Compton Scattering at HIGS with Polarized Photons

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Compton Scattering at HIGS with Polarized Photons
Compton_at_HIGS Collaboration

• George Washington University
• Jerry Feldman
• Mark Sikora
• Duke University/TUNL
• Luke Myers
• Henry Weller
• Mohammad Ahmed
• Jonathan Mueller
• Seth Henshaw

• University of Kentucky
• Mike Kovash

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Outline

• What (and where) is HIGS?
• What have we done so far at HIGS?
• polarized Compton scattering study of IVGQR
• elastic Compton scattering on 6Li
• high energy (60-86 MeV) and low energy (3-5 MeV)
• What are we planning to do at HIGS?
• elastic Compton scattering on deuterium
• neutron polarizability
• polarized Compton scattering on proton
• proton electric polarizability
• double-polarized Compton scattering on proton
• proton spin polarizability

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Background Information on HIGS
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United States
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North Carolina
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Duke University
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TUNL
HIGS
TUNL
Triangle Universities Nuclear Laboratory
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Duke Free-Electron Laser Lab
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Storage Ring and Booster
Circularly and linearly polarized g rays, nearly
monoenergetic (Eg 290 MeV) Utilizes Compton
backscattering to generate g rays
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HIGS Photon Beam
to target room
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HIGS Photon Beam

• monoenergetic photons up to 90 MeV
• energy will reach 160 MeV by 2015
• 100 linear or circular polarization
• high photon beam intensity
• 107 Hz at 20-60 MeV
• 108 Hz below 15 MeV
• low beam-related background
• no bremsstrahlung typical of tagged photons

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Polarized Compton Scattering for IVGQR
Systematics
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Giant Resonances

• collective nuclear excitations
• GDR and ISGQR well known
• IVGQR poorly known
• photon as isovector probe

• use pol. photons for IVGQR
• map systematics vs. A
• nuclear symmetry energy
• neutron star eqn. of state

• ratio of H/V scattered photons is sensitive to
  E1/E2 interference
• sign difference in interference term at
  forward/backward angles

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Photon Asymmetry in IVGQR
E1/E2 interference
pure E1
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HINDA Array
HIGS NaI Detector Array
55o
55o
125o
125o
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Results for 209Bi
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Results for 89Y

• extend measurements to 89Y

• measured 124Sn last month!
• lease 142Nd target from ORNL for 15k
• other targets include A 56, 180, 238

89Y
preliminary
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Results for 124Sn
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IVGQR Systematics
89Y
124Sn
209Bi
Pitthan 1980
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Compton Scattering on 6Li
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World Data SetD(g,g)D

• Lucas Illinois (1994)
• Eg 49, 69 MeV
• Hornidge SAL (2000)
• Eg 85-105 MeV
• Lundin Lund (2003)
• Eg 55, 66 MeV

• Myers and Shonyozov
• (coming 2013)
• Illinois, GW, UK, Lund
• Eg 58-115 MeV

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EFT Fits to Deuteron Data
Griesshammer 2012
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Summary of Neutron Results

• Neutron scattering
• Schmiedmayer (91)

an 12.6 ? 1.5(stat) ? 2.0(syst)
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Experiment on 6Li at HIGS

• experiment motivation
• exploit higher nuclear cross section to measure
  a and b
• cross section scales as Z2, so factor of 9x
  higher than 2H
• solid 6Li target is simple
• provided by Univ. of Saskatchewan
• no previous Compton data on 6Li exists
  (except Pugh 1957)

• energies Eg 60, 86 MeV
• angles qg 40-160 (Dq 17)
• target solid 12.7 cm long 6Li cylinder (plus
  empty)
• detectors eight 10?12 NaIs (HINDA array)
• good photon energy resolution (DEg/Eg lt 5)

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HINDA Array
HIGS NaI Detector Array
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Experimental Setup
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Sample Spectra
Full and Empty Targets
Full ? Empty subtraction
6Li(g,g)6Li
Eg 60 MeV
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Cross Section for 16O(g,g)16O
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Cross Section for 6Li(g,g)6Li
L. Myers et al. Phys. Rev. C86 (2012)
Eg 60 MeV
sum rule a b 14.5
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sum rule a b 14.5
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Cross Section for 6Li(g,g)6Li
Eg 86 MeV
preliminary
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LIT Method for Compton Scattering
Bampa 2011
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Nuclear Polarizability of 6Li (and 4He?)
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Nuclear Polarizability

• nuclear polarizability affects energy levels of
  light atoms
• non-negligible corrections for high-precision
  tests of QED
• extraction of nuclear quantities from atomic
  spectroscopy
• nuclear charge radius from Lamb shift in muonic
  atoms
• usually determined from photoabsorption sum rule

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Nuclear Polarizability of 6Li
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q 55? f 90?
q 125? f 90?
6Li(g,g)6Li
Eg 3.0 MeV
q 55? f 0?
q 125? f 0?
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q 55? f 90?
q 125? f 90?
6Li(g,g)6Li
Eg 4.2 MeV
q 55? f 0?
q 125? f 0?
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Compton Scattering on the Proton and Deuteron
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Compton Scattering on Deuterium

• unpolarized photon beam and unpolarized
  deuterium target
• first use of our new LD2 cryogenic target
• scattering angles 45o, 80o, 115o, 150o (Eg
  65, 100 MeV)
• requires 300 hrs (65 MeV) 100 hrs (100 MeV)
• detectors eight 10?12 NaIs (HINDA array)
• arranged symmetrically left/right

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Cryogenic Target
LH2/LD2/LHe
(3.5 K ? 24 K)

• paid by GWU and TUNL
• procured from vendors
• assembled at HIGS
• first run Oct. 2013?

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HINDA Array
HIGS NaI Detector Array
55o
55o
125o
125o
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Sum-Rule-Independent Measurement of ap

• linearly polarized photon beam (unpolarized
  target)
• scintillating active target (detect recoils in
  coincidence)
• measure scattered photons at 90o (Eg 82 MeV)
• scattering cross section is independent of bp
• extraction of ap is independent of the Baldin
  sum rule
• extraction of ap is model-independent
• requires 300 hrs for 5 uncertainty in ap
• detectors four 10?12 NaIs (HINDA array)
• located left, right, up, down

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Sum-Rule-Independent Measurement of ap
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Polarizability of the Proton
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Scintillating Target
simulations R. Miskimen
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Nucleon Spin Polarizability
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Polarization Observables
Circular polarization
Circular polarization
Linear polarization
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Spin Polarizabilities of the Proton

• measure S2x for first determination of proton
  gE1E1
• circularly polarized photon beam
• scintillating active transverse polarized target
  (P 80)
• scattering angles 65o, 90o, 115o (Eg 100 MeV)
• requires 800 hrs for DgE1E1 ?1
• detectors eight 10?12 NaIs
• 4 in plane, 4 out of plane

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Spin Polarizabilities of the Proton
expand
simulations R. Miskimen
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(No Transcript)
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Summary

• Early measurements of Compton scattering at HIGS
• polarized A(g,g)A for A 89-209 (IVGQR
  systematics)
• 6Li(g,g)6Li at 60, 86 MeV (nucleon
  polarizability)
• polarized 6Li(g,g)6Li at 3.0-4.2 MeV (nuclear
  polarizability)
• Next generation of experiments on light nuclei
• D(g,g)D at 65 and 100 MeV (neutron
  polarizability)
• polarized p(g,g)p at 82 MeV (proton electric
  polarizability)
• polarized 4He(g,g)4He at 3-15 MeV (nuclear
  polarizability)
• double-polarized Compton scattering on
  proton/deuteron
• nucleon spin polarizability
• HIGS can contribute high-quality polarized data!
• stay tuned for further developments in the
  future

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Extra slides
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Phenomenological Formalism
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Cross-Section Ratios for Deuterium
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Level Scheme of 6Li
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Nuclear Polarizability
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Calculations by Trento Group

• photoabsorption on 6Li
• Lorentz Integral Transform method
• extend calculations to case of Compton
  scattering

Bacca 2002
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NaI Detectors
Paraffin n shield
10" ? 10" NaI core detector
3" thick optically isolated NaI shield segments
(8 in total)
Pb collimator
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q 55?
q 125?
Eg 3.0 MeV
q 55?
q 125?
Eg 4.2 MeV
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Nuclear Polarizability of 4He
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Nuclear Polarizability of 4He
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Compton Scattering with scintillating target
deuteron
proton
simulations R. Miskimen
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Nucleon Spin Polarizability

• classical analogy Faraday rotation of linearly
  polarized light in a spin-polarized medium
• four spin polarizabilities g1, ,g4
• forward spin polarizability g0 g1 g2
  2g4
• backward spin polarizability gp g1 g2
  2g4
• expt. asymmetries with circularly polarized
  photons
• Sx target spin ? photon helicity (in reaction
  plane)
• Sz target spin parallel to photon helicity