Transcript Electron and Ion Polarimetry for EIC Wolfgang Lorenzon (Michigan)

Electron and Ion Polarimetry
for EIC
Wolfgang Lorenzon
(Michigan)
Electron-Ion Collider Workshop
Hampton University
20 May 2008
Thanks to Yousef Makdisi
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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
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Bunch separation: 3 - 35 ns
•
Luminosity: L(ep) ~1033 - 1034 cm-2 s-1 per IP
Goal: 50 fb-1 in 10 years
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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
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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 – 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 ?)
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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:
AN <50% for
p+/
3. p - carbon:
inclusive pion production
p-
12 – 200 GeV
at xF ~0.8, but is it large over entire EIC energy range?
p-C elastic (CNI region)
24 – 250 GeV
AN <5% (“calculable”), but high cross section & weak dependence on Ep
4. p - hydrogen:
p-p elastic (CNI region)
24 – 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).
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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
LPol Compton
1.6%
analyzing power
TPol Compton
3.1%
focus correction + analyzing power
Cavity LPol Compton
?
still unknown
Mott
~3%
Sherman function + detector response
Transmission
>4%
analyzing power
Compton
~4%
analyzing power
Compton
0.5%
analyzing power
HERA
MIT-Bates
SLAC
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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
Compton
Møller B
Møller A
1%
1.6%
3%
Even including systematic errors, discrepancy still significant
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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
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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
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Electron
Polarimetry
Kent Paschke
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9/14/2007
W. Lorenzon
PSTP 2007
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
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EIC Compton Polarimeter
chicane
separates polarimetry from accelerator
pair spectrometer (counting mode)
e+e- pair production in variable converter
dipole magnet separates/analyzes e+ e-
scattered electron
momentum analyzed in dipole magnet
measured with Si or diamond strip detector
sampling calorimeter (integrating mode)
count rate independent
insensitive to calorimeter response
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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
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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 beam
2.32 eV laser
Compton photon detection
- Sampling calorimeter (W, pSi) modeled in Geant
- based on HERA calorimeter
No additional smearing
additional smearing: 5%
- study effect of additional
additional smearing: 10%
energy smearing
additional smearing: 15%
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RHIC Polarized Collider
RHIC pC Polarimeters
BRAHMS & PP2PP
Absolute Polarimeter (H jet)
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
200 MeV Polarimeter
AGS
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
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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
RHIC @ 100 GeV
•
The single-flip hadronic amplitude is
unknown, estimated at ~15 % uncertainty
→ absolute calibration necessary
A simple apparatus (detect the slow recoil protons
or carbon @ ~ 900)
|r5|=0
PLB 638 (2006) 450
•
Concept test: first at IUCF and later at the AGS
C targets survive RHIC beam heating
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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
Hyperfine states
(1),(2),(3),(4)
(1),(2)
• 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
Pz+ : (1),(4)
SFT ON (2)(4)
Pz- : (2),(3)
WFT ON (1)(3)
Pz0: (1),(2),(3),(4)
(SFT&WFT ON )
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
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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%}
Psyst/Psyst = 2.8%
• 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
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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
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