Measuring Proton Spin- Polarizabilities with the Crystal Ball

1. Measuring Proton Spin-Polarizabilities with the Crystal Ball Rory Miskimen University of Massachusetts Amherst • Compton scattering and nucleon polarizabilities • Measuring proton spin-polarizabilitieswith the Crystal Ball • How well can we measure the proton spin-polarizabilities? • A polarized scintillating target for Compton scattering studies below pion threshold.

2. Compton scattering from the proton

3. Dispersion Model for RCS and VCS† g* g g* g p = Im N N N N • Connects pion electroproduction amplitudes from MAID with VCS • Unconstrained asymptotic contributions to two of the 12 VCS amplitudes are fit to the data. Valid up to Enhanced sensitivity to the polarizabilities †B. Pasquini, et al., Eur. Phys. J. A11 (2001) 185, and D. Drechsel et al., Phys. Rep. 378 (2003) 99.

4. Measuring nucleon spin-polarizabilities in polarized real Compton scattering • At O(w3) four new nucleon structure terms that involve nucleon spin-flip operators enter the RCS expansion. Spin polarizabilities tell us about the response of the nucleon spin to the photon polarization. The “stiffness” of the spin can be thought of as arising from the nucleon’s spin interacting with the pion cloud.

5. Proton spin polarizability p+ Rotating electric field induces pion current. Lorentz force moves pion outward Spin polarizability: “Pionic” Faraday effect

6. Proton spin polarizability p+ Rotating electric field induces pion current. Lorentz force moves pion inward Spin polarizability: “Pionic” Faraday effect

7. Experiments The GDH experiments at Mainz and ELSA used the Gell-Mann, Goldberger, and Thirring sum rule to evaluate the forward S.-P. g0 Backward spin polarizability from dispersive analysis of backward angle Compton scattering

8. Proton spin-polarizability measurements and predictions in units of 10-4 fm4 † The pion-pole contribution has been subtracted from the experimental value for gp Calculations labeled O(pn) are ChPT LC3 and LC4 are O(p3) and O(p4) Lorentz invariant ChPT calculations SSE is small scale expansion Other calculations are dispersion theory Lattice QCD calculation by Detmold is in progress

9. and nature is always full of surprises! Prior to the 1991 publication of Federspielet al., it was surmised that a ≈ 10 ×10-4 fm3 Proton electric and magnetic polarizabilities from real Compton scattering† † M. Schumacher, Prog. Part. and Nucl. Phys. 55, 567 (2005).

10. Virtual Compton Scattering N* ? Courtesy of Helene Fonvieille

11. Measuring proton spin-polarizabilities at MAMI • Crystal ball detector, ≈ 4p photon detection detection • Eg≈ 280 MeV (large sensitivity to g’s) • Possible problems: • Photon backgrounds from p0 production • Coherent and incoherent scattering on 12C in the butanol target. Solution detect recoil protons from Compton events.

12. Signal and Background Reactions Require recoil proton Require only two energy clusters Require correct opening angle between proton and photon, and co-planarity Proton π0 Proton Compton Coherent π0 Coherent Compton Incoherent π0 Incoherent Compton

13. Polarization observables in real Compton scattering Circular polarization Circular polarization Linear polarization

14. Polarization observables in real Compton scattering Circular polarization Circular polarization Linear polarization

15. Polarization observables in real Compton scattering Circular polarization Circular polarization Linear polarization

16. Polarization observables in real Compton scattering Circular polarization Circular polarization Linear polarization

17. Sensitivity Study I: • Hold g0 and gp fixed at experimental values • Vary gE1E1 or gM1M1 • Do this at photon energies of 240 and 280 MeV.

18. Eg=240 MeV Eg=280 MeV is mostly sensitive to

19. Eg=240 MeV Eg=280 MeV is mostly sensitive to

20. Eg=240 MeV Eg=280 MeV is mostly sensitive to and

21. Sensitivity Study II: • Hold g’s fixed at values given by Pasquiniet al. • Vary g’s individually • Do this at photon energies of 240 and 280 MeV.

22. Eg=240 MeV Eg=280 MeV is mostly sensitive to

23. Eg=240 MeV Eg=280 MeV is mostly sensitive to

24. Eg=240 MeV Eg=280 MeV is mostly sensitive to and

25. Sensitivity Study III: • Study what happens when you vary two polarizabilities simultaneously? • Vary a primary S.P. by ± 1, and • vary a secondary S.P. by 0, or +1. • Do this at photon energies of 240 and 280 MeV.

26. Eg=240 MeV Eg=280 MeV is mostly sensitive to

27. Eg=240 MeV Eg=280 MeV is mostly sensitive to

28. Eg=240 MeV Eg=280 MeV is mostly sensitive to and

29. Sensitivity Study IV: • Produce pseudo-data for the asymmetries S2x, S2z, S3 with the expected statistical errors • Fit the pseudo-data with gE1E1, gE1M2, gM1E2, gM1M1, a and b. • Option 1: constrain the fit with the experimental values of g0 and gp • Option 2: no constraint on g0 or gp

30. Projected Errors Option 1: Constrain the fit with the experimental values of g0 and gp Option 2: No constraint on g0 or gp Least well constrained of the 4 S.P.’s.

31. S3y: Linearly polarized photons, target polarization perpendicular to scattering plane is mostly sensitive to and Pasquini, et al., Phys. Rev. C, 76, 015203

32. After doing everything you can do with butanol: an active target for the A2 frozen spin target? • We would like to extend Compton measurements below pion threshold, ≈100 MeV, not to measure the S.P.’s, but rather to test theoretical models for Compton scattering: • HBChPT, dispersion theory, effective field theories. • An active polarized target will be required to do this. • A polarized scintillator target for Compton scattering at HIGS/TUNL is under construction • Probably not possible to reach polarizations or relaxation times equal to those routinely attained for butanol. However, what is achievable might be good enough for Compton studies @ 100 MeV, where asymmetries are large.

33. Existing A2 target with active insert Photodetector Fused silica shell Scintillator foils suspended on graphite rods BCF-92 WLS fibers on outside of transparent shell

34. Polarization studies of polystyrene scintillators DNP measurements at UVa EPR measurements at UMass T≈2° K • Data are consistent with a loss of oxo-tempo in the fabrication process at the level of 0.8×1019 molecules/cc. • More polarization studies are planned at UVa and at JLab using the FROST target.

35. Photo-detector: Radiation Monitoring Devices SS-PMT 3 mm 3 mm QE ≈ 30% Gain = 103 to 104

36. Summary • We can measure all four proton spin-polarizabilities with the crystal ball and the frozen-spin target with a sensitivity at the level of ≈0.5 x 10-4 fm4. • One of the spin observables, S3, requires only linearly polarized photons and a liquid hydrogen target. • We have responded to all of the critical comments of the PAC, and have submitted a detailed report to the A2 Steering Committee. • Polarized Compton scattering below pion threshold will require an active target. The HIGS scintillating insert can probably be adapted to the A2 target. • We look forward to data taking in 2010 !