New Measurement of Beam Asymmetry from Pion Photoproduction

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… v Nucleon resonance spectrum v Pseudo-scalar Meson Photoproduction v Polarisation observables – Beam Asymmetry v The attraction of pions and importance of neutrons v CLAS and details of the g 13 experiment v Highlights of analysis v Preliminary Beam Asymmetry in 1. 6 < W < 2. 3 Ge. V Daria Sokhan JLab Users’ Meeting – 8 June 2010

Nucleon excitation spectrum v Little known - predictions primarily from calculations on the Lattice and phenomenological models (eg: constituent quark-model, di-quark model, quark – flux tube model…) v Clear indication of resonances Daria Sokhan Free proton photoabsorbtion cross-sections from various meson channels (PDG 2002) JLab Users’ Meeting – 8 June 2010

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 Daria Sokhan JLab Users’ Meeting – 8 June 2010

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

Meson Photoproduction v Real photons – well understood EM interaction, giving access to EM properties of resonances. v 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 Beam Target & 12 double-polarisation observables Recoil v Partial Wave Analysis (PWA) fits to observables are used to extract resonance parameters (eg: angular momentum, parity), eg: SAID, MAID Daria Sokhan JLab Users’ Meeting – 8 June 2010

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

Beam Asymmetry v Beam asymmetry, S, from linearly polarised photons – crucial observable to constrain PWA. v Many wide, overlapping resonances expected to couple to the pion channel. v Current experiment: v Advantages of pion photoproduction: sensitivity to many resonances, large cross-section, easy detection. Daria Sokhan JLab Users’ Meeting – 8 June 2010

The importance of the Neutron v EM interaction does not conserve isospin, so multipole amplitides contain isoscalar and isovector contributions of EM current: Proton Neutron v Proton data alone does not allow separation of the isoscalar, A(0), and isovector, A(1), components. v Need data on both proton and neutron! Daria Sokhan JLab Users’ Meeting – 8 June 2010

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

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

The Photon Beam v Produced via bremsstrahlung of electron beam in a radiator v Energies up to 6 Ge. V v Photons “tagged” in the Tagger → culprit photon causing reaction can be identified and its energy measured. Daria Sokhan JLab Users’ Meeting – 8 June 2010

Linearly polarised photons v Coherent bremsstrahlung from unpolarised electrons passing through a highly ordered crystalline radiator, typically 20 – 50 µm diamond. v Crystal orientation chosen to produce a “coherent” peak of polarised photons at the required energy. v Polarisation up to > 90%. Daria Sokhan JLab Users’ Meeting – 8 June 2010

The g 13 b Experiment v Experimental run: March – June 2007 v Electron energies: 3. 3 – 5. 2 Ge. V v Linearly polarised photons produced via coherent bremsstrahlung v Six photon energy settings in range: 1. 1 - 2. 3 Ge. V, with two orthogonal polarisation orientations. v Target: liquid Deuterium v Single charged particle trigger. Total of 3∙ 1010 events Daria Sokhan JLab Users’ Meeting – 8 June 2010

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

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

v Cut on low “missing momentum” below 0. 12 Ge. V where quasi-free contribution dominates. v Cut on proton and pion back-to-back in CMS: coplanarity. Daria Sokhan JLab Users’ Meeting – 8 June 2010

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

Extracting the Asymmetry v Reaction axes: in the Centre of Momentum System (CMS) v φ: angle of beam polarisation plane in CMS w. r. t. reaction plane. v Asymmetry from cos(2φ) fit to the φ-distribution of pions: Daria Sokhan JLab Users’ Meeting – 8 June 2010

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

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

Preliminary S Measurement I SAID 09 Daria Sokhan MAID 07 c 2 from new SAID PWA fit: 2. 6 JLab Users’ Meeting – 8 June 2010

Preliminary S Measurement II SAID 09 Daria Sokhan MAID 07 c 2 from new SAID PWA fit: 2. 6 JLab Users’ Meeting – 8 June 2010

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). Daria Sokhan JLab Users’ Meeting – 8 June 2010

To conclude… v Beam Asymmetry S measured in range 1. 6 < W < 2. 3 Ge. V, for the channel v A sizeable asymmetry changing both with scattering angle and energy can be observed. v Data included in a new SAID PWA analysis – good c 2, significant changes from previous SAID PWA observed v Greatly expanded the sparse world data set on the neutron with > 1000 additional points v Will aid in constraining amplitudes of PWAs, en route to a “complete measurement” of polarisation observables. v Shed new light on the nucleon excitation spectrum! Daria Sokhan JLab Users’ Meeting – 8 June 2010

THANK YOU! Daria Sokhan JLab Users’ Meeting – 8 June 2010