Discussion of the opportunities for precision electron beam polarimetry at the Electron-Ion Collider. Delivered at the International Workshop on Accelerator Science and Technology at Jefferson Lab on March 19, 2014.

1. Requirements Techniques New Concepts Conclusion
Sub-Percent Electron Polarimetry for the EIC
Wouter Deconinck
College of William & Mary
(Supported by the NSF under Grant No. PHY-1206053)
International Workshop on Accelerator Science and
Technology for Electron-Ion Collider 2014
March 19, 2014
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2. Requirements Techniques New Concepts Conclusion
Outline
Polarization Precision Requirements
Overview of Electron Polarimetry Techniques
New Concepts in Electron Polarimetry
Conclusion
2

3. Requirements Techniques New Concepts Conclusion
Outline
Polarization Precision Requirements
Luminosity Measurements
Overview of Electron Polarimetry Techniques
New Concepts in Electron Polarimetry
Conclusion
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4. Requirements Techniques New Concepts Conclusion
Polarization
The need for polarized electron and nucleon beams
• New frontiers in QCD:
• Correlations of nucleon spin with confined sea quark and gluons
• Spin-dependent structure functions gp
1 (x, Q2
)
• Access to generalized parton distributions H and E
• Fundamental symmetries at the intensity frontier
• Not achievable at any existing or other proposed facility
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5. Requirements Techniques New Concepts Conclusion
Polarization
The need for polarized electron and nucleon beams
• New frontiers in QCD:
• Correlations of nucleon spin with confined sea quark and gluons
• Spin-dependent structure functions gp
1 (x, Q2
)
• Access to generalized parton distributions H and E
• Fundamental symmetries at the intensity frontier
• Not achievable at any existing or other proposed facility
4

6. Requirements Techniques New Concepts Conclusion
Polarization
Relevant design parameters
• High luminosities of ≈ 1033−34 cm−2s−1
• High beam electron polarization ≈ 80% as injected (no
reliance on Sokolov-Ternov build-up)
• Longitudinal electron polarization at interaction point,
alternating for different bunches
• Variable center of mass energy from 20–100 GeV, even up to
150 GeV
• Possibility of multiple interaction regions, separated by spin
precession
• Strictest requirements on electron polarimetry from
fundamental symmetries program: significantly better than
1%, expected precision at 0.5%
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7. Requirements Techniques New Concepts Conclusion
Coupling of Polarization and Luminosity
Luminosity measurements
• Use bremsstrahlung ep → epγ as reference process
• Normally only γ are measured
• Reached 1-2% uncertainty at HERA using this method
Spin dependence of bremsstrahlung
• Bremsstrahlung cross section for polarized beam:
σ = σ0(1 + aPePp)
• Measured polarizations may limit precision of absolute and
relative Luminosity measurements
• Also will need to measure correlation between bunch current
and polarization
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8. Requirements Techniques New Concepts Conclusion
Outline
Polarization Precision Requirements
Overview of Electron Polarimetry Techniques
Mott Polarimetry
Møller Polarimetry
Compton Polarimetry
New Concepts in Electron Polarimetry
Conclusion
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9. Requirements Techniques New Concepts Conclusion
Electron Beam Polarimetry Techniques
Mott polarimetry: e + A → e + A
• Transverse spin-orbit coupling in high-Z elements
• Limited to low energies in the injector, few MeV
Møller polarimetry: e + e(Fe) → e + e
• Scattering off atomic electrons in magnetized iron foil
• Limited to separate, low current running (I ≈ 1 µA at JLab)
Compton polarimetry: e + γ → e + γ
• Compton scattering of electrons from circularly polarized laser
• Continuous, non-destructive, high precision measurements
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10. Requirements Techniques New Concepts Conclusion
Mott Polarimetry
Jefferson Lab’s 5 MeV Mott Polarimeter
• Located in injector, set spin launch angle for end stations
• Measures transverse polarization at low energies (3–8 MeV)
• Polarization P and normal to scattering plane n
• Scattering cross section σ = σ0(1 + S(θ) P · n), depends on
Sherman function S(θ), up-down asymmetry AUD ∝ S(θ) P
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11. Requirements Techniques New Concepts Conclusion
Mott Polarimetry
Jefferson Lab’s 5 MeV Mott Polarimeter
• Located in injector, set spin launch angle for end stations
• Measures transverse polarization at low energies (3–8 MeV)
• Polarization P and normal to scattering plane n
• Scattering cross section σ = σ0(1 + S(θ) P · n), depends on
Sherman function S(θ), up-down asymmetry AUD ∝ S(θ) P
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12. Requirements Techniques New Concepts Conclusion
Mott Polarimetry
Systematic uncertainties
• Limited by knowledge of effective Sherman function
• Contribution of 1.1% systematic uncertainty
• Improvements are currently being made:
• Reduction of backgrounds (e.g. scatter from beam dump
behind Au target)
• Improvements to theoretical description of Mott scattering
• Better Geant4 modeling incorporating the theoretical modeling
• Technique should allow for increased precision
THAF1131: “Precision Tests of Mott Polarimetry in the MeV
Energy Range,” Joe Grames, Thursday 08:30
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13. Requirements Techniques New Concepts Conclusion
Møller Polarimetry: JLab
JLab Hall C Møller
• High-field brute-force polarized pure iron foils in saturation
• Less sensitive to iron properties (Hall A adopted similar setup)
• No need to measure target polarization, calculation sufficient
• Bremsstrahlung background
• Coincidence detection
• Movable collimator system
• Lead-glass total absorption
detectors
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14. Requirements Techniques New Concepts Conclusion
Møller Polarimetry: JLab
Uncertainties of JLab Hall C Møller polarimeter
• Extrapolation to larger currents than Møller runs: 0.5%
• Levchuk effect = scattering of internal shell electrons: 0.33%
• Total systematic uncertainty of 0.85%
Relevance of Mott and Møller polarimetry to EIC
• Møller kicker system difficult in practice
• Limited to high precision measurements at injector
• Still need continuous measurements in interaction region
THAF1133: “Percent-Level Polarimetry in JLab Hall C,” Dave
Gaskell, Thursday 9:30
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15. Requirements Techniques New Concepts Conclusion
Compton Polarimetry: HERA
HERA: two electron polarimeters
• TPOL: position asymmetry in back-scattered photons for
transversely polarized electrons
• LPOL: energy-dependent cross-section asymmetry for
longitudinally polarized electrons
• 27.5 GeV
• Single pass
• 80 m for laser
• Laser analyzer
• Uncertainty
due to laser
transport
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16. Requirements Techniques New Concepts Conclusion
Compton Polarimetry: HERA
HERA: two electron polarimeters
• TPOL: position asymmetry in back-scattered photons for
transversely polarized electrons
• LPOL: energy-dependent cross-section asymmetry for
longitudinally polarized electrons
Polarization for stored bunches
• Uncertainty 1.4% at 27.5 GeV
• Beam-beam interaction
• Bunch by bunch at EIC?
• Single pass laser?
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17. Requirements Techniques New Concepts Conclusion
Compton Polarimetry: JLab
Compton polarimeter in Hall C (as used for Qweak
experiment)
• Beam: 150 µA at 1.165 GeV
• Chicane: interaction region 57 cm below straight beam line
• Laser system: 532 nm green laser
• 10 W CW laser with low-gain cavity
• Photons: PbWO4 scintillator in integrating mode
• Electrons: Diamond strips with 200 µm pitch
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18. Requirements Techniques New Concepts Conclusion
Compton Polarimetry: JLab
Uncertainties for Hall C system
• High current 180 µA, statistical precision easily reached
• Limited by uncertainty on laser polarization intra-cavity,
better than 1%
THAF1132: “Percent-Level Polarimetry in JLab Hall A,” Gregg
Franklin, Thursday 9:00
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19. Requirements Techniques New Concepts Conclusion
Compton vs. Møller Polarimetry
Compton-Møller-Compton in Hall C
• Excellent agreement of two polarimeters within quoted
uncertainties
THAF1133: “Percent-Level Polarimetry in JLab Hall C,” Dave
Gaskell, Thursday 9:30
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20. Requirements Techniques New Concepts Conclusion
Spin Dances at Jefferson Lab
Changing spin rotation in injector’s Wien filters
• Excellent consistency, better than 1%
• No full spin dance across the halls has been performed since
the Hall C Compton was installed
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21. Requirements Techniques New Concepts Conclusion
Compton vs. Møller Polarimetry Projections
Møller Polarimetry
• ee → ee (magnetized Fe)
• Low current because
temperature induces
demagnetization
• High asymmetry but low
target polarization
• Levchuk effect: scattering
off internal shell electrons
• Intermittent measurements
at different beam conditions
• Total systematics ∼ 0.65%
Compton Polarimetry
• eγ → eγ (polarized laser)
• Detection e and/or γ
• Only when beam energy
above few hundred MeV
• High photon polarization
but low asymmetry
• Total systematics ∼ 0.5%
• laser polarization
• detector linearity
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22. Requirements Techniques New Concepts Conclusion
Outline
Polarization Precision Requirements
Overview of Electron Polarimetry Techniques
New Concepts in Electron Polarimetry
Spin-Light Polarimetry
Atomic Hydrogen Møller Polarimetry
Conclusion
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23. Requirements Techniques New Concepts Conclusion
Spin-Light Polarimetry
Spin-dependence in synchrotron radiation
• Synchrotron light emitted in forward boosted direction ∝ γ4
• Sokolov-Ternov: spin-dependence in synchrotron light
emission (including a no-spin-flip term and a spin-flip term)
Concept for polarimeter
• Detect the spin dependence in differential ion chamber
• Sensitivity of differential ion chamber to spatial asymmetry
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24. Requirements Techniques New Concepts Conclusion
Spin-Light Polarimetry
Introduce 3-magnet wiggler to induce radiation
• Radiated power for transversely polarized beam: different
from unpolarized beam
• Radiated power for longitudinally polarized beam: different
above and below the orbital plane
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25. Requirements Techniques New Concepts Conclusion
Spin-Light Polarimetry
Conceptual design
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26. Requirements Techniques New Concepts Conclusion
Spin-Light Polarimetry
Calculated and simulated spectrum and asymmetry
• Red: polarization independent background
• Blue: spin-dependent “spin light”, 4–5 orders of magnitude
suppressed
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27. Requirements Techniques New Concepts Conclusion
Spin-Light Polarimetry
Small asymmetry and large deposited power
• Asymmetries of 10−5 are routinely measured by JLab PV
experiments (Qweak: 10−5 in 1 second), 1% statistical
precision in order of minutes
• Systematic uncertainty below 1%, dominated by
bremsstrahlung background dilution
• Large deposited power is challenging for high current and high
energy beams
THAF1134: “Spin-Light Polarimetry at the EIC,” Dipangkar
Dutta, Thursday 10:00
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28. Requirements Techniques New Concepts Conclusion
Atomic Hydrogen Polarimetry
Møller polarimetry
• 300 mK cold atomic H
• 8 T solenoid trap
• 3 · 1016 atoms/cm2
• 3 · 1015−17 atoms/cm3
• 100% polarization of e in
the atomic hydrogen
Advantages
• High beam currents
• No Levchuk effect
• Non-invasive, continuous
Reference: E. Chudakov, V. Luppov, IEEE Trans. on Nucl. Sc.
51, 1533 (2004).
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29. Requirements Techniques New Concepts Conclusion
Atomic Hydrogen Polarimetry: 100% e Polarization
Hyperfine Splitting in Magnetic Field
• Force (−µ · B) will pull
|a and |b into field
• Energy splitting of ∆E = 2µB:
↑ / ↓= exp(−∆E/kT) ≈ 10−14
• Low energy states with |sesp :
• |d = |↑⇑
• |c = cos θ |↑⇓ + sin θ |↓⇑
• |b = |↓⇓
• |a = cos θ |↓⇑ − sin θ |↑⇓
• with sin θ ≈ 0.00035
• Pe(↓) ≈ 1 with only 105 dilution
from |↑⇓ in |a at B = 8 T
• Pp(⇑) ≈ 0.06 because 53% |a
and 47% |b
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30. Requirements Techniques New Concepts Conclusion
Atomic Hydrogen Polarimetry: Projected Uncertainties
Projected Systematic Uncertainties ∆Pe
Source Fe-foil Hydrogen
Target polarization 0.63% 0.01%
Analyzing power 0.30% 0.10%
Levchuk effect 0.50% 0.00%
Deadtime 0.30% 0.10%
Background 0.30% 0.10%
Other 0.30% 0.00%
Unknown unknowns 0.00% 0.30%(?)
Total 1.0% 0.35%
• Big question: how will high current beam impact the stored
hydrogen? depolarization. . .
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31. Requirements Techniques New Concepts Conclusion
Outline
Polarization Precision Requirements
Overview of Electron Polarimetry Techniques
New Concepts in Electron Polarimetry
Conclusion
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32. Requirements Techniques New Concepts Conclusion
Polarization Measurements
Questions for accelerator experts to consider
• Does the polarization vary from bunch to bunch?
• Yes → was done at HERA, but needs a concept to measure
this in an ERL
• No → similar to current JLab setup, measure the mean of all
bunches
• Do the bunches have a polarization profile (transverse,
longitudinal)?
• Yes → how do we even measure this?
• No → similar to current JLab setup, measure a mean over the
entire bunch
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33. Requirements Techniques New Concepts Conclusion
Conclusion
Importance of sub-1% electron polarimetry at the EIC
• Redundant and precise electron polarimetry necessary for
QCD, certainly for physics beyond the Standard Model
Various approaches well-tested
• Mott and Møller polarimetry: injector and fixed target,
absolute precision
• Møller polarimetry: injector and fixed targets
• Compton polarimetry: high-power cavity or single-pass laser,
simultaneous electron/photon detection
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34. Requirements Techniques New Concepts Conclusion
Conclusion
Compton polarimetry efforts for EIC
• Single-pass laser (“HERA-like”) at interaction region (BNL)
• High gain cavity (“JLab-like”) in magnetic chicane (JLab)
New approaches to keep an eye on
• Spin-light polarimeter: challenging measurement, large power
synchrotron power deposit
• Atomic hydrogen Møller polarimetry: promising R&D project,
beam current may be too high
Thanks to Elke Aschenauer, Dave Gaskell, Marty McHugh,
Amrendra Narayan for figures.
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