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Double Polarization Virtual Compton Scattering at MAMI

24 th Students' Workshop on Electromagnetic Interactions Bosen (Saar), 9-14 Sept. 2007. Double Polarization Virtual Compton Scattering at MAMI. Luca Doria for the A1 Collaboration Institut fuer Kernphysik, Mainz (Germany). Talk Outline. Physical Motivations Theoretical Aspects

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Double Polarization Virtual Compton Scattering at MAMI

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  1. 24th Students' Workshop on Electromagnetic Interactions Bosen (Saar), 9-14 Sept. 2007 Double Polarization Virtual Compton Scattering at MAMI Luca Doria for the A1 Collaboration Institut fuer Kernphysik, Mainz (Germany)

  2. Talk Outline Physical Motivations Theoretical Aspects Experimental Setup Data Analysis and Results Conclusions and Outlook Luca Doria, KPH Mainz Bosen Workshop 2007

  3. Polarizabilities: Classical Picture Electric Polarizability α Displacement of electric charges Induced dipole moment p=αE For atomic systems α/V~1 Nucleons: α~10-4 fm3, V~1fm3 Magnetic Polarizability β ChPT: Pions are relevant degrees of freedom „Pion Cloud“ : Induced currents of spinless charged particles Diamagnetism β<0 Paramagnetism β>0 Resonance Structure of the Nucleon Example: N->Delta Transition Luca Doria, KPH Mainz Bosen Workshop 2007

  4. Form Factors Analogy Elastic eN Scattering Q2=0 : Charge/magnetic moment of the nucleon FT: Charge/magnetic dipole spatial distribution Luca Doria, KPH Mainz Bosen Workshop 2007

  5. Form Factors Analogy Elastic eN Scattering Q2=0 : Charge/magnetic moment of the nucleon FT: Charge/magnetic dipole spatial distribution (Virtual) Compton Scattering Q2=0 : Electric/magnetic polarizabilities of the nucleon FT: Spatial distribution of the polarizabilities inside the nucleon α(Q2), β(Q2) +4 new observables: Generalized Polarizabilities RCS ? Luca Doria, KPH Mainz Bosen Workshop 2007

  6. Virtual Compton Scattering Full electron photoproduction amplitude Bethe-Heitler Contribution Non separable from VCS Lorentz Boost: g in the direction of e Cross Section proportional to 1/k Knowledge of the Form Factos needed Born Contribution First Order Term Suppressed by 1/m VCS Contribution Access to the Generalized Polarizabilities Luca Doria, KPH Mainz Bosen Workshop 2007

  7. Virtual Compton Scattering Luca Doria, KPH Mainz Bosen Workshop 2007

  8. Observables (LEX approach) Cross Section Double Polarization Observable Generalized Polarizabilities P.A.M. Guichon, at al., Nucl Phys A 591 (1995) 606-638 Luca Doria, KPH Mainz Bosen Workshop 2007

  9. Observables (LEX approach) Cross Section Double Polarization Observable Generalized Polarizabilities α β P.A.M. Guichon, at al., Nucl Phys A 591 (1995) 606-638 Luca Doria, KPH Mainz Bosen Workshop 2007

  10. Observables (Dispersion Relations) VCS defined by Im ν Amplitudes analytical in ν, Unitarity, Crossing Symmetry Analytical continuation Re ν νth -νth πN part given by MAID2000 π-photoproduction amplituds Parameterization of α and β Fit to the experimental data Prediction for the 4 Spin GPs Dipole Form B.Pasquini et al, Eur. Phys J. 11 (2001) 185-208 Luca Doria, KPH Mainz Bosen Workshop 2007

  11. Theoretical Models NRQM: P.A.M. Guichon, at al., Nucl Phys A 591 (1995) 606-638 HBChPT O(3): T.R.Hemmert et al. Phys. Rev. D 62 (2000) 014013 HBChPT O(4/5): Kao et al, Phys.Rev.D 70 (2004) 114004 LSM: A.Metz, D.Drechsel, Z. der Physik A356 (1996) 351 ELM: M.Vanderhaeghen, Phys.Lett. B368 (1996) 13 DR: B.Pasquini et al, Eur. Phys J. 11 (2001) 185-208 Luca Doria, KPH Mainz Bosen Workshop 2007

  12. Experimental Status Unpolarized Cross Section 2 Combinations of GPs Measurements from three laboratories Bates point: LEX not applicable α and β extracted with the DR model Not all the data can be described Dipole parameterization Two distinct regions of β LEX and DR consistent for Jlab points DR analysis for MAMI data coming soon Polarizability rms radius consistent with the pion cloud interpretation MAMI: J. Roche et al, Phys. Rev. Lett. 85 (2000) 708-711JLAB: Phys.Rev.Lett., 93 (2004) 122001 MIT-Bates: P.Bourgeois et al., Phys.Rev.Lett 97 (2006) 212001 Luca Doria, KPH Mainz Bosen Workshop 2007

  13. Experimental Setup (1) (A.Jankowiak) MAMI Accelerator: 3 Cascaded Racetrack Microtrons: E = 855 MeV/c2 Max. Current = 20 Duty Cycle: 100% Energy Spread 30 keV (FWHM) Detector Package for each Spectrometer: Cerenkov Detector (e/p id.) Vertical Drift Chambers (4 planes) 2 Scintillator Planes (Photo: M.Weiss) Luca Doria, KPH Mainz Bosen Workshop 2007

  14. Experimental Setup (2) Spectrometer A: >20 p<735 MeV/c =28msr p/p=20% Spectrometer B: >8 p<870 MeV/c =5.6msr p/p=15% Spectrometer C: >55 p<655 MeV/c =28msr p/p=25% Out-of-Plane Capability for Spectrometer B Moller Polarimeter - 70%-80% Beam Polarization - 5 Min. pro Measurement - 1% Systematic Uncertainty Luca Doria, KPH Mainz Bosen Workshop 2007

  15. Focal Plane Proton Polarimeter Systematic Error Px ~ 1% Pz , Py ~ 2% Polarization Reconstruction Three polarization components accesible in principle Maximum Likelihood Fit Full spin precession taken into account Luca Doria, KPH Mainz Bosen Workshop 2007

  16. Data Analysis: Cross Section Particle ID: Time Coincidence Scintillators (A) Cerenkov Detector (B) Reaction ID Missing Mass2 Background Random Coincidences Other Particles Luca Doria, KPH Mainz Bosen Workshop 2007

  17. Data Analysis: Cross Section Mergell et al. Friedrich-Walcher HBChPT [1] P.Mergell et al. , Nucl.Phys. A596 (1996) 367-391 [2] J. Friedrich and Th. Walcher, Eur. Phys. J. A 17 (2003) 607-623 Luca Doria, KPH Mainz Bosen Workshop 2007

  18. Data Analysis: Double Polarization Observable Px Py θγγ θγγ Theoretical Curves based on the mean exp. Kinematics Projection to nominal kinematics needed False asymmetries and background asymmetries negligible Px higher than BH+B Py consistent with 0 Pz constrained to BH+B Very low correlation with Px Luca Doria, KPH Mainz Bosen Workshop 2007

  19. Data Analysis: Extraction of PLT Largest effect given by PLT Less information from Py Pz not measured with sufficent accuracy Luca Doria, KPH Mainz Bosen Workshop 2007

  20. Data Analysis: Extraction of PLT Mergell et al. Friedrich-Walcher HBChPT Luca Doria, KPH Mainz Bosen Workshop 2007

  21. Summary and Outlook Virtual Compton Scattering Intuitive physical interpretation of the GPs Fundamental as the form factors New test for the thoretical models Experimental activity Accessed through photon electroproduction First unpolarized experiment at MAMI (Q2=0.33 GeV2/c2) Experiments by MIT-Bates (Q2=0.05 GeV2/c2) and JLab (Q2 = 0.92 , 1,76 GeV2/c2) Single Spin Asymmetry measured at MAMI I. Bensafa et al., Eur. Phys. J. A 32, (2007) 69-75 NOW: Double Polarization Observable The Future Enhance the statistics/accuracy for the double polarization experiment Measure new kinematical points for improving the knowledge of α and β Herzlichen Dank an: H. Fonvieille (UBP Clermont-Ferrand, France) P. Janssens (Ghent University, Belgium) N. d'Hose and the Saclay Group (Paris) Collaboration Accelerator Staff

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