1. Electron and Ion Polarimetry for EIC
Wolfgang Lorenzon
(Michigan)
Electron-Ion Collider Workshop Hampton 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 (~ GeV) electrons/positrons
250 GeV (~ GeV) protons
100 GeV/u (~ GeV/u) heavy ions (eRHIC) / (~ GeV/u) light ions (3He)
Polarized lepton, proton and light ion beams
Longitudinal polarization at Interaction Point (IP): ~70% or better
Bunch separation: ns
Luminosity: L(ep) ~ cm-2 s-1 per IP Goal: 50 fb-1 in 10 years 2

3. Electron Ion Collider ELIC
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 – 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 > 1 GeV laser 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 – 100 GeV AN (2%-10%) at low t( ): drops with 1/Ep
2. p - hydrogen: inclusive pion production – 200 GeV AN <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) – 250 GeV AN <5% (“calculable”), but high cross section & weak dependence on Ep
4. p - hydrogen: p-p elastic (CNI region) – 250 GeV AN <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).

6. e-/e+ Polarimeter Roundup
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% (?)
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%
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%
Compton
~4%
SLAC
0.5%

7. The “Spin Dance” Experiment (2000)
Phys. Rev. ST Accel. Beams 7, (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 %
Møller A %
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
9/14/2007
W. Lorenzon PSTP 2007

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 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 11

12. EIC Compton Polarimeter
chicane
separates polarimetry from accelerator
scattered electron momentum 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 12

13. 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 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: cm (10 GeV), 6.7 cm (3 GeV) deflection at “zero-crossing”: 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 beam 2.32 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
Absolute Polarimeter (H jet)
RHIC pC Polarimeters
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
Show layout & point of polarimeters and experimental locations
Emphasize bunch polarizations prepared at source going through AGS etc. to RHIC
Time spacing between them ~100 ns
Transverse polarization in ring, longitudinal at experiments (optional) using spin rotators
Emphasize none of these techniques existed just 4-5 years ago, all developed here at RHIC with international collaborations led by US and Japanese teams.
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 is unknown, estimated at ~15 % uncertainty
→ absolute calibration necessary
A simple apparatus (detect the slow recoil protons or ~ 900)
PLB 638 (2006) 450
|r5|=0
100 GeV
Concept test: first at IUCF and later at the AGS C targets survive RHIC beam heating

17. Hyperfine state (1),(2),(3),(4)
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)
(SFT&WFT 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
100 GeV
32 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