MEASUREMENT OF THE PROTON SPIN POLARIZABILITIES AT MAMI G.M.Gurevich, V.P.Lisin (INR RAS) EMIN 2015 Moscow, October 7, 2015 1 1
Outline ● Motivation ● Compton scattering & nucleon polarizabilities ● Experiment ● Results ● Summary Measurement of the proton spin polarizabilities at MAMI 2
Motivation Electromagnetic polarizabilities are fundamental properties of composite systems such as molecules, atoms, nuclei, and hadrons. They describe the response of the system to applied electric or magnetic field. Whereas magnetic moments provide information about the ground-state properties of a system, polarizabilities provide information about the excited states of the system. ■ Polarizability of the ideally conducting sphere ~1/4 volume. ■ Polarizability of the hydrogen atom ~1/10 volume. ■ For hadrons, polarizabilities are much smaller than the volume, typically of order 10 -4 fm 3 , because of the greater strength of the QCD force as compared to the electromagnetic force. ■ Measurements of polarizabilities provide an essential test for theories of hadron structure and QCD. Measurement of the proton spin polarizabilities at MAMI 3
Compton scattering & nucleon polarizabilities Hadron polarizabilities are best measured in Compton scattering experiments, where the polarizabilities cause a deviation of the cross section from the prediction of Compton scattering from a structureless Dirac particle. q ′ p q p ′ γ(q) + p(p) → γ(q′) + p(p′) Measurement of the proton spin polarizabilities at MAMI 4
Compton scattering Hamiltonian (expansion in incident photon energy) Measurement of the proton spin polarizabilities at MAMI 5
Scalar polarizabilities Proton between charged parallel plates: Proton between poles of a magnet: d ind = 4 πα E m ind = 4 πβ B α = 12 ± 0.6 ∙ 10 -4 fm 3 β = 1.9 ± 0.5 ∙ 10 -4 fm 3 Measurement of the proton spin polarizabilities at MAMI 6
Compton scattering Hamiltonian (expansion in incident photon energy) Subscripts give multipolarities of incident and scattered photons • Spin-polarizabilities describe the response of the nucleon spin to an incident polarized photon. Analogous to a classical Faraday effect. • To date, these have not been individually determined. However, two linear combinations of them have been. Measurement of the proton spin polarizabilities at MAMI 7
Forward and backward spin polarizabilities Measurement of the proton spin polarizabilities at MAMI 8
Spin polarizabilities of the proton – Theory Extracting the proton spin polarizabilities would provide the useful test of nucleon structure. χ EFT γ L χ ChPT1 ChPT2 K-matrix DR1 DR2 DR3 E1E1 -5.4 -1.3 -4.8 -3.7 -1.1 -3.4 -4.3 -3.8 M1M1 1.4 3.3 3.5 2.5 2.2 2.7 2.9 2.9 E1M2 1.0 0.2 1.8 1.2 -0.4 0.3 -0.02 0.5 M1E2 1.0 1.8 1.1 1.2 1.9 1.9 2.2 1.6 0 1.9 -3.9 2.0 -1.2 -2.6 -1.5 -0.8 -1.1 π 6.8 6.1 11.2 6.1 5.6 7.8 9.4 7.8 Values of proton spin polarizabilities (in units 10 -4 fm 4 ): ChPT – Chiral Perturbation Theory calculations, L χ - chiral Lagrangian calculations, χ EFT – chiral effective field theory calculations, DR – Dispersion Relation calculations. Measurement of the proton spin polarizabilities at MAMI 9
Spin polarizabilities - Measurement Use Compton scattering with polarization degrees of freedom (3 asymmetries): Measurement of the proton spin polarizabilities at MAMI 10
Spin polarizabilities - Measurement Use Compton scattering with polarization degrees of freedom (3 asymmetries): Measurement of the proton spin polarizabilities at MAMI 11
Spin polarizabilities - Measurement Use Compton scattering with polarization degrees of freedom (3 asymmetries): Measurement of the proton spin polarizabilities at MAMI 12
Status of the experiment A2 collaboration measured 3 asymmetries of the Compton scattering on the proton in Δ (1232)-resonance region: ■ Σ 2x (circularly pol. beam, transversely pol. Butanol target): ~550 hours of measurement (Sept. 2010 & February 2011) ■ Σ 2z (circularly pol. beam, longitudinally pol. Butanol target): ~1000 hours of measurement (April – May 2014, July 2015) ■ Σ 3 (linearly pol. beam, liquid hydrogen target): ~500 hours of measurement (Dec. 2012, May – June 2013) Measurement of the proton spin polarizabilities at MAMI 13
MAMI electron accelerator Measurement of the proton spin polarizabilities at MAMI 14
Upgraded A2 Tagging system (Glasgow, Mainz) N γ ~ 10 7 s -1 MeV -1 linear circular polatization polarization Measurement of the proton spin polarizabilities at MAMI 15
Unpolarized proton target 10 cm liquid hydrogen target Measurement of the proton spin polarizabilities at MAMI 16
Frozen spin polarized target (Dubna, Moscow, Mainz) Butanol C 4 H 10 O Measurement of the proton spin polarizabilities at MAMI 17
Frozen spin polarized target (Dubna, Moscow, Mainz) General view of the target in the experimental hall ► DNP to achieve ~90% proton, Polarization reversed approximately ~80% deuteron polarization once per week to remove systematic errors ► Relaxation time >2000 hours Measurement of the proton spin polarizabilities at MAMI 18
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. Measurement of the proton spin polarizabilities at MAMI 19
Detecting system ■ Final-state particles were detected in the Crystal Ball and TAPS detectors, both of which are outfitted with charged particle identication systems. Together these detectors cover 97% of 4 π 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. Measurement of the proton spin polarizabilities at MAMI 20
4 π Spectrometer TAPS: Crystal Ball: 366 BaF 2 detectors 672 NaI detectors 72 PbWO 4 detectors Max. kin. energy: m +- : 233MeV Max. kin. energy: p +- : 180 MeV p +- : 240 MeV K +- : 280 MeV K +- : 341 MeV P : 360 MeV P : 425 MeV Vertex detector: 2 Cylindr. MWPCs 480 wires, 320stripes PID detector: 24 thin plastic Measurement of the proton spin polarizabilities at MAMI 21 detectors
4 π Spectrometer elements Crystal Ball NaI BaF 2 MWPCs PID Measurement of the proton spin polarizabilities at MAMI 22
π 0 photoproduction backgrounds The cross section for π 0 photoproduction is about 100 times that of Compton scattering in Δ (1232) region Measurement of the proton spin polarizabilities at MAMI 23
Background subtraction Missing mass for E γ =273 – 303 MeV, and θ γ =100 – 120 . The different colors indicate the various backgrounds in the missing-mass distribution. Light blue is for tagger accidentals. Blue is for carbon/cryostat background. Magenta, red and yellow were constructed from data to mimic where a π 0 decay photon was lost in the upstream CB hole, the region between CB and TAPS, and the downstream TAPS hole, respectively. Green shows the final subtracted result. Measurement of the proton spin polarizabilities at MAMI 24
Background subtraction Missing mass for E γ = 273 – 303 MeV, and θ γ = 100 – 120 after removing the background contribution. Red line indicates the missing-mass cut value used in the analysis. Measurement of the proton spin polarizabilities at MAMI 25
Σ 2x asymmetry For a given incoming photon energy E, Compton scattering polar angle θ , and azimuthal angle Φ relative to the target polarization direction, the asymmetry Σ 2x was formed by: where P T is the target polarization, P γ is the beam polarization, and N R (N L ) are the counts in the specified bin with a right (left) helicity beam. Measurement of the proton spin polarizabilities at MAMI 26
Σ 2x asymmetry (theory and experiment) Σ 2x for E γ =273 – 303 MeV. The curves are from a dispersion theory calculation* with α, β, γ 0 , and γ π held fixed at their experimental values, and γ M1M1 fixed at 2.9. The green, blue, brown, red and magenta bands are for γ E1E1 equal to – 6.3, – 5.3, – 4.3, – 3.3, and – 2.3, respectively. The width of each band represents the propagated errors from α, β, γ 0 , and γ π combined in quadrature. * D. Drechsel, B. Pasquini, and M. Vanderhaeghen, Phys. Rep. 378, 99 (2003). 27 Measurement of the proton spin polarizabilities at MAMI
Σ 2x asymmetry (theory and experiment) Measurement of the proton spin polarizabilities at MAMI 28
Σ 2z asymmetry (theory and experiment) Measurement of the proton spin polarizabilities at MAMI 29
Σ 2z asymmetry (theory and experiment) Measurement of the proton spin polarizabilities at MAMI 30
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