Electron Polarimetry Working Group Update

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• Electron Polarimetry Working GroupUpdate

• Wolfgang Lorenzon
• (Michigan)
• EIC Collaboration MeetingStony Brook
• Dec 7-8, 2007

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• 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 Participants
BNL 3 / HERA 4 / Jlab 7 /
MIT-Bates 1 Accelerator/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 gt
  GeVlaser photons scatter off lepton beam

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

• 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
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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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Compton vs Møller Polarimetry

• Detect g at 0, e- lt Ee
• Strong ? need ltlt1
• at Ee lt 20 GeV
• Plaser 100
• non-invasive measurement
• syst. Error 3 ? 50 GeV (1 ? 0.5) hard at
  lt 1 GeV (Jlab project 0.8)
• rad. corr. to Born lt 0.1

• Detect e- at qCM 90
• ? good systematics
• beam energy independent
• ferromagnetic target PT 8
• beam heating (Ie lt 2-4 mA), Levchuck eff.
• invasive measurement
• syst. error 2-3 typically 0.5 (1?) at high
  magn. field
• rad. corr. to Born lt 0.3

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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 31015 cm-3 , stat. 1 in 10 min at 100
  mA)
• expected depolarization for 100 mA CEBAF lt
  10-4
• limitations beam heating ?
  continuous beam complexity of target
• advantages expected accuracy lt 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

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Hybrid Electron Compton Polarimeterwith online
self-calibration
W. Deconinck, A. Airapetian
chicane separates polarimetry from
accelerator scattered electronmomentum analyzed
in dipole magnet measured with Si or diamond
strip detector
pair spectrometer (counting mode) ee pair
production in variable converter dipole magnet
separates/analyzes e e sampling calorimeter
(integrating mode)count rate independent Insensit
ive 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 335 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 starting

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Longitudinal Polarimetry
Pair Spectrometer Geant simulations with pencil
beams (10 GeV leptons on 2.32 eV
photons) Coincidence Mode - acceptance (from
lt1.51 GeV (zero crossing) to gt2.63 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
(a, b functions) - fix configuration (dipole
strength, length, position, hodoscope
position and sizes, - estimate efficiencies,
count rates
ee coincidence mode
ee single arm mode
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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
7.5 GeV beam2.32 eV laser

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

No additional smearing
additional smearing 5
additional smearing 10
additional smearing 15
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Transverse Polarimetry
Energy 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 1010 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 Polarimetry

• Hydrogen 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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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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