Presentation on theme: "Rory Miskimen University of Massachusetts, Amherst"— Presentation transcript:
1 Measuring Spin-Polarizabilities of the Proton in Polarized Compton Scattering at MAMI-Mainz Rory MiskimenUniversity of Massachusetts, Amherstfor the Mainz A2 CollaborationChiral Dynamics 2012Compton scattering and nucleon spin-polarizabilitiesFirst measurements of double-polarized Compton scattering asymmetries on the proton
2 Measuring nucleon spin-polarizabilities in polarized Compton scattering At O(w3) four nucleon structure terms involving nucleon spin-flip operators enter the Real Compton Scattering 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.
3 ExperimentsThe GDH experiments at Mainz and ELSA used the Gell-Mann, Goldberger, and Thirring sum rule to evaluate the forward S.-P. g0Backward spin polarizability from dispersive analysis of backward angle Compton scatteringThe pion-pole contribution has been subtracted from gp
4 Proton spin-polarizability measurements and predictions in units of 10-4 fm4 O(p3)O(p4)LC3LC4SSEBGLMNHDPVKSDPVDTheoryExperimentgE1E1-5.7-1.4-1.8-3.2-2.8-3.4-4.3-5.04.3No datagM1M1-184.108.40.206-220.127.116.11.46.5gE1M18.104.22.168.8.980.3-0.01gM1E22.214.171.124.1g04.6-3.9-126.96.36.199-1.5-.72.3-1.01 ±0.08 ±0.10gp6.35.8-.188.8.131.521.38.0± 1.8†Calculations labeled O(pn) are ChPTLC3 and LC4 are O(p3) and O(p4) Lorentz invariant ChPT calculationsSSE is small scale expansionOther calculations are dispersion theory
5 Polarization observables in real Compton scattering Circular polarizationCircular polarizationLinear polarization
6 Polarization observables in real Compton scattering Circular polarizationCircular polarizationLinear polarization
7 Polarization observables in real Compton scattering Circular polarizationCircular polarizationLinear polarization
8 Polarization observables in Compton scattering Circular polarizationCircular polarizationLinear polarization
9 g* g g* g p = Im Dispersion Model for RCS and VCS† N N N N Connects pion electroproduction amplitudes from MAID with VCSUnconstrained asymptotic contributions to two of the 12 VCS amplitudes are fit to the data. Valid up toEnhanced sensitivity to the polarizabilities†B. Pasquini, et al., Eur. Phys. J. A11 (2001) 185, and D. Drechsel et al., Phys. Rep. 378 (2003) 99.
10 Sensitivity Study for S2x Vary a, b, g0 and gp within experimental error bars, andvary gE1E1 holding gM1M1 fixed, orvary gM1M1 holding gE1E1 fixedEg = 280 MeVDgE1E1 = ±1DgM1M1 = ±1S2xS2x
11 Polarization observables in real Compton scattering Circular polarizationSensitive to gE1E1Circular polarizationSensitive to gM1M1Linear polarizationSensitive to gE1E1 and gM1M1
12 Measurements of S2x at MAMI-Mainz Circular polarizationSensitive to gE1E1Phil Martel’s Ph.D. thesis, UMass Amherst
19 Proton detection efficiency measured in the gp → p0 p reaction Peak efficiency ~60%Low energy cutoff ~ 75 MeV
20 Signal and Background Reactions Require only two tracks in the detector, one neutral and one charged, andrequire correct opening angle between Compton scattered photon and charged track, and co-planarityProton ComptonProton π0Coherent π0Coherent ComptonIncoherent ComptonIncoherent π0
30 Asymmetry with transverse polarized target and circularly polarized photons Eg~ 285 MeVPRELIMINARYS2xChanging gE1E1
31 SummaryFirst measurement of a double-polarized Compton scattering asymmetry on the nucleon, S2xData have sensitivity to the gE1E1 spin-polarizabilityOutlookData taking on S3 later this year at MAMI ( for gE1E1 and gM1M1 )Data taking on S2z in 2013 ( for gM1M1 )A global analysis of all polarized Compton scattering data on the proton using dispersion analysis treatment is in progressDevelopment of an active polarized target has been approved for MAMI. Polarizable scintillators have been developed at UMass.
33 Measuring the spin polarizabilities of the proton in double-polarized Compton scattering at Mainz: PRELIMINARY results from P. Martel (Ph.D. UMass)Transverse target asymmetry S2x and sensitivity to gE1E1Frozen spin targetPRELIMINARYCrystal Ball
35 IntegrateHave used a very conservative cut on the missing mass spectrum, E < 930 MeVUse monte carlo constrained Compton scattering peak-shapes to extract yieldsMonte carlo simulation of Compton scattering peak-shape
36 Proton spin polarizability Rotating electric field induces pion current. Lorentz force moves pion orbit outwardSpin polarizability: “Pionic” Faraday effect
37 Proton spin polarizability Rotating electric field induces pion current. Lorentz force moves pion orbit inwardSpin polarizability: “Pionic” Faraday effect