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

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. 9/14/2007 W. Lorenzon PSTP 2007

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: 3 Polarimetry: 12 9/14/2007 W. Lorenzon PSTP 2007

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 9/14/2007 W. Lorenzon PSTP 2007

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 9/14/2007 W. Lorenzon PSTP 2007

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 – 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 9/14/2007 W. Lorenzon PSTP 2007

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 9/14/2007 W. Lorenzon PSTP 2007

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

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%

The “Spin Dance” Experiment (2000) Phys. Rev. ST Accel. Beams 7, 042802 (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 9/14/2007 W. Lorenzon PSTP 2007

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

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

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! 9/14/2007 W. Lorenzon PSTP 2007

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

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 9/14/2007 W. Lorenzon PSTP 2007

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

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

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