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 LorenzonUniversity of Michigan

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.

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Workshop Participants

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First Name Last Name Affiliation

Kieran Boyle Stony Brook

Abhay Deshpande RIKEN-BNL / Stony Brook

Christoph Montag BNL

Brian Ball Michigan

Wouter Deconinck Michigan

Avetik Hayrapetyan Michigan

Wolfgang Lorenzon Michigan

Eugene Chudakov Jefferson Lab

Dave Gaskell Jefferson Lab

Joseph Grames Jefferson Lab

Jeff Martin University of Winnipeg

Anna Micherdzinska University of Winnipeg

Kent Paschke University of Virginia

Yuhong Zhang Jefferson Lab

Wilbur Franklin MIT Bates

BNL: 3 / HERA: 4 / Jlab: 7 / MIT-Bates: 1Accelerator/Source: 3 Polarimetry: 12

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

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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

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ELIC

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

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Electron Polarimetry

Many polarimeters are, have been in use, or a planned:

• Compton Polarimeters: LEP mainly used as machine tool for resonant depolarizationSLAC SLD 46 GeV DESY HERA, storage ring 27.5 GeV (three polarimeters)JLab Hall A < 8 GeV / Hall C < 12 GeVBates South Hall Ring < 1 GeV Nikhef AmPS, storage ring < 1 GeV

• Møller / Bhabha Polarimeters:Bates linear accelerator < 1 GeVMainz Mainz Microtron MAMI < 1 GeVJlab Hall A, B, C

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532 nm HERA (27.5 GeV)

EIC (10 GeV)

Jlab

HERAEIC

-7/9

x 2maeE E E Compton edge:

Compton vs Moller Polarimetry

Laboratory Polarimeter Relative precision Dominant systematic uncertainty

JLab 5 MeV Mott ~1% Sherman function

JLab Hall A Møller ~2-3% target polarization

JLab Hall B Møller 1.6% (?) target polarization, Levchuk effect

JLab Hall C Møller 0.5% (→1.3%) target polarization, Levchuk effect, high current extrapolation

JLab Hall A Compton 1% (@ > 3 GeV) detector acceptance + response

HERA LPol Compton 1.6% analyzing power

HERA TPol Compton 3.1% focus correction + analyzing power

HERA Cavity LPol Compton ? still unknown

MIT-Bates Mott ~2% Sherman function + detector response

MIT-Bates Transmission >5% analyzing power

MIT-Bates Compton ~3-4% analyzing power

SLAC Compton 0.5% analyzing power

Polarimeter Roundup

The “Spin Dance” Experiment (2000) SourceStrained GaAs photocathode (= 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

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Polarimeter I ave Px Py Pz

Injector Mott 2 A x xHall A Compton 70 A xHall A Moller 1 A x xHall B Moller 10 nA x xHall C Moller 1 A x

Phys. Rev. ST Accel. Beams 7, 042802 (2004)

Polarization ResultsResults shown include statistical errors only→ some amplification to account for non-sinusoidal behavior

Statistically significant disagreement

Even including systematic errors, discrepancy still significant

Systematics shown:

MottMøller C 1% ComptonMøller B 1.6%Møller A 3%

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!

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New Ideas

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New Fiber Laser Technology (Hall C)

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30 ps pulses at 499 MHz

- external to beamline vacuum (unlike Hall A cavity) → easy access- excellent stability, low maintenance

Electron Beam LaserBeamJeff Martin

Gain switched

Electron Polarimetry

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Kent Paschke

Hybrid Electron Compton Polarimeterwith online self-calibration

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W. Deconinck, A. Airapetian

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

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