1. University of Michigan
9/14/2007
Summary of EIC Electron Polarimetry Workshop August 23-24, hosted by the University of Michigan (Ann Arbor)
Wolfgang Lorenzon
University of Michigan
PSTP 2007
W. Lorenzon PSTP 2007

2. Goals of Workshop
Which design/physics processes are appropriate for EIC?
What difficulties will different design parameters present?
What is required to achieve sub-1% precision?
What resources are needed over next 5 years to achieve CD0 by the next Long Range Plan Meeting (2013?)
→ Exchange of ideas among experts in electron polarimetry and source & accelerator design to examine existing and novel electron beam polarization measurement schemes.
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3. Workshop Participants
First Name
Last Name
Affiliation
Kieran
Boyle
Stony Brook
Abhay
Deshpande
RIKEN-BNL / Stony Brook
Christoph
Montag
BNL
Brian
Ball
Michigan
Wouter
Deconinck
Avetik
Hayrapetyan
Wolfgang
Lorenzon
Eugene
Chudakov
Jefferson Lab
Dave
Gaskell
Joseph
Grames
Jeff
Martin
University of Winnipeg
Anna
Micherdzinska
Kent
Paschke
University of Virginia
Yuhong
Zhang
Wilbur
Franklin
MIT Bates
BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1
Accelerator/Source: Polarimetry: 12
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4. EIC Objectives e-p and e-ion collisions cm energies: 20–100 GeV
10 GeV (~3–20 GeV) electrons/positrons
250 GeV (~30–250 GeV) protons
100 GeV/u (~ GeV/u) heavy ions (eRHIC) / (~ GeV/u) light ions (3He)
Polarized lepton, proton and light ion beams
Longitudinal polarization at Interaction Point (IP): ~70% or better
Bunch separation: 3–35 ns
Luminosity: L(ep) ~ cm-2 s-1 per IP Goal: 50 fb-1 in 10 years
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5. Electron Ion Collider ELIC
Addition of a high energy polarized electron beam facility to the existing RHIC [eRHIC]
Addition of a high energy hadron/nuclear beam facility at Jefferson Lab [ELectron Ion Collider: ELIC]
will drastically enhance our ability to study fundamental and universal aspects of QCD
ELIC
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6. How to measure polarization of e-/e+ beams?
Macroscopic:
Polarized electron bunch: very weak dipole (~10-7 of magnetized iron of same size)
Microscopic:
spin-dependent scattering processes simplest → elastic processes:
- cross section large
- simple kinematic properties
- physics quite well understood
three different targets used currently:
1. e- - nucleus: Mott scattering – 300 keV (5 MeV: JLab) spin-orbit coupling of electron spin with (large Z) target nucleus
2. e - electrons: Møller (Bhabha) Scat MeV – GeV atomic electron in Fe (or Fe-alloy) polarized by external magnetic field
3. e  - photons: Compton Scattering > GeV laser photons scatter off lepton beam
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7. Electron Polarimetry
Many polarimeters are, have been in use, or a planned:
Compton Polarimeters:
LEP mainly used as machine tool for resonant depolarization
SLAC SLD 46 GeV
DESY HERA, storage ring 27.5 GeV (three polarimeters)
JLab Hall A < 8 GeV / Hall C < 12 GeV
Bates South Hall Ring < 1 GeV
Nikhef AmPS, storage ring < 1 GeV
Møller / Bhabha Polarimeters:
Bates linear accelerator < 1 GeV
Mainz Mainz Microtron MAMI < 1 GeV
Jlab Hall A, B, C
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8. Compton vs Moller Polarimetry
-7/9
Jlab
HERA
EIC
532 nm
HERA (27.5 GeV)
EIC (10 GeV)
Compton edge:
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9. Polarimeter Roundup Laboratory Polarimeter Relative precision
Dominant systematic uncertainty
JLab
5 MeV Mott
~1%
Sherman function
Hall A Møller
~2-3%
target polarization
Hall B Møller
1.6% (?)
target polarization, Levchuk effect
Hall C Møller
0.5% (→1.3%)
target polarization, Levchuk effect, high current extrapolation
Hall A Compton
1% > 3 GeV)
detector acceptance + response
HERA
LPol Compton
1.6%
analyzing power
TPol Compton
3.1%
focus correction + analyzing power
Cavity LPol Compton ?
still unknown
MIT-Bates
Mott
~2%
Sherman function + detector response
Transmission
>5%
Compton
~3-4%
SLAC
0.5%

10. The “Spin Dance” Experiment (2000)
Phys. Rev. ST Accel. Beams 7, (2004)
Source
Strained GaAs photocathode (l = 850 nm, Pb >75 %)
Accelerator
5.7 GeV, 5 pass recirculation
Wien filter in injector was varied from -110o to 110o to vary degree of longitudinal polarization in each hall
→ precise cross-comparison of JLab polarimeters
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11. Polarization Results
Results shown include statistical errors only
→ some amplification to account for non-sinusoidal behavior
Statistically significant disagreement
Systematics shown:
Mott
Møller C 1%
Compton
Møller B %
Møller A 3%
Even including systematic errors, discrepancy still significant

12. Additional Cross-Hall Comparisons (2006)
During G0 Backangle, performed “mini-spin dance” to ensure purely longitudinal polarization in Hall C
Hall A Compton was also online use, so they participated as well
Relatively good agreement between Hall C Møller and Mott and between Hall C Møller and Compton

13. Lessons Learned
Include polarization diagnostics and monitoring in beam lattice design
minimize bremsstrahlung and synchrotron radiation
Measure beam polarization continuously
protects against drifts or systematic current-dependence to polarization
Providing/proving precision at 1% level very challenging
Multiple devices/techniques to measure polarization
cross-comparisons of individual polarimeters are crucial for testing systematics of each device
at least one polarimeter needs to measure absolute polarization, others might do relative measurements
Compton Scattering
advantages: laser polarization can be measured accurately – pure QED – non-invasive, continuous monitor – backgrounds easy to measure – ideal at high energy / high beam currents
disadvantages: at low beam currents: time consuming – at low energies: small asymmetries – systematics: energy dependent
Møller Scattering
advantages: rapid, precise measurements – large analyzing power – high B field Fe target: ~0.5% systematic errors
disadvantages: destructive – low currents only – target polarization low (Fe foil: 8%) – Levchuk eff.
New ideas are always welcome!
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14. New Ideas
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15. New Fiber Laser Technology (Hall C)
Jeff Martin
Electron Beam
LaserBeam
30 ps pulses at 499 MHz
Gain switched
external to beamline vacuum (unlike Hall A cavity) → easy access
excellent stability, low maintenance
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16. Electron Polarimetry
Kent Paschke
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17. Hybrid Electron Compton Polarimeter with online self-calibration
W. Deconinck, A. Airapetian
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18. Summary
Electron beam polarimetry between 3 – 20 GeV seems possible at 1% level: no apparent show stoppers (but not easy)
Imperative to include polarimetry in beam lattice design
Use multiple devices/techniques to control systematics
Issues:
crossing frequency 3–35 ns: very different from RHIC and HERA
beam-beam effects (depolarization) at high currents
crab-crossing of bunches: effect on polarization, how to measure it?
measure longitudinal polarization only, or transverse needed as well
polarimetry before, at, or after IP
dedicated IP, separated from experiments?
Workshop attendees agreed to be part of e-pol task force
W. Lorenzon coordinator of initial activities and directions
design efforts and simulations just started
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