particles. • Very large angular coverage: Nearfull coverage in azimuthal angle and from 8° to 140° in scattering angle. • 1.4 kmracetrack accelerator with two LinAc sections. • Operation at up to 6 GeV. • Linearly polarised photon beam via Coherent Bremsstrahlung.

9. The g13b Experiment • Experimental run: March – June 2007 • Linearly polarised beam produced by Coherent Bremsstrahlung off a diamond lattice, then “tagged” in the Tagger→ culprit photon causing reaction can be identified and its energy measured. • Electron energies: 3.3 – 5.2 GeV • 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.

10. Reaction Identification • Deuterium target – quasi-free reaction with spectator proton: • Identify the channel: • Cut on events with two particles, momentum-dependent mass cut on proton and pion. • Cut on “missing mass” – for the spectator proton.

11. 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.

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

13. 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:

14. Preliminary! • To reduce systematics, beam polarisation plane rotated between two orthogonal directions during experiment. Fit with: Where B = PΣ, assuming P = 1

15. Status of Analysis • Final calibrations are underway. • Polarisation has not yet been calculated. The plots to follow assume P = 100% and will be scaled down accordingly. • In final analysis, statistical uncertainty expected be tiny – goal is to reduce systematics to ~ 5%. • As an illustration of current analysis, first look at the data on following slide…

16. Preliminary Results: 1.67 – 2.25 GeV VERY PRELIMINARY! ~ 30 % of the data set Assumed polarisation P = 1 Existing data: • Alspector, PRL 28, 1403 (1972). • Abrahamian,SJNP 32, 69 (1980). • Adamyan, JPG 15, 1797 (1989). Only statistical error is shown!

17. VERY PRELIMINARY! Six W points ~ 0.1 GeV apart Only statistical error is shown!

18. Epilogue • Initial analysis shows that the data quality is good and a sizeable asymmetry changing both with scattering angle and energy can be observed. • Indication of probable overall shape of the final result. • Illustration of statistics – low statistical uncertainty on a third of the data. • Final calibrations are currently under way – full analysis to follow soon in the entire energy range 1.65 < W < 2.3 GeV. • Will greatly expand the sparse world data set on neutron and aid in constraining amplitudes of PWA’s, shedding light on the nucleon excitation spectrum. Thank you!