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

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

4. Form Factors Analogy Luca Doria, KPH Mainz Bosen Workshop 2007 Elastic eN Scattering Q 2 =0 : Charge/magnetic moment of the nucleon FT: Charge/magnetic dipole spatial distribution

5. Form Factors Analogy Luca Doria, KPH Mainz Bosen Workshop 2007 Elastic eN Scattering (Virtual) Compton Scattering Q 2 =0 : Charge/magnetic moment of the nucleon FT: Charge/magnetic dipole spatial distribution Q 2 =0 : Electric/magnetic polarizabilities of the nucleon FT: Spatial distribution of the polarizabilities inside the nucleon α (Q 2 ), β (Q 2 ) +4 new observables: Generalized Polarizabilities ? RCS

6. Virtual Compton Scattering Luca Doria, KPH Mainz Bosen Workshop 2007 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 Full electron photoproduction amplitude

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

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

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

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

11. Theoretical Models Luca Doria, KPH Mainz Bosen Workshop 2007 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

12. Experimental Status Luca Doria, KPH Mainz Bosen Workshop 2007 MAMI: J. Roche et al, Phys. Rev. Lett. 85 (2000) 708-711 JLAB: Phys.Rev.Lett., 93 (2004) 122001 MIT-Bates: P.Bourgeois et al., Phys.Rev.Lett 97 (2006) 212001 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

13. Experimental Setup (1) (Photo: M.Weiss) (A.Jankowiak) MAMI Accelerator: 3 Cascaded Racetrack Microtrons: E = 855 MeV/c 2 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 Luca Doria, KPH Mainz Bosen Workshop 2007

14. Experimental Setup (2) Out-of-Plane Capability for Spectrometer B Moller Polarimeter 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% - 70%-80% Beam Polarization - 5 Min. pro Measurement - 1% Systematic Uncertainty Luca Doria, KPH Mainz Bosen Workshop 2007

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

16. Data Analysis: Cross Section Luca Doria, KPH Mainz Bosen Workshop 2007 Particle ID: Time Coincidence Scintillators (A) Cerenkov Detector (B) Reaction ID Missing Mass 2 Background Random Coincidences Other Particles

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

18. Data Analysis: Double Polarization Observable Luca Doria, KPH Mainz Bosen Workshop 2007 θ γγ PxPx Theoretical Curves based on the mean exp. Kinematics Projection to nominal kinematics needed False asymmetries and background asymmetries negligible PyPy θγγθγγ P x higher than BH+B P y consistent with 0 P z constrained to BH+B Very low correlation with P x

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

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

21. Summary and Outlook 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 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 (Q 2 =0.33 GeV 2 /c 2 ) Experiments by MIT-Bates (Q 2 =0.05 GeV 2 /c 2 ) and JLab (Q 2 = 0.92, 1,76 GeV 2 /c 2 ) 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