1. Summary of EIC Electron Polarimetry Workshop August 23-24, 2007 hosted by the University of Michigan (Ann Arbor) http://eic.physics.lsa.umich.edu/ Wolfgang Lorenzon University of Michigan 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. 9/14/20072W. Lorenzon PSTP 2007

3. Workshop Participants 9/14/20073W. Lorenzon PSTP 2007 First NameLast NameAffiliation Kieran *BoyleStony Brook AbhayDeshpandeRIKEN-BNL / Stony Brook ChristophMontagBNL Brian * BallMichigan Wouter * DeconinckMichigan Avetik * HayrapetyanMichigan WolfgangLorenzonMichigan EugeneChudakovJefferson Lab DaveGaskellJefferson Lab JosephGramesJefferson Lab JeffMartinUniversity of Winnipeg Anna * MicherdzinskaUniversity of Winnipeg KentPaschkeUniversity of Virginia YuhongZhangJefferson Lab WilburFranklinMIT Bates BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1 Accelerator/Source: 3 / Polarimetry: 12 / students/postdocs (*): 5

4. EIC Objectives e-p and e-ion collisions c.m. energies: 20 - 100 GeV – 10 GeV (~3 - 20 GeV) electrons/positrons – 250 GeV (~30 - 250 GeV) protons – 100 GeV/u (~50-100 GeV/u) heavy ions (eRHIC) / (~15-170 GeV/u) light ions ( 3 He) Polarized lepton, proton and light ion beams Longitudinal polarization at Interaction Point (IP): ~70% or better Bunch separation: 3 - 35 ns Luminosity: L(ep) ~10 33 - 10 34 cm -2 s -1 per IP Goal: 50 fb -1 in 10 years 9/14/20074W. Lorenzon PSTP 2007

5. Electron Ion Collider 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 9/14/20075W. Lorenzon PSTP 2007 ELIC

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 30 – 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 9/14/20076W. Lorenzon PSTP 2007

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 9/14/20077W. Lorenzon PSTP 2007

8. 9/14/20078W. Lorenzon PSTP 2007 532 nm HERA (27.5 GeV) EIC (10 GeV) Jlab HERA EIC -7/9 Compton edge: Compton vs Møller Polarimetry

9. LaboratoryPolarimeterRelative precisionDominant systematic uncertainty JLab5 MeV Mott~1%Sherman function Hall A Møller~2-3%target polarization Hall B Møller1.6% (?)target polarization, Levchuk effect Hall C Møller1.3% (best quoted) 0.5% (possible ?) target polarization, Levchuk effect, high current extrapolation Hall A Compton1% (@ > 3 GeV)detector acceptance + response HERALPol Compton1.6%analyzing power TPol Compton3.1%focus correction + analyzing power Cavity LPol Compton?still unknown MIT-BatesMott~3%Sherman function + detector response Transmission>4%analyzing power Compton~4%analyzing power SLACCompton0.5%analyzing power Polarimeter Roundup

10. The “Spin Dance” Experiment (2000) Source Strained GaAs photocathode (  = 850 nm, P b >75 %) Accelerator 5.7 GeV, 5 pass recirculation Wien filter in injector was varied from -110 o to 110 o to vary degree of longitudinal polarization in each hall → precise cross-comparison of JLab polarimeters 9/14/200710W. Lorenzon PSTP 2007 Phys. Rev. ST Accel. Beams 7, 042802 (2004)

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 1.6% 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-April 2006 Spin Dance Summary

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 effect New ideas are always welcome! 9/14/200713W. Lorenzon PSTP 2007

14. New Ideas 9/14/200714W. Lorenzon PSTP 2007

15. New Fiber Laser Technology (Hall C) 9/14/200715W. Lorenzon PSTP 2007 30 ps pulses at 499 MHz - external to beamline vacuum (unlike Hall A cavity) → easy access - excellent stability, low maintenance Electron BeamLaserBeam Jeff Martin Gain switched

16. Electron Polarimetry 9/14/200716W. Lorenzon PSTP 2007 Kent Paschke

17. Hybrid Electron Compton Polarimeter with online self-calibration 9/14/200717W. Lorenzon PSTP 2007 W. Deconinck, A. Airapetian

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 9/14/200718W. Lorenzon PSTP 2007