1. At a Future Linear Collider
(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

2. Improved Weak Mixing angle measurement at Giga-Z
(positrons unpolarized)

3. 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)

4. Polarized Moller Scattering with e-e- Collisions
From F. Cuypers and P. Gambino,
Phys. Lett. B388: , 1996,
Measure 3 asymmetries:
They considered
to determine:
With 250 fb-1, could expect to achieve

5. SM Asymmetries in e+e- Polarimetry to 0.5% or better desirable
From Snowmass ‘96 study,
Consider,
Final State #events ALR
W+W K %
q q K %
l+l K %
Polarimetry to 0.5% or better desirable

6. Other Considerations for Precision of Polarimetry
Background suppression of W pairs in e+e-
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
0 2.4%
1% 2.6%
2% 3.1%
An excess of 20 events is observed above the expected W pair background.
Would like 1% polarimetry in order to achieve a 4s signal.

7. Polarimetry Options at NLC/TESLA
Mott Polarimeter at Source for commissioning
Compton 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

8. 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 e- e+ W- W+ n

9. 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)

10. 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+)<2x SLD’s ‘posipol’ measurement
achieved d(P+)=7x (This is relevant for electron-only ALR measurement,
which has a goal a factor 5 better than SLD’s result.)

11. Compton Polarimetry
at SLD

12. Features of Compton polarimetry
Physics well understood and radiative corrections <0.1%
- 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

13. Compton Scattering Kinematics
The cross section for Compton scattering is
where
The measured asymmetry in a channel is
Where the analyzing power is calculated from the Compton
Cross section and the channel response function, Ri.

14. Raw Data from CKV Detector

15. Laser Polarization Systematic error
Abox PD (adc counts)
CP Voltage (Volts)
Ability to extinguish laser light after
Helicity filter determines polarization purity
Residuals (adc counts)
CP Voltage (Volts)
CP Voltage (Volts)
CKV7 Raw Asymmetry
Monitor phase shifts
in laser polarization
with frequent Pockels
cell scans
Laser Polarization at Compton IP (%)
0.1% systematic error

16. 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

17. Linearity Systematic error
Measurement of electronics linearity
with pulser
Study measured Compton asymmetry as a
Function of background level.
0.2% linearity systematic

18. Goals for Compton Polarimetry Systematics
At NLC (mainly ‘Giga-Z’)
Uncertainty
SLD
dP/P
NLC Goal
Laser Polarization
0.1%
Analyzing Power
0.4%
0.2%
Linearity
Electronic Noise
0.05%
TOTAL
0.50%
0.25%
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
(original design goal for SLD was 1%)
- detailed systematics data taking
- cross-checks (if possible): Compton gamma detectors,
W-pair asymmetries, polarized Moller asymmetries
Electronic Noise includes crosstalk and laser pickup: reduce by careful setup and accurate measurements

19. 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)

20. 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)

21. Depolarization Results from SLAC PUB 8716, K. Thompson Jan. 2001
BMT: spin precession effects
ST: Sokolov-Ternov spin flip effects

22. 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

23. 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?

24. 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