Electron Polarimetry Working GroupUpdate

Wolfgang Lorenzon(Michigan)

EIC Collaboration MeetingStony Brook

Dec 7-8, 2007

1W. Lorenzon SBU Dec-2007

EIC Electron Polarimetry Workshop

August 23-24, 2007 hosted by the University of Michigan (Ann Arbor)

http://eic.physics.lsa.umich.edu/(A. Deshpande, W. Lorenzon)

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Workshop ParticipantsFirst 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

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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 (2012)

→ 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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How to measure polarization of e-/e+ beams?

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 – GeVatomic electron in Fe (or Fe-alloy) polarized by external magnetic field

3. e - photons: Compton scattering > GeVlaser photons scatter off lepton beam

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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% (quoted)2-3% (realistic ?)

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% (~2%) 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

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The “Spin Dance” Experiment (2000)Phys. Rev. ST Accel. Beams 7, 042802 (2004)

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Results 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

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?

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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

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• Detect at 0°, e- < Ee

• Strong need <<1

• at Ee < 20 GeV

• Plaser ~100%• non-invasive measurement• syst. Error: 3 → 50 GeV (~1 → 0.5%) hard at < 1 GeV: (Jlab project: ~0.8%)• rad. corr. to Born < 0.1%

dA

dE/E E

• Detect e- at CM ~90°

• good systematics• beam energy independent• ferromagnetic target PT ~8%

• beam heating (Ie < 2-4 A), Levchuck eff.• invasive measurement• syst. error 2-3% typically 0.5% (1%?) at high magn. field• rad. corr. to Born < 0.3%

~ 090oCM

dA

d

eA EE

W. Lorenzon SBU Dec-2007

New Ideas• Polarized Hydrogen in a cold magnetic trap (E. Chudakov et al., IEEE Trans. Nucl. Sci. 51, 1533 (2004) )

– use ultra-cold traps (at 300 mK: Pe ~ 1-10-5, density ~ 3∙1015 cm-3 , stat. 1% in 10 min at 100 A)– expected depolarization for 100 A CEBAF < 10-4

– limitations: beam heating → “continuous” beam & complexity of target– advantages: expected accuracy < 0.5% & non-invasive, continuous, the same beam– Problem: very unlikely to work for high beam currents for EIC (due to gas and cell heating)– Jet Target: avoids these problems

– VEPP-3 100 mA, transverse– stat 20% in 8 minutes (5 ∙ 1011 e- /cm2 , 100% polarization)– What is electron polarization in a jet?

• New fiber laser technology (Jeff Martin for Hall C)– Gain switched fiber laser

– huge luminosity boost when locked to Jlab beam structure (30 ps pulses at 499 MHz)– lower instantaneous rates than high power pulsed lasers– external to beam line vacuum → easy access

– in-house experience (Jlab source group)– excellent stability, low maintenance

• Compton e- analysis (Kent Paschke for PV-DIS experiments)– dominant challenge: determination of analyzing power Az – zero-crossing e- analysis: two points of well-defined energy (Compton edge, zero crossing)

– linear fit of zero crossing: integrate between two points– absolute calibration (only input is QED)– weak dependence of energy resolution & no need to calibrate calorimeter

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2bI

Hybrid Electron Compton Polarimeterwith online self-calibration

W. Deconinck, A. Airapetian

chicaneseparates polarimetry from accelerator

scattered electronmomentum analyzed in dipole magnet measured with Si or diamond strip detector

pair spectrometer (counting mode)e+e– pair production in variable converterdipole magnet separates/analyzes e+ e–

sampling calorimeter (integrating mode)count rate independentInsensitive to calorimeter response

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A2 Workshop 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 working group– coordination of initial activities and directions: W. Lorenzon– members: A. Airapetian, D. Gaskell (long. polar.), W. Franklin (trans. polar.), E. Chudakov (Møller targets)

• Design efforts and simulations just starting1212W. Lorenzon SBU Dec-2007

Longitudinal PolarimetryPair Spectrometer

Geant simulations with pencil beams (10 GeV leptons on 2.32 eV photons)

Coincidence Mode: - acceptance (from <1.51 GeV (“zero crossing”) to >2.63 GeV (Compton edge) - resolution (2%-3.5%)

Single Arm Mode: - analyzing magnet relates momentum and position of pair produced e - e+

- provide well defined e - or e+ beams to calibrate the Compton photon calorimeter

Plans: - include beam smearing ( functions) - fix configuration (dipole strength, length, position, hodoscope position and sizes, … - estimate efficiencies, count rates

e+e– coincidence mode

e+e– single arm mode

single hodo channels

all 18 hodo channels

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Longitudinal Polarimetry (II)Compton electron detection

- using chicane design, max deflection from e- beam: 22.4 cm (10 GeV), 6.7 cm (3 GeV) deflection at “zero-crossing”: 11.1 cm (10 GeV), 3.3 cm (3 GeV) → e- detection should be easy Plans: - include realistic beam properties → study bkgd rates due to halo and beam divergence - adopt Geant MC from Hall C Compton design - learn from Jlab Hall C new Compton polarimeter

Compton photon detection - Sampling calorimeter (W, pSi) modeled in Geant- based on HERA calorimeter- study effect of additional energy smearing

No additional smearingadditional smearing: 5%additional smearing: 10%additional smearing: 15%

7.5 GeV beam2.32 eV laser

Transverse PolarimetryEnergy Dependence

- analyzing power as function of scattered photon energy - large variation in energy of peak analyzing power

20 GeV studies - using pencil beams - peak asymmetry in gamma spectrum at ~6 GeV for 20 GeV electron beam of - resolution of ~1 m needed in vertical centroid for 1% polar. measurement for 50 m flight path

3 GeV studies - peak asymmetry in gamma spectrum at ~200 MeV for 3 GeV electron beam - position sensitive detector of 10*10 cm2 will subtend relevant region for asymmetry at lowest energy for 50 m flight path

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Transverse Polarimetry (II)Plans:

• Asymmetries appear adequate for transverse polarimetry, even at low energies.

• Inclusion of transverse electron polarimetry within IP polarimeter appears feasible with compact position-sensitive detector in photon arm. Flight path greater than 50 m desirable.

• Next steps:– Include beta functions and emittance at IP – Projection of asymmetry vs. position for asymmetry for EIC energies– Begin simulation to determine effective analyzing power of

calorimeter– Use of electron vertical information?

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Møller PolarimetryHydrogen Atomic Jet

• Just started investigations• Several problems to address:

– Breit-Rabi measurement analyzes only part of jet → uniformity of jet has to be understood – large background from ions in the beam: most of them associated with

jet (hard to measure)– origin of background observed in Novosibirsk still unclear (in contact

with them)– clarification of depolarization by beam RF needed → might be considerable

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W. Lorenzon SBU Dec-2007

Conclusions

• Electron Polarimetry working group has been formed– kick-off at A2 Workshop in Aug 2007– design efforts and simulations have started– dialog with accelerator groups at BNL / JLab

• There are issues that need attention (crossing frequency 3-35 ns; beam-beam effects at high currents; crab crossing effect on polarization)

• JLAB at 12 GeV will be a natural testbed for future EIC Polarimeter tests– evaluate new ideas/technologies for the EIC

• No serious obstacles are foreseen to achieve 1% precision for electron beam polarimetry at the EIC (3-20 GeV)

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