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Wolfgang Lorenzon University of Michigan PSTP 2007

Wolfgang Lorenzon University of Michigan PSTP 2007. Summary of EIC Electron Polarimetry Workshop August 23-24, 2007 hosted by the University of Michigan (Ann Arbor) http://eic.physics.lsa.umich.edu/. Goals of Workshop. Which design/physics processes are appropriate for EIC?

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Wolfgang Lorenzon University of Michigan PSTP 2007

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

  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. W. Lorenzon PSTP 2007

  3. Workshop Participants BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1 Accelerator/Source: 3 Polarimetry: 12 W. Lorenzon PSTP 2007

  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 (~50-100 GeV/u) heavy ions (eRHIC) / (~15-170 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) ~1033-34 cm-2 s-1 per IP Goal: 50 fb-1 in 10 years W. 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 ELIC W. Lorenzon PSTP 2007

  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 100 – 300 keV (5 MeV: JLab)spin-orbit coupling of electron spin with (large Z) target nucleus 2. e - electrons: Møller (Bhabha) Scat. MeV – GeVatomic electron in Fe (or Fe-alloy) polarized by external magnetic field 3. e  - photons: Compton Scattering > GeVlaser photons scatter off lepton beam W. 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 W. Lorenzon PSTP 2007

  8. Compton vs Moller Polarimetry HERA EIC Jlab -7/9 9/14/2007 W. Lorenzon PSTP 2007 8

  9. Polarimeter Roundup

  10. Phys. Rev. ST Accel. Beams 7, 042802 (2004) The “Spin Dance” Experiment (2000) 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 W. Lorenzon PSTP 2007

  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. 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! W. Lorenzon PSTP 2007

  14. New Ideas W. Lorenzon PSTP 2007

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

  16. Kent Paschke Electron Polarimetry W. Lorenzon PSTP 2007

  17. W. Deconinck, A. Airapetian Hybrid Electron Compton Polarimeterwith online self-calibration W. Lorenzon PSTP 2007

  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 9/14/2007 W. Lorenzon PSTP 2007 18

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