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 / students/postdocs (*): 5

EIC Objectives

• e-p and e-ion collisions

• c.m. 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 - 1034 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 30 – 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 Møller Polarimetry

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 1.3% (best quoted)0.5% (possible ?)

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 ~3% Sherman function + detector response

Transmission >4% analyzing power

Compton ~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

Systematics shown:

MottMøller C 1% ComptonMøller 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

13-April 2006 Spin Dance Summary

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 effect

• 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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