Measurements of DoublePolarized Compton Scattering Asymmetries and a

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 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′ q p p′ γ(q) + p(p) → γ(q′) + p(p′) G. M. Gurevich, EMIN-2018 2

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

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

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

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

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

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

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

Detecting system ■ Final-state particles were detected in the Crystal Ball and TAPS detectors, both of which are outfitted with charged particle identification systems. Together these detectors cover 97% of 4π sr. ■ The target was placed inside the aperture of the Crystal Ball detector. ■ Events were selected where a single neutral and a single charged cluster of detector element hits were observed in coincidence with an event in the photon tagger. G. M. Gurevich, EMIN-2018 10

4π Spectrometer Crystal Ball: 672 Na. I detectors TAPS: 366 Ba. F 2 detectors 72 Pb. WO 4 detectors G. M. Gurevich, EMIN-2018 Vertex detector: 2 Cylindr. MWPCs 480 wires, 320 stripes PID detector: 24 thin plastic 11 detectors

4π Spectrometer elements Crystal Ball Na. I Ba. F 2 MWPCs PID G. M. Gurevich, EMIN-2018 12

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

Frozen spin polarized target (Dubna, Moscow, Mainz) Butanol C 4 H 10 O 14 G. M. Gurevich, EMIN-2018

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 15

Cryostat background subtraction To remove backgrounds from the cryostat and from the non-hydrogen nucleons in the butanol target and He bath, separate running was performed on a carbon foam target with density 0. 55 g/cm 3. The density of the carbon foam was such that a cylinder of identical geometric size to the butanol target provided a close approximation to the number of non-hydrogen nucleons in the butanol target, allowing for a simple 1: 1 subtraction accounting only for differences in luminosity. G. M. Gurevich, EMIN-2018 16

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

Active polarized target Material - polystyrene T=45 m. K, Proton polarization ~65%, relaxation time ~100 hours at 0. 4 T G. M. Gurevich, EMIN-2018 18

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

Active polarized target First count rate asymmetries from ϕ distribution for π0 production Positive target polarization Negative target polarization Eγ = 450 Me. V G. M. Gurevich, EMIN-2018 20

π0 photoproduction backgrounds The cross section for π0 photoproduction is about 100 times that 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

Background subtraction • Experimental missing mass spectrum for θγ = 125 - 140° and Eγ = 285 - 305 Me. V (blue). π0 photoproduction (black). Green shows the final subtracted result. Two vertical lines represent the missing mass integration limit. G. M. Gurevich, EMIN-2018 22

Σ 2 x asymmetry (theory and experiment) Σ 2 x for Eγ =273 – 303 Me. V. The curves are from a dispersion theory calculation* with α, β, γ 0, and γπ held fixed at their experimental values, and γM 1 M 1 fixed at 2. 9∙ 10 -4 fm 4. The green, blue, brown, red and magenta bands are for γE 1 E 1 equal to – 6. 3, – 5. 3, – 4. 3, – 3. 3, and – 2. 3, respectively (in 10 -4 fm 4). The width of each band represents the propagated errors from α, β, γ 0, and γπ combined in quadrature. * D. Drechsel, B. Pasquini, and M. Vanderhaeghen, Phys. Rep. 378, 99 (2003). G. M. Gurevich, EMIN-2018 23

Σ 2 x asymmetry (theory and experiment) G. M. Gurevich, EMIN-2018 24

Σ 2 z asymmetry (theory and experiment) G. M. Gurevich, EMIN-2018 25

Σ 2 z asymmetry (theory and experiment) G. M. Gurevich, EMIN-2018 26

Σ 3 asymmetry (theory and experiment) Σ 3 Asymmetry for incident energy range 297. 0 ± 10. 1 Me. V. Curves from: B. Holstein, D. Drechsel, B. Pasquini, and M. Vanderhaeghen Phys. Rev. C. , vol. 61, 2000. V. Lensky and V. Pascalutsa Eur. Phys. J. C. , vol. 65, 2010. G. M. Gurevich, EMIN-2018 27

Σ 3 asymmetry (theory and experiment) Σ 3 Asymmetry for incident energy range 277. 1 ± 10. 1 Me. V. Curves from: B. Holstein, D. Drechsel, B. Pasquini, and M. Vanderhaeghen Phys. Rev. C. , vol. 61, 2000. V. Lensky and V. Pascalutsa Eur. Phys. J. C. , vol. 65, 2010. G. M. Gurevich, EMIN-2018 28

Extraction of spin polarizabilities ■ In principle, one can measure two asymmetries, e. g. Σ 2 x and Σ 3, and extract all four spin polartizabilities, using experimental values for αE 1, βM 1, γ 0, 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 γE 1 E 1, γM 1 M 1, γE 1 M 2, γM 1 E 2, to extract all four spin polarizabilities independently with small statistical, systematic and model-dependent errors. G. M. Gurevich, EMIN-2018 29

Comparison with our previous results Data Sets Polarizability γE 1 E 1 γM 1 M 1 γE 1 M 2 γM 1 E 2 Σ 2 x(MAMI) + Σ 3(LEGS*) Σ 2 x(MAMI) + Σ 3(MAMI) Σ 2 x(MAMI) + Σ 2 z(MAMI) + Σ 3(MAMI) -3. 5± 1. 2 3. 16± 0. 85 -0. 7± 1. 2 1. 99± 0. 29 -5. 0± 1. 5 3. 13± 0. 88 1. 7± 1. 7 1. 26± 0. 43 -4. 24± 0. 39 3. 25± 0. 40 0. 76± 0. 83 1. 24± 0. 39 Polarizabilities in 10 -4 fm 4. 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 30

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

Summary ■ Three asymmetries of Compton scattering cross section Σ 2 x, Σ 2 z, and Σ 3 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 32

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

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