Polarimetry at NLC

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Polarimetry at NLC How Precise? e-e- 99
Workshop, UC Santa Cruz Dec. 10-12,
1999
Mike Woods SLAC

• Standard Model asymmetries in ee- and e-e-
• testing for physics beyond SM
• polarimetry from SM asymmetries
• running at Z0 resonance
• Other considerations for precision polarimetry
• background suppression of W pairs in ee-
• depolarization in beam-beam interaction
• design of extraction line and beam losses

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Assumptions for Machine Performance
Parameter ee- e-e- 500 GeV 500 GeV 80
fb-1 25 fb-1 P1 0 90 P2 90 90
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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
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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
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SM Asymmetries in ee- (cont.)

• Notes
• 1. Better than 1 polarimetry is needed to
  fully exploit
• these measurements for SM tests.
• 2. 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.)
• advantage wrt Compton polarimetry is that any
• depolarization in beam-beam interaction is
  properly
• accounted for
• disadvantage wrt Compton polarimetry is Compton
  can
• achieve 1 accuracy in a few minutes

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From F. Cuypers and P. Gambino, Phys. Lett.
B388 211-218, 1996,
Measure 3 asymmetries
Consider,
For comparison, i) SLD has achieved
ii) E158 at SLAC will achieve (at Q20.02 GeV2)
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SM Asymmetries in e-e- (cont.)
Notes 1. Achieves better than 1 polarimetry
using a SM physics asymmetry. Again, has
advantage wrt Compton polarimetry that it
properly takes into account any depolarization
due to beam-beam effects. But disadvantage is
that Compton can achieve 1 accuracy in a few
minutes.
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The Linear Collider Z-factory option
Some anomalies remain from the LEP/SLC
era (sin2qWeff, Ab, Nn)
May be very desirable to accumulate a large Z
sample (gtgt10M) with polarized beam(s) (ex. Monig
and Hawkings, DESY-99-157)
Ideally the positron beam has P0.6, and can
then use Blondel scheme for polarimetry from the
measured physics asymmetries in the
detector. However, if positron beam is
unpolarized then will want a very precise Compton
polarimeter, better than the 0.5 accuracy
achieved with SLDs Compton. And will want the
Compton to measure any beam-beam polarization
effects.
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Other Considerations for Precision 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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• Depolarization in beam-beam interaction
• need Compton polarimeter in extraction line to
  measure
• polarization with and without collisions, or
• polarization measured from a physics
  asymmetry
• need to emphasize that depolarization should be
  included in
• parameter tables for the Interaction Region
• need to encourage the simulation programs
  Guinea-Pig and
• CAIN to include polarization effects

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• Design of Extraction Line effect of beam
  losses
• ideally, want to have a large number of
  diagnostic devices for
• measuring and optimizing luminosity,
• polarization and energy measurements
• in practice, need to balance this with cleanly
  transporting the
• beams to the dumps. Want to minimize beam
  losses
• and backgrounds for the detector.
• ZDR approach allowed for a Compton polarimeter,
  a wire scanner and
• other devices

• Increased disruption effects in higher luminosity
  schemes or e-e- option,
• may lead to elimination of some extraction
  line diagnostics
• important to point out how this may limit the
  physics capability
• important to still try to incorporate
  polarization and energy diagnostics
• in the extraction line

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Summary
Standard Model asymmetries - better than 1
polarimetry is needed for testing SM
and probing for new physics - SM asymmetries
in e-e- e-e- and in ee- WW-
should achieve better than 1 polarimetry
(very good detector coverage and capability
needed for forward angles)
Other considerations for precision polarimetry -
should have a Compton polarimeter in the
extraction line - depolarization effects should
be calculated in beam-beam simulations and
tabulated in IR paramater tables - high
luminosity scenarios and e-e- option
significantly complicate the design for a
Compton polarimeter in the extraction line, and
could make it impractical