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Wolfgang Lorenzon (Michigan) Electron-Ion Collider Workshop Hampton University 20 May 2008. Electron and Ion Polarimetry for EIC. Thanks to Yousef Makdisi. EIC Objectives. e-p and e-ion collisions c.m. energies: 20 - 100 GeV 10 GeV (~3 - 20 GeV) electrons/positrons

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electron and ion polarimetry for eic

Wolfgang Lorenzon


Electron-Ion Collider WorkshopHampton University

20 May 2008

Electron and Ion Polarimetry for EIC

Thanks to Yousef Makdisi

eic objectives

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


electron ion collider

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



how to measure polarization of e e beams

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 > 1 GeVlaser photons scatter off lepton beam

Goal: measure DP/P ≈ 1% (realistic ?)

how to measure polarization of p beams

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 GeVAN <50% 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 GeVAN <5% (“calculable”), but high cross section & weak dependence on Ep

4. p - hydrogen: p-p elastic (CNI region) 24 – 250 GeVAN <5% (“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).

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

The “Spin Dance” Experiment (2000)

Systematics shown:


Møller C 1%


Møller B 1.6%

Møller A 3%

Even including systematic errors, discrepancy still significant

lessons learned

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

dominant challenge determine a z

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
e e polarimetry at eic

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


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?

Design efforts and simulations have started


eic compton polarimeter

EIC Compton Polarimeter


separates 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 converter

dipole magnet separates/analyzes e+ e-

sampling calorimeter (integrating mode)count rate independent

insensitive to calorimeter response



Possible Compton IP Location (ELIC)

  • ~85 m available for electron polarimetry
  • ~20 m needed for chicane
  • simulations started for IP location at s=161 m
  • location can be shifted due to cell structure (8.2m) of lattice design

Alex Bogacz


compton polarimetry

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%)


- 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


- 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%


RHIC Polarized Collider

RHIC pC Polarimeters

Absolute Polarimeter (H jet)



Siberian Snakes

Siberian Snakes


DPb/Pb = 5%



Spin Rotators

(longitudinal polarization)

Spin Rotators

(longitudinal polarization)

Pol. H- Source



Helical Partial Siberian Snake


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


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 @ ~ 900)

RHIC @ 100 GeV


PLB 638 (2006) 450

Concept test: first at IUCF and later at the AGS C targets survive RHIC beam heating


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



Pz+ : (1),(4)

SFT ON (2)(4)

Pz- : (2),(3)

WFT ON (1)(3)

Pz0: (1),(2),(3),(4)


Hyperfine state (1),(2),(3),(4)


p-C polarimeter vs Hydrogen Jet (2006)

p-C CNI data

Fill Number

H-Jet calibration data

p-C CNI data

100 GeV

32 GeV

issues with p polarimetry at rhic

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%


e e p ion polarimetry at eic

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