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Compton scattering & nucleon structure

Measurements of Double-Polarized Compton Scattering Asymmetries and a Study of the Proton Spin Structure at MAMI G.M.Gurevich (INR RAS) EMIN-2018 Moscow, October 8-11, 2018. Compton scattering & nucleon structure.

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Compton scattering & nucleon structure

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  1. Measurements of Double-Polarized Compton Scattering Asymmetries anda Study of the Proton Spin Structure at MAMI G.M.Gurevich (INR RAS) EMIN-2018 Moscow, October 8-11, 2018

  2. Compton scattering & nucleon structure In general, structure observables of the nucleons are experimentally accessible by the scattering of real photons from the nucleon in Real Compton Scattering (RCS). This process is best described using an effective Hamiltonian, expanded in terms of the incident photon energy. q′ p q p′ γ(q) + p(p) → γ(q′) + p(p′) G.M.Gurevich, EMIN-2018 2

  3. Compton scattering Hamiltonian (expansion in incident photon energy) G.M.Gurevich, EMIN-2018 3

  4. Compton scattering Hamiltonian (expansion in incident photon energy) • These internal structure constants are manifestations of the nucleon spin structure, which parameterize the “stiffness” of the nucleon’s spin against the electromagnetically induced deformations relative to the spin axis. Could be extracted from the measurements of RCS asymmetries. • To date, these have not been individually determined. However, two linear combinations of them have been. G.M.Gurevich, EMIN-2018 4

  5. Forward and backward spin polarizabilities G.M.Gurevich, EMIN-2018 5

  6. Spin polarizabilities - Measurement Use Compton scattering with polarization degrees of freedom (3 asymmetries): G.M.Gurevich, EMIN-2018 6

  7. Spin polarizabilities - Measurement Use Compton scattering with polarization degrees of freedom (3 asymmetries): G.M.Gurevich EMIN-2018 7

  8. Spin polarizabilities - Measurement Use Compton scattering with polarization degrees of freedom (3 asymmetries): G.M.Gurevich, EMIN-2018 8

  9. Polarized photon beam Nγ ~ 107 s-1 MeV-1 linear polarization circular polarization G.M.Gurevich, EMIN-2018 9

  10. Detecting system ■ Final-state particles were detected in the Crystal Balland TAPS detectors, both of which are outfittedwith charged particle identificationsystems. Togetherthese detectors cover 97% of 4π sr. ■The target was placed inside the aperture of the Crystal Ball detector. ■ Events wereselected where a single neutral and a single charged clusterof detectorelement hitswere observed incoincidence with an event inthephotontagger. G.M.Gurevich, EMIN-2018 10

  11. Vertex detector: 2 Cylindr. MWPCs 480 wires, 320stripes PID detector: 24 thin plastic detectors 4π Spectrometer Crystal Ball: 672 NaI detectors TAPS: 366 BaF2 detectors 72 PbWO4 detectors G.M.Gurevich, EMIN-2018 11

  12. 4π Spectrometer elements Crystal Ball NaI BaF2 MWPCs PID G.M.Gurevich, EMIN-2018 12

  13. Unpolarized proton target 10 cm liquid hydrogen target G.M.Gurevich, EMIN-2018 13

  14. Frozen spin polarized target (Dubna, Moscow, Mainz) Butanol C4H10O G.M.Gurevich, EMIN-2018

  15. Frozen spin polarized target (Dubna, Moscow, Mainz) General view of the target in the experimental hall ►DNP to achieve ~90% proton, ~80%deuteron polarization ►Relaxation time >2000 hours Polarization reversed approximately once per week to remove systematic errors G.M.Gurevich, EMIN-2018

  16. Cryostat background subtraction To remove backgrounds from the cryostat and fromthe non-hydrogen nucleons in the butanol target and He bath,separate running was performed on a carbon foam targetwith density 0.55 g/cm3. The density of the carbon foamwas such that a cylinder of identical geometric size tothe butanol target provided a close approximation to thenumber of non-hydrogen nucleons in the butanol target,allowing for a simple 1:1 subtraction accounting only fordifferences in luminosity. G.M.Gurevich, EMIN-2018 16

  17. New development: Active Polarized Target Cryostat insert G.M.Gurevich, EMIN-2018

  18. Active polarized target Material - polystyrene T=45mK, Proton polarization ~65%, relaxation time ~100 hours at 0.4 T G.M.Gurevich, EMIN-2018

  19. Active polarized target Detector electronics at 150 K G.M.Gurevich, EMIN-2018

  20. Active polarized target First count rate asymmetries from ϕdistribution forπ0production Positive target polarization Negative target polarization Eγ = 450 MeV G.M.Gurevich, EMIN-2018

  21. π0 photoproduction backgrounds The cross section for π0photoproduction is about 100 timesthat of Compton scattering in Δ(1232) region Events where a single neutral and a single charged cluster of detector element hits were observed in coincidence with an event in the photon tagger were selected as a Compton scattering G.M.Gurevich, EMIN-2018 21

  22. Background subtraction Experimental missing mass spectrum for θγ = 125 - 140° and Eγ = 285 - 305 MeV (blue). π0photoproduction (black). Green shows the final subtracted result. Two vertical lines represent the missing mass integrationlimit. G.M.Gurevich, EMIN-2018 22

  23. Σ2xasymmetry (theory and experiment) Σ2x for Eγ =273 – 303 MeV. The curves are from a dispersion theory calculation* with α, β, γ0, and γπ held fixed at their experimental values, and γM1M1 fixed at 2.9∙10-4 fm4. The green, blue, brown, red and magenta bands are for γE1E1 equal to –6.3, –5.3, –4.3, –3.3, and –2.3, respectively (in 10-4 fm4). The width of each band represents the propagated errors from α, β, γ0, and γπ combined in quadrature. * D. Drechsel, B. Pasquini, and M. Vanderhaeghen, Phys. Rep. 378, 99 (2003). 23 G.M.Gurevich, EMIN-2018

  24. Σ2xasymmetry (theory and experiment) G.M.Gurevich, EMIN-2018 24

  25. Σ2zasymmetry (theory and experiment) G.M.Gurevich, EMIN-2018 25

  26. Σ2zasymmetry (theory and experiment) G.M.Gurevich, EMIN-2018 26

  27. Σ3asymmetry (theory and experiment) Σ3 Asymmetry for incident energy range 297.0 ± 10.1 MeV. Curves from: B. Holstein, D. Drechsel, B. Pasquini, and M. VanderhaeghenPhys. Rev. C., vol. 61, 2000. V. Lensky and V. PascalutsaEur. Phys. J. C., vol. 65, 2010. G.M.Gurevich, EMIN-2018 27

  28. Σ3asymmetry (theory and experiment) Σ3 Asymmetry for incident energy range 277.1 ± 10.1 MeV. Curves from: B. Holstein, D. Drechsel, B. Pasquini, and M. VanderhaeghenPhys. Rev. C., vol. 61, 2000. V. Lensky and V. PascalutsaEur. Phys. J. C., vol. 65, 2010. G.M.Gurevich, EMIN-2018 28

  29. Extraction of spin polarizabilities ■In principle, one can measure two asymmetries, e.g. Σ2x and Σ3, and extract all four spin polartizabilities, using experimental values for αE1, βM1, γ0, γπ. Results contain model-dependent errors. ■ When all three asymmetries are measured at different energies and angles, a global χ2 fitting can be performed using the multipole basis γE1E1, γM1M1, γE1M2, γM1E2, to extract all four spin polarizabilities independently with small statistical, systematic and model-dependent errors. G.M.Gurevich, EMIN-2018 29

  30. Comparison with our previous results Polarizabilities in 10-4 fm4. The fitting performed using the fixed-t dispersion relation**. Fitting errors of the model to the data are shown. *G. Blanpied et al. (The LEGS Collaboration), Phys. Rev.C 64, 025203 (2001). ** D. Drechsel, B. Pasquini, and M. Vanderhaeghen, Phys. Rep. 378, 99 (2003). G.M.Gurevich, EMIN-2018

  31. Comparison with theory models G.M.Gurevich, EMIN-2018

  32. Summary ■ Three asymmetries of Compton scattering cross section Σ2x, Σ2z, and Σ3 measured in Δ(1232) region. ■Global analysis of 3 measured asymmetries completed and the values of four leading-order spin polarizabilities extracted. ■ The accuracies of the results are improved by a factor of two to four. ■First experiment with the Active polarized target performed. Analysis of first data is on the way. Measurements below π0 threshold are planned. G.M.Gurevich, EMIN-2018

  33. Thanks for your attention! G.M.Gurevich, EMIN-2018 33

  34. Backup

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