Electron and Ion Polarimetry for EIC

1

• Electron and Ion Polarimetry for EIC

• Wolfgang Lorenzon
• (Michigan)
• Electron-Ion Collider WorkshopHampton University
• 20 May 2008

Thanks to Yousef Makdisi
2
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

2
3
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

ELIC
3
4
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
  1 GeVlaser photons scatter off lepton beam
• Goal measure DP/P 1 (realistic ?)

5
How to measure polarization of p beams?

• For transverse beam polarization
• 1. p - hydrogen p-p elastic scattering
  10 100 GeV AN (2-10) at low
  t(0.1-0.3) drops with 1/Ep
• 2. p - hydrogen inclusive pion
  production 12 200 GeV AN lt50 for p/
  p- at xF 0.8, but is it large over entire EIC
  energy range?
• 3. p - carbon p-C elastic (CNI
  region) 24 250 GeV AN lt5
  (calculable), but high cross section weak
  dependence on Ep
• 4. p - hydrogen p-p elastic (CNI region)
  24 250 GeV AN lt5
  (calculable), but high cross section weak
  dependence on Ep
• Goal measure DP/P 2-3 (challenging)
• Note unlike e-/e polarimeters (where QED
  processes are calculable), proton polarimeters
  rely on experimental verifications (especially at
  high energies).

6
e-/e Polarimeter Roundup
7
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
8
Lessons Learned

• Providing/proving precision at 1 level
  challenging
• Including polarization diagnostics/monitoring in
  beam lattice design crucial
• Measure polarization at (or close to) IP
• Measure beam polarization continuously
• protects against drifts or systematic
  current-dependence to polarization
• Flip electron and laser polarizations
• fast enough to protect against drifts
• 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
• absolute measurement does not have to be fast
• 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
• New ideas

9
Dominant Challenge determine Az

• Best tool to measure e- polarization
• ? Compton e- (integrating mode)
• Traditional approach
• use a dipole magnet to momentum analyze Compton
  e-
• accurate knowledge of ?Bdl
• must calibrate the electron detector
• fit the asymmetry shape or use Compton Edge

10
Electron Polarimetry
Kent Paschke
11
e-/e Polarimetry at EIC

• 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?
• Design efforts and simulations have started

11
12
EIC Compton Polarimeter
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
12
13
Possible Compton IP Location (ELIC)

• 85 m available for electron polarimetry
• 20 m needed for chicane
• simulations started for IP location at s161
  m
• location can be shifted due to cell structure
  (8.2m) of lattice design

Alex Bogacz
13
14
Compton Polarimetry
Pair Spectrometer- Geant simulations with pencil
beams (10 GeV leptons on 2.32 eV photons)-
including beam smearing (a, b functions)
resolution (2-3.5) Plans - fix
configuration (dipole strength, length, position,
hodoscope position and sizes, - estimate
efficiencies, count rates
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
14
14
additional smearing 15
15
RHIC Polarized Collider
RHIC pC Polarimeters
Absolute Polarimeter (H? jet)
BRAHMS PP2PP
PHOBOS
Siberian Snakes
Siberian Snakes
Goal DPb/Pb 5
PHENIX
STAR
Spin Rotators (longitudinal polarization)
Spin Rotators (longitudinal polarization)
Pol. H- Source
LINAC
BOOSTER
Helical Partial Siberian Snake
AGS
200 MeV Polarimeter
AGS pC Polarimeter
Strong AGS Snake
Source Lamb Shift
Polarimeter Linac (200 MeV) p-C scattering
(calibrated with p-D elastic scattering) Ap-X
0.50
16
p-p and p-C elastic scattering in CNI region

• The asymmetry is calculable
• J. Schwinger, Phys. Rev. 69,681 (1946)
• Weak beam momentum dependence
• Analyzing power is few percent ( 5)
• Cross section is high
• The single-flip hadronic amplitude isunknown,
  estimated at 15 uncertainty
• ? absolute calibration necessary
• A simple apparatus (detect the slow recoil
  protons or carbon _at_ 900)

RHIC _at_ 100 GeV
Concept test first at IUCF and later at the AGS
C targets survive RHIC beam
heating
17
The RHIC Polarized Hydrogen Jet Target

• pumps 1000 l/s compression 106 for H
• nozzle temperature 70K
• sextupoles 1.5T pole field and 2.5T/cm grad.
• RF transitions SFT (1.43GHz) WFT (14MHz)
• holding field 1.2 kG ?B/B 10-3
• vacuum 10-8 Torr (Jet on) / 10-9 Torr (Jet off)
  
• molecular hydrogen contamination 1.5
• overall nuclear polarization dilution of 3
• Jet beam intensity 12.4 x 1016 H atoms /sec
• nuclear polarization (BRP) 95.8 0.1
• Jet beam polarization measured (after
  corrections) 92.4 1.8
• Jet beam size 6.6 mm FWHM
• In 2006 the Jet measured the beam to jet
  polarization ratio to 10 per 6-hr store

Hyperfine states (1),(2),(3),(4)
(1),(2) Pz (1),(4) SFT ON (2)?(4) Pz-
(2),(3) WFT ON (1)?(3) Pz0 (1),(2),(3),(4)
(SFTWFT ON )
Hyperfine state (1),(2),(3),(4)
18
p-C polarimeter vs Hydrogen Jet (2006)
p-C CNI data
Fill Number
H-Jet calibration data
p-C CNI data
32 GeV
100 GeV
19
Issues with p Polarimetry at RHIC

• Beam Polarization desired goal for RHIC 5 ?
  DPb/Pb 4.2
• largest syst uncertainties
• beam polarization profile 5
• improvement in C target mechanism is expected to
  eliminate this uncertainty
• molecular H fraction 1.8
• residual gas background 2.1
• H-Jet Pb measurements per fill 10 (stat) in 6
  hr
• increase Si t-range acceptance
• open up the holding field magnet aperture
• p-C polarimeter 2-3 (stat) per min
• replace Si strips with APDs (better energy
  resolution)
• improve beam profile and polarization profile
  measurements
• Molecular H component
• molecular H fraction is 1.5 ? 3 nuclear
  dilution (if H2 is unpolarized)
• H2 content confirmed with electron beam ionizing
  jet beam and analyzing it with magnet
• repeat those measurements using proton beam
  luminescence and a CCD camera ? H lines seen,
  but not H2 lines more work needed

DPsyst/Psyst 2.8
19
20
e-/e p/ion Polarimetry at EIC

• No serious obstacles are foreseen to achieve 1
  precision for electron beam polarimetry at the
  EIC (3-20 GeV)
• JLAB at 12 GeV will be a natural testbed for
  future EIC e-/e Polarimeter tests
• evaluate new ideas/technologies for the EIC
• There are issues that need attention (crossing
  frequency 3-35 ns beam-beam effects at high
  currents crab crossing effect on polarization)
• Proton beam polarimetry between 24 GeV
  (injection) 250 GeV (top energy) seems
  possible at 2-3 level (but not easy)
• if goal is at 1-2 level there is a long way to
  go
• major challenges are closer bunch spacing at the
  EIC and reducing the H jet molecular fraction
  to below 2
• Studies for 3He beams have started
• Design efforts and simulations have started for
  e-/e p/ion polarimetry

20