Beam Asymmetry measurement in Pion Photoproduction on the neutron using CLAS

Transcription

1 Beam Asymmetry measurement in Pion Photoproduction on the neutron using CLAS University of Glasgow, UK on behalf of the CLAS Collaboration MENU2013, Rome, Italy 1st October 2013

2 Meson Photoproduction Meson photoproduction for pseudo-scalar mesons: 4 invariant complex reaction amplitudes Experimentally, 16 single and double polarisation observables Polarised: Beam Target Recoil Beam Target Recoil Σ T R & 12 double-polarisation observables Partial Wave Analysis (PWA) fits to observables are used to extract resonance parameters (angular momentum, parity, etc), eg: SAID, MAID, Bonn-Gatchina

3 From observables to the resonance spectrum New measurement of observable World data set, all measured observables Partial Wave Analyses Multipoles Resonance parameters

4 Polarisation Observables a) Notation: { P ; P ; P } P L( ) C Beam polarisation Linear polarisation at angle θ to scattering plane Circular polarisation T R Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975) P T P R Direction of target polarisation Component of recoil polarisation measurement

5 Polarisation Observables Complete measurement requires cross-section, Σ, T, R and four doublepolarisation observables! W.-T. Chiang and F. Tabakin, Phys. Rev. C 55, 2054 (1997). a) Notation: { P ; P ; P } P L( ) C Beam polarisation Linear polarisation at angle θ to scattering plane Circular polarisation T R Barker, A. Donnachie, J. Storrow, Nucl. Phys. B 95, 347 (1975) P T P R Direction of target polarisation Component of recoil polarisation measurement

6 What makes up a cross-section

7 Focus on: Beam Asymmetry Σ Experiment: linearly polarised beam, unpolarised target

8 Measuring Beam Asymmetry Beam asymmetry, Σ, from linearly polarised photons crucial observable to constrain PWA. d 0 (1 Plin cos2 ) d Many wide, overlapping resonances expected to couple to the pion channel. Large cross-section: precision data. Experiment using JLab exclusive measurement of Σ in 1.6 < W < 2.3 GeV on a quasi-free neutron : d p ( p spectator )

9 Why neutrons? EM interaction does not conserve isospin, so multipole amplitides contain isoscalar and isovector contributions of EM current: Proton I 1 2 ( 2 (0) (1) I ) 3 A p p 3 A 3 A 3 A I 1 2 ( 2 (0) (1) I ) 3 A p n 2 3 A 3 A 3 A Neutron I 1 2 ( 2 (0) (1) I ) 3 A n n 3 A 3 A 3 A I 1 2 ( 2 (0) (1) I ) 3 A n n 2 3 A 3 A 3 A 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 Jefferson Lab (Virginia, USA) CEBAF: Continuous Electron Beam Accelerator Facility: Duty cycle: ~ 100% Energy up to ~6 GeV Electron polarisation up to ~85% CLAS in Hall B: Drift chambers Toroidal magnetic field Cerenkov Counters Scintillator Time of Flight Electromagnetic Calorimeters Very large angular coverage near full azimuthal and 8 to 140 scattering angle

11 The Photon Beam e - Goniometer for tilting radiator Linearly polarised (up to > 90%) photon beam: bremsstrahlung of unpolarised electrons in a highly ordered crystalline radiator, typically µm diamond. Crystal orientation chosen to produce a coherent peak of polarised photons at the required energy.

12 The Photon Beam e - e - Tagger momentum analyses deflected electrons γ Goniometer for tilting radiator Culprit photon causing reaction can be identified and its energy calculated Linearly polarised (up to > 90%) photon beam: bremsstrahlung of unpolarised electrons in a highly ordered crystalline radiator, typically µm diamond. Crystal orientation chosen to produce a coherent peak of polarised photons at the required energy.

13 The Photon Beam e - e - Tagger momentum analyses deflected electrons γ γ Goniometer for tilting radiator Culprit photon causing reaction can be identified and its energy calculated Target within CLAS Linearly polarised (up to > 90%) photon beam: bremsstrahlung of unpolarised electrons in a highly ordered crystalline radiator, typically µm diamond. Crystal orientation chosen to produce a coherent peak of polarised photons at the required energy.

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

15 World Data: Σ off the Neutron Alspector, PRL 28, 1403 (1972). Abrahamian, SJNP 32, 69 (1980). Adamyan, JPG 15, 1797 (1989). Mandaglio, PRC 82, (2010). has very few points from the neutron (most data is from proton). is in a limited polarangle and energy range.

16 World Data: Σ off the Neutron Alspector, PRL 28, 1403 (1972). Abrahamian, SJNP 32, 69 (1980). Adamyan, JPG 15, 1797 (1989). Mandaglio, PRC 82, (2010). has very few points from the neutron (most data is from proton). is in a limited polarangle and energy range. Our experiment has added ~1180 new data points to the previous set of 166

17 Reaction Identification Deuterium target quasi-free reaction with spectator proton: Identify the d p ( p spectator ) p channel: n(, ) Identify p, π - final state Cut on missing mass for the spectator proton.

18 Cut on low missing momentum below 0.12 GeV where quasifree contribution dominates. P P P P P deuterium p spectator Cut on proton and pion back-to-back in CMS: coplanarity. p x CM angle

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

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

21 Measuring the Beam Asymmetry Σ Reaction axes: z p p y p p p p Reaction plane 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: d 0 (1 Plin cos2 ) d

22 Σ from fits To reduce systematics, beam polarisation plane rotated between two orthogonal directions during experiment. N = N θ (1 PΣcos2φ) N = N θ (1 + PΣcos2φ) PΣcos2φ = N N N + N At its simplest, fit with: Where B = PΣ ABcos(2 C) Actual fit function more complicated, to account for imperfect normalisation, etc

23 Σ from fits To reduce systematics, beam polarisation plane rotated between two orthogonal directions during experiment. N = N θ (1 PΣcos2φ) N = N θ (1 + PΣcos2φ) PΣcos2φ = N N N + N At its simplest, fit with: Where B = PΣ ABcos(2 C) Actual fit function more complicated, to account for imperfect normalisation, etc

24 Check of FSI Quasi-free nucleon good approximation to a free nucleon: V. Vegna et al., Chin. Phys. C 33, 1249 (2009), PRELIMINARY!!! < 30 MeV missing momentum MeV missing momentum

25 Preliminary Σ Measurement: θ > 90º ~ 1/4 of the g13 dataset shown below MAID 07 SAID 11 BoGa 11 Graal: Madaglio et al, PRC (2010)

26 Preliminary Σ Measurement: θ < 90º ~ 1/4 of the g13 dataset shown below MAID 07 SAID 11 BoGa 11 Graal: Madaglio et al, PRC (2010)

27 A closer took at discrepancy with GRAAL Graal: Madaglio et al, PRC (2010) Discrepancies too large to be due entirely to systematics! Different treatment of Fermi momentum? CLAS data: Fermi-momentum calculated from quasi-exclusive reconstruction of the reaction: γ + d π + p + X. GRAAL data: Fermi momentum reconstructed through an iterative match to a Monte Carlo simulation.

28 A closer took at discrepancy with GRAAL Incompatible bins? The beam asymmetry varies dramatically with the angle slight differences in the composition of the bin may yield significant differences in Σ PRELIMINARY!!! c 2 from SAID-09 PWA fit including GRAAL data: 89 c 2 from SAID-09 PWA fit including g13 data, but not GRAAL: 2.6 Reasons for CLAS GRAAL discrepancy under investigation!

29 In Conclusion Beam Asymmetry Σ measured in range 1.6 < W < 2.3 GeV, for the channel d p ( p ) spectator A sizeable asymmetry changing rapidly both with scattering angle and energy can be observed. Comparison with the most recent PWA predictions shows significant differences in certain W regions. Inclusion of preliminary data into SAID09 PWA gave a good c 2 Discrepancy with Graal measurement under investigation. 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.

30 THANK YOU!