Some Aspects of Polarimetry

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(Some) Aspects of Polarimetry At a Future Linear
Collider
M. Woods SLAC

• Polarimetry Requirements
• precision electroweak
• measurements of ALR for weak mixing angle
• determinations from Giga-Z, Bhabha/Moller
  scattering,
• fixed target Moller scattering
• W-pair asymmetry and other Standard Model
  Asymmetries
• background estimations
• Polarimetry for the SLD Experiment at SLC
• design and systematic errors
• Polarimeter Design Issues at NLC

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Improved Weak Mixing angle measurement at Giga-Z
(positrons unpolarized)
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Fixed Target Polarized Moller Scattering at NLC
from SLAC-PUB-8725, L. Keller et al. (2001)
Moller measurements at NLC
(requires 0.3 polarization measurement)
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Polarized Moller Scattering with e-e- Collisions
From F. Cuypers and P. Gambino, Phys. Lett.
B388 211-218, 1996,
Measure 3 asymmetries
They considered
With 250 fb-1, could expect to achieve
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SM Asymmetries in ee-
From Snowmass 96 study,
Consider,
Final State events ALR WW -
560K 100 q q 250K
45 0.005 ll - 120K 10
0.032
Polarimetry to 0.5 or better desirable
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Other Considerations for Precision of Polarimetry

• Background suppression of W pairs in ee-
• most important is to achieve high polarization
• increasing P from 80 to 90 allows for a
• factor 2 further background reduction
• need more precise polarimetry as P increases

An example P 90 Observe 400 events -- after
analysis cuts, but no polarization cut Observe
40 events -- after additional requirement on
polarization state
An excess of 20 events is observed above the
expected W pair background. Would like 1
polarimetry in order to achieve a 4s signal.
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Polarimetry Options at NLC/TESLA
Mott Polarimeter at Source for commissioning Comp
ton Polarimeter before IP after IP after
energy collimation for fixed target Blondel
scheme if both beams polarized W-pair
asymmetry - with only electrons polarized -
better with both beams polarized
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Polarimetry using W-pair asymmetries

• Can we use asymmetry in forward W pairs as a
  polarimeter?
• Yes, if can achieve backgrounds below 1.
• (This level of backgrounds is achieved for
  LEP200 W mass
• measurements, if require one W to decay to ee or
  mm.)
• If positron beam is also polarized, can use
  Blondel-type scheme to
• fit for beam polarizations as well as physics
  asymmetry and
• eliminate sensitivity to backgrounds
• advantage wrt Compton polarimetry is that any
• depolarization in beam-beam interaction is
  properly
• accounted for also need to be above W-pair
  threshold
• disadvantage wrt Compton polarimetry is Compton
  can
• achieve 1 accuracy in a few minutes

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Blondel scheme with electron and positron beams
both polarized
Can also use Blondel scheme to determine beam
polarizations directly
- using Blondel technique, just need Compton
polarimeters for measuring polarization
differences between L,R states and only need to
spend approx. 10 of running time in NRR, NLL
states - this technique directly measures
lum-wted polarizations (any depolarization effect
properly taken into account)
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Polarized Positrons?
Need to understand relative detector efficiency
for RL and LR modes at level of 10-4 Need
to measure polarization difference, PR-PL, at
level of 10-3
This will be difficult unless can measure these
modes simultaneously, ie. can switch positron
polarization randomly pulse-to-pulse, as is
done for electrons.
Note even if positrons are nominally
unpolarized, need to verify this!
For d(ALR)4 x 10-4, want d(P)lt2x10-4. SLDs
posipol measurement achieved d(P)7x10-4.
(This is relevant for electron-only ALR
measurement, which has a goal a factor 5 better
than SLDs result.)
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Compton Polarimetry at SLD
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Features of Compton polarimetry
Physics well understood and radiative corrections
lt0.1 - no atomic or nuclear physics corrections
(ex. Levchuk effect in Moller
polarimetry) Easy to measure backgrounds with
laser off pulses Polarimetry data taken
parasitic to physics data Scattering rate is
high and can achieve small statistical errors in
short amount of time (1 in a few minutes or
less is possible) Easy to reverse laser
polarization quickly Laser polarization can be
determined to 0.1 With Polarimeter after IP,
can measure beam-beam depolarization effects by
comparing polarization with and without
collisions
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Compton Scattering Kinematics
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Raw Data from CKV Detector
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Laser Polarization Systematic error
Ability to extinguish laser light after Helicity
filter determines polarization purity
Residuals (adc counts)
CP Voltage (Volts)
Monitor phase shifts in laser polarization with
frequent Pockels cell scans
Laser Polarization at Compton IP ()
0.1 systematic error
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Analyzing Power Systematic Error
Estimate CKV7 systematic error in analyzing power
at 0.3 from table scan data that determines
location of Compton edge and accuracy of
modeling detector response function
Results from cross-check Polarimeters
0.4 systematic error
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Linearity Systematic error
Study measured Compton asymmetry as a Function of
background level.
0.2 linearity systematic
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Goals for Compton Polarimetry Systematics At NLC
(mainly Giga-Z)
Improve linearity by - higher resolution adcs
(16-bit rather than 11-bit) - blue diode
laser calibration and bias system (provide
large stable background) - use of Compton
laser power scans Improve analyzing power by -
more segmentation of channels near Compton edge
in CKV detector - careful design and
simulation of spectrometer and detector (origi
nal design goal for SLD was 1) - detailed
systematics data taking - cross-checks (if
possible) Compton gamma detectors, W-pair
asymmetries, polarized Moller asymmetries Electron
ic Noise includes crosstalk and laser pickup
reduce by careful setup and accurate measurements
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Compton Kinematics and Cross Sections at NLC/TESLA

• can have a large separation between the kinematic
  endpoint energy
• the beam energy
• - large Compton asymmetry at high energy

Spectrum of Compton-scattered electrons
(500 GeV electrons, 1.165 eV photons)
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Depolarization Effects at NLC (see SLAC PUB 8716,
K. Thompson Jan. 2001)
Lum-wted depolarization is approx. 25 of
average depolarization Depolarization can be
large for beam particles that lose significant
energy to beamstrahlung Depolarization can be
large for large vertical offset (may have large
effect for first bunches in train)
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Depolarization Results from SLAC PUB 8716, K.
Thompson Jan. 2001
BMT spin precession effects ST
Sokolov-Ternov spin flip effects
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Extraction Line Design at NLC (Y. Nosochkov)
Location for Compton IP (also for wire scanner to
measure beam energy distribution) - secondary
focus with 20mm dispersion Detailed design for
polarimeter still to be done
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Additional comments on Beam Delivery Issues for
E158 at NLC
New strained super-lattice photocathodes will
hopefully reach 90 polarization and with less
(x3?) anisotropy wrt linear polarization of
incident laser light (good for minimizing false
beam asymmetries) Damped Beams eliminate any
helicity-dependent position asymmetries should
only have charge asymmetries after
DR Intensity and Position Asymmetries. Will
want to zero charge asym after DR. Beam losses
in extraction line will introduce intensity
jitter and significant charge asymmetry due to
this jitter (expect 1ppm over run). Do not want
charge asym in Linac leads to energy asym due to
beam loading and position asym due to residual
Linac dispersion and any wakefield
amplification. Momentum Slits Will have (1-2)
slits in extraction line before target. During
colliding beam running, will only accept 70 of
extracted beam from IP. - intensity jitter may
be large also position and energy jitter? -
will need 0.1 linearity for detectors and
toroids (1 for E158) - want polarimeter
downstream of slits additional chicane for
Compton IP? Spotsize Jitter and Tails Spotsize
jitter can cause target density fluctuations. In
E158, taking advantage of SR emittance growth in
A-line to mitigate effects. Sensitivity to Linac
emittance and beam-beam effects on emittance may
be significant Depolarization in Target? -
Compton IP and spectrometer/detector after target
to measure effects?
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Intensity-Position Correlation Observed during
E158 engineering run (April-May 2001)
Stripline BPM in S30
Stripline BPM in S30
Head of pulse
Tail of pulse
45 GeV 2.3e11 in 130ns train 0.3ns microbunch
spacing (low intensity NLC beam!)
rf bpm in front of E158 target
2 bands related to K02 problem Position-intensity
correlation believed to be due to beam
loading (5) and dispersion in Linac, with
possible wakefield amplification We minimize
position-intensity correlation with careful
steering in injector, but corrections vary in
time.
Entire pulse average