Summary of Workshop on Precision Electron Beam Polarimetry Newport News June 9-10, 2003 workshop summary by Dave Gaskell, Jefferson Lab Richard Jones, Connecticut

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.

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)

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 Beam Kicking System for Higher Beam Current Operation –Joe Grames (JLab)Accelerator Tools for Improving Polarimetry –M. Poelker (JLab)The JLab Polarized Source

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

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...

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 m A) –could be overcome with slow foil rotation (?) –Chen Yan suggests iron wire + kicker magnet to get to higher currents and do “continuous” measurement.

Moller Polarimetry II Hall A: Eugene Chudakov and Vladimir Luppov –replace iron foil with polarized atomic hydrogen target –saturate magnetic field at 8T –P target = 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

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.

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 E beam > 2.5 GeV –shorter wavelengths needed to control systematics –an upgrade from an IR to green laser is planned for lead-parity experiment E beam < 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

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

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?

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 Q weak 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.