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

Compton scattering from the proton

  NN   NN  = Im Dispersion Model for RCS and VCS † 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.

At O(  3 ) four new nucleon structure terms that involve nucleon spin-flip operators enter the RCS expansion. Measuring nucleon spin-polarizabilities in polarized real Compton scattering 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.

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

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

Experiments The GDH experiments at Mainz and ELSA used the Gell-Mann, Goldberger, and Thirring sum rule to evaluate the forward S.-P.  0 Backward spin polarizability from dispersive analysis of backward angle Compton scattering

O(p 3 )O(p 4 ) LC3LC4SSEBGLMNHDPVKSDPVExperiment  E1E No data  M1M No data  E1M No data  M1E No data 0 ±0.08 ±0.10  ± 1.8 † † The pion-pole contribution has been subtracted from the experimental value for   Calculations labeled O(p n ) are ChPT LC3 and LC4 are O(p 3 ) and O(p 4 ) Lorentz invariant ChPT calculations SSE is small scale expansion Other calculations are dispersion theory Lattice QCD calculation by Detmold is in progress Proton spin-polarizability measurements and predictions in units of fm 4

and nature is always full of surprises! Proton electric and magnetic polarizabilities from real Compton scattering † Prior to the 1991 publication of Federspiel et al., it was surmised that  ≈ 10 ×10 -4 fm 3 † M. Schumacher, Prog. Part. and Nucl. Phys. 55, 567 (2005).

Courtesy of Helene Fonvieille Virtual Compton Scattering N* ?

Measuring proton spin-polarizabilities at MAMI Crystal ball detector, ≈ 4  photon detection detection E  ≈ 280 MeV (large sensitivity to  ’s) Possible problems: i.Photon backgrounds from  0 production ii.Coherent and incoherent scattering on 12 C in the butanol target. Solution detect recoil protons from Compton events.

Signal and Background Reactions Coherent Compton Incoherent Compton Proton π 0 Coherent π 0 Incoherent π 0 Proton Compton i.Require recoil proton ii.Require only two energy clusters iii.Require correct opening angle between proton and photon, and co-planarity

Polarization observables in real Compton scattering Circular polarization Linear polarization

Polarization observables in real Compton scattering Circular polarization Linear polarization

Polarization observables in real Compton scattering Circular polarization Linear polarization

Polarization observables in real Compton scattering Circular polarization Linear polarization

Sensitivity Study I: Hold  0 and   fixed at experimental values Vary  E1E1 or  M1M1 Do this at photon energies of 240 and 280 MeV.

is mostly sensitive to E  =240 MeV E  =280 MeV

is mostly sensitive to E  =240 MeV E  =280 MeV

is mostly sensitive to and E  =240 MeV E  =280 MeV

Sensitivity Study II: Hold  ’s fixed at values given by Pasquini et al. Vary  ’s individually Do this at photon energies of 240 and 280 MeV.

is mostly sensitive to E  =240 MeV E  =280 MeV

is mostly sensitive to E  =240 MeV E  =280 MeV

is mostly sensitive to and E  =240 MeV E  =280 MeV

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.

is mostly sensitive to E  =240 MeV E  =280 MeV

is mostly sensitive to E  =240 MeV E  =280 MeV

is mostly sensitive to and E  =240 MeV E  =280 MeV

Sensitivity Study IV: Produce pseudo-data for the asymmetries  2x,  2z,  3 with the expected statistical errors Fit the pseudo-data with  E1E1,  E1M2,  M1E2,  M1M1,  and . Option 1: constrain the fit with the experimental values of  0 and   Option 2: no constraint on  0 or  

E  (Mev)  E1E1  E1M2  M1E2  M1M Option 1: Constrain the fit with the experimental values of  0 and   Projected Errors E  (Mev)  E1E1  E1M2  M1E2  M1M Option 2: No constraint on  0 or   Least well constrained of the 4 S.P.’s.

 3y : Linearly polarized photons, target polarization perpendicular to scattering plane is mostly sensitive to and Pasquini, et al., Phys. Rev. C, 76,

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 100 MeV, where asymmetries are large.

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

Polarization studies of polystyrene scintillators DNP measurements at UVaEPR measurements at UMass Data are consistent with a loss of oxo-tempo in the fabrication process at the level of 0.8×10 19 molecules/cc. More polarization studies are planned at UVa and at JLab using the FROST target. T≈2° K

Photo-detector: Radiation Monitoring Devices SS-PMT QE ≈ 30%Gain = 10 3 to mm

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 fm 4. One of the spin observables,  3, 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 !