EMIN 2015 Moscow, October 7, 2015 1 1 Outline Motivation - - PowerPoint PPT Presentation

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G.M.Gurevich, V.P.Lisin (INR RAS) EMIN 2015 Moscow, October 7, 2015 MEASUREMENT OF THE PROTON SPIN POLARIZABILITIES AT MAMI

Measurement of the proton spin polarizabilities at MAMI 2

Outline

• Motivation
• Compton scattering & nucleon polarizabilities
• Experiment
• Results
• Summary

Measurement of the proton spin polarizabilities at MAMI 3

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

• f order 10-4 fm3, 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 4

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.

γ(q) + p(p) → γ(q′) + p(p′)

q p p′ q′

Measurement of the proton spin polarizabilities at MAMI 5

Compton scattering Hamiltonian (expansion in incident photon energy)

Measurement of the proton spin polarizabilities at MAMI 6

Scalar polarizabilities

Proton between charged parallel plates:

dind = 4παE α = 12 ± 0.6 ∙ 10-4 fm3

Proton between poles of a magnet:

mind = 4πβB

β = 1.9 ± 0.5 ∙ 10-4 fm3

Measurement of the proton spin polarizabilities at MAMI 7

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 8

Forward and backward spin polarizabilities

Measurement of the proton spin polarizabilities at MAMI 9

Spin polarizabilities of the proton – Theory

γ ChPT1 ChPT2 K-matrix Lχ

χEFT

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 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 fm4): ChPT – Chiral Perturbation Theory calculations, Lχ - chiral Lagrangian calculations, χEFT – chiral effective field theory calculations, DR – Dispersion Relation calculations.

Extracting the proton spin polarizabilities would provide the useful test of nucleon structure.

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

Spin polarizabilities - Measurement

Use Compton scattering with polarization degrees of freedom (3 asymmetries):

Measurement of the proton spin polarizabilities at MAMI

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Status of the experiment A2 collaboration measured 3 asymmetries of the Compton scattering

• n the proton in Δ(1232)-resonance region:

■ Σ2x (circularly pol. beam, transversely pol. Butanol target): ~550 hours of measurement (Sept. 2010 & February 2011) ■ Σ2z (circularly pol. beam, longitudinally pol. Butanol target): ~1000 hours of measurement (April – May 2014, July 2015) ■ Σ3 (linearly pol. beam, liquid hydrogen target): ~500 hours of measurement (Dec. 2012, May – June 2013)

Measurement of the proton spin polarizabilities at MAMI 14

MAMI electron accelerator

Measurement of the proton spin polarizabilities at MAMI 15

Upgraded A2 Tagging system (Glasgow, Mainz)

circular polarization linear polatization

Nγ ~ 107 s-1 MeV-1

Measurement of the proton spin polarizabilities at MAMI 16

Unpolarized proton target

10 cm liquid hydrogen target

Measurement of the proton spin polarizabilities at MAMI 17

Butanol C4H10O

Frozen spin polarized target (Dubna, Moscow, Mainz)

Measurement of the proton spin polarizabilities at MAMI 18

Frozen spin polarized target (Dubna, Moscow, Mainz)

General view of the target in the experimental hall Polarization reversed approximately

• nce per week to remove systematic

errors ►DNP to achieve ~90% proton, ~80% deuteron polarization ►Relaxation time >2000 hours

Measurement of the proton spin polarizabilities at MAMI 19

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/cm3. 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 20

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 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.

Measurement of the proton spin polarizabilities at MAMI 21

4π Spectrometer TAPS: 366 BaF2 detectors 72 PbWO4 detectors

• Max. kin. energy:

p+- : 180 MeV K+- : 280 MeV P : 360 MeV Crystal Ball: 672 NaI detectors

• Max. kin. energy:

m+- : 233MeV p+- : 240 MeV K+- : 341 MeV P : 425 MeV Vertex detector: 2 Cylindr. MWPCs 480 wires, 320stripes PID detector: 24 thin plastic detectors

Measurement of the proton spin polarizabilities at MAMI 22

4π Spectrometer elements Crystal Ball MWPCs PID

NaI BaF2

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π0 photoproduction backgrounds

The cross section for π0 photoproduction is about 100 times that of Compton scattering in Δ(1232) region

Measurement of the proton spin polarizabilities at MAMI 24

Background subtraction

Missing mass for Eγ=273 – 303 MeV, and θγ=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 π0 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 25

Background subtraction

Missing mass for Eγ = 273 – 303 MeV, and θγ = 100 – 120 after removing the background contribution. Red line indicates the missing-mass cut value used in the analysis.

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Σ2x 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 Σ2x was formed by: where PT is the target polarization, Pγ 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

Measurement of the proton spin polarizabilities at MAMI 27

Σ2x asymmetry (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. 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. The width

• f 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).

Measurement of the proton spin polarizabilities at MAMI

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Σ2x asymmetry (theory and experiment)

Measurement of the proton spin polarizabilities at MAMI

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Σ2z asymmetry (theory and experiment)

Measurement of the proton spin polarizabilities at MAMI

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Σ2z asymmetry (theory and experiment)

Measurement of the proton spin polarizabilities at MAMI

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Σ3 asymmetry (theory and experiment)

Σ3 Asymmetry for incident energy range 297.0 ± 10.1 MeV.

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.

Measurement of the proton spin polarizabilities at MAMI 32

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.

Measurement of the proton spin polarizabilities at MAMI 33

Spin polarizabilities – Theory and Experiment γ

ChP T1 ChP T2 K- matrix Lχ χEFT DR1 DR2 DR3 Experiment with LEGS Σ3 data* Experiment with Mainz Σ3 data** E1E1

• 5.4
• 1.3
• 4.8
• 3.7
• 1.1
• 3.4
• 4.3
• 3.8
• 3.5 ± 1.2

−5.0 ± 1.5 M1M1 1.4 3.3 3.5 2.5 2.2 2.7 2.9 2.9 3.16 ± 0.85 3.13 ± 0.88 E1M2 1.0 0.2 1.8 1.2

• 0.4

0.3

• 0.02

0.5

• 0.7 ± 1.2

1.7 ± 1.7 M1E2 1.0 1.8 1.1 1.2 1.9 1.9 2.2 1.6 1.99 ± 0.29 1.26 ± 0.43

Experimental results for the multipole basis spin-polarizabilities are from the combined analysis of Σ2x (Mainz data) and Σ3 (LEGS* or Mainz data**) asymmetries using a dispersion model calculation***. The size of the experimental uncertainties is too large to discriminate between these various models. One of the largest uncertainties in the analysis is due to the error on γπ, ±1.8. The data on the asymmetry Σ2z, should reduce the uncertainty on γπ by a factor of approximately four.

* G. Blanpied et al. (The LEGS Collaboration), Phys. Rev.C 64, 025203 (2001). ** Preliminary. *** D. Drechsel, B. Pasquini, and M. Vanderhaeghen, Phys. Rep. 378, 99 (2003).

Measurement of the proton spin polarizabilities at MAMI 34

Summary

Done: ■ Three asymmetries of Compton scattering cross section Σ2x, Σ2z, and Σ3 were measured in Δ(1232) region ■ Values of all four spin polarizabilities were obtained from the analyses of Σ2x and Σ3 data In progress: ■ Combined analysis of 3 measured asymmetries ■ Allows discrimination between various models Perspectives: ■ Measurements below π0-meson threshold – most model- independent extraction of SP’s ■ Requires recoil proton detection with low energy threshold and high efficiency ■ Solution: Active (scintillating) polarized target R&D in progress

Measurement of the proton spin polarizabilities at MAMI 35

Thank you!