Daria Users meeting 2010

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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

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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

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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)

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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

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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!

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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: S T R Polarised: Beam Target Recoil Partial Wave Analysis (PWA) fits to observables are used to extract resonance parameters (eg: angular momentum, parity), eg: SAID, MAID

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Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975) 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).

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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.

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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! Proton Neutron

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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

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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.

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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.

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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

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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

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

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

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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.

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

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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:

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

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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),

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Preliminary S Measurement I c2 from new SAID PWA fit: 2.6 SAID 09 MAID 07

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Preliminary S Measurement II c2 from new SAID PWA fit: 2.6 SAID 09 MAID 07

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

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To conclude… Beam Asymmetry S 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!

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THANK YOU!