Summary of Workshop on Precision Electron Beam Polarimetry Newport News June 9-10, 2003

1. Summary of Workshop on Precision Electron Beam PolarimetryNewport NewsJune 9-10, 2003 workshop summary by Dave Gaskell, Jefferson Lab Richard Jones, Connecticut

2. Polarimetry Workshop • from the Qweak Cost and Technical Review: “High precision polarimetry is a lab--wide concern at JLab. ...JLab should consider sponsoring a workshop of polarimeter experts to examine the prospects and techniques for achieving such high precision polarization measurements.” • JLab, University of Connecticut, and Hampton University jointly sponsored such a workshop this summer.

3. Workshop held June 9-10 at JLab • 15 speakers over 2 days • significant contribution from non-local experts (SLAC, HERMES, Basel) • about 60 participants overall • organizing committee • Dave Gaskell (JLab) • Richard Jones and Kyungseon Joo (Connecticut) • Cynthia Keppel (Hampton)

4. Workshop Agenda I • Monday, June 9 • Bob Michaels (JLab) Physics Intro • Ingo Sick (Basel) The Hall C Moller Polarimeter • Andrei Afanasev (JLab) Radiative Corrections for Moller and Compton Asymmetries • Vladimir Luppov (UMich) Storage of Polarized Atomic Hydrogen • Eugene Chudakhov (JLab) Moller Polarimetry with Atomic Hydrogen Targets • Dave Mack (JLab) Alkali Atom Moller • Chen Yan (JLab) Iron Wire Basel Moller Plus BeamKicking System for Higher Beam Current Operation • Joe Grames (JLab) Accelerator Tools for Improving Polarimetry • M. Poelker (JLab) The JLab Polarized Source

5. Workshop Agenda II • Tuesday, June 10 • Wolfgang Lorenzon (UMich) The Longitudinal Polarimeter at HERA • Mike Woods (SLAC) SLD Compton Polarimeter • David Lhuillier (CEA-Saclay) Compton Polarimetry at JLab Hall A • Townsend Zwart (MIT Bates) Electron Polarimetry at MIT Bates • Simon Taylor (MIT) Polarimetry for the Mainz A4 Experiment • Richard Jones (UConn) Hall C Compton Polarimeter Preliminary Design • Dave Armstrong (W & M) Summary

6. Summary:Why precision polarimetry? • Approved experiments requiring high precision (<2%) polarimetry • Lead Parity (1%) • Qweak (1%) • Future (12 GeV) experiments that require precision polarimetry • Moller • DIS-Parity • More are undoubtedly on the way...

7. Moller Polarimetry I • High precision requires good knowledge of target polarization • Typically targets are iron or iron-alloy foils so measurements are invasive • Hall C: Ingo Sick and Chen Yan • brute force magnetization of iron foil to saturation using 4 T solenoid • beam heating of foil limits to low currents (2-10 mA) • could be overcome with slow foil rotation (?) • Chen Yan suggests iron wire + kicker magnet to get to higher currents and do “continuous” measurement.

8. Moller Polarimetry II • Hall A: Eugene Chudakov and Vladimir Luppov • replace iron foil with polarized atomic hydrogen target • saturate magnetic field at 8T • Ptarget = 80% (vs. about 8% for iron foil) • target is thin so it would allow non-invasive measurement • target is hard to build • Dave Mack: look at other options • alkali atoms • good for doing in-situ diagnostics • one day for 1% measurement

9. Compton Polarimetry I • Compton polarimetry provides a continuous, non-destructive measurement. • Compton polarimetry works better at higher beam energies where the Compton asymmetry is quite large. • Lorenzon and Woods: Even at much higher energies (HERMES and SLAC) achieving 1% systematics is not trivial. Dominant systematic seems to be characterization of detector response.

10. Compton Polarimetry II • Lhuillier: Hall A Compton is closing in on 1% measurement. • small asymmetry, low beam current requires high-gain laser cavity • coincident electron detection allows good characterization of photon detector • at lower energies, the systematics grow • so far 2% precision achieved for Ebeam > 2.5 GeV • shorter wavelengths needed to control systematics • an upgrade from an IR to green laser is planned for lead-parity experiment Ebeam< 1 GeV • Jones: Qweak is working on Compton for Hall C • preliminary chicane design and optics solution exists • considering going with a pulsed laser option would give more options for choice of high-power laser • choices are between green and UV laser

11. Accelerator • Polarized Source (Poelker) • The source configuration is constantly changing - pay attention • Polarization is sensitive to spot on photocathode, quantum efficiency • Measure the polarization often, and if possible under running conditions • New Ti-Sapphire lasers will improve bleedthrough situation • Accelerator Tools (Grames) • Spin-dance can provide useful cross check of Hall polarimeters • 5-MeV Mott will be back in action some time soon • Transmission polarimeter might be nice complement (used at Bates and Mainz) • High gun current polarimetry may help nail down current dependence without sending large currents to the Hall

12. Points of Discussion • Is a “continuous” measurement of the polarization really necessary? Perhaps that is overkill? • How can we make Compton polarimetry work better at low energies? • How can we make Moller polarimetry work better at high currents?

13. Impact on Hall C • a proto-type kicker+wire target is being installed this fall • Ingo Sick is testing the feasibility of rotating an iron foil in an inhomogeneous magnetic field. • It became clear that a carbon copy of the Hall A Compton will not work for Qweak • A design that decouples the laser from the magnet chicane has been developed - this will allow us to take advantage of improvements in laser technology.