Introduction What, who

1
Polarized Positrons at a Linear Collider
and FFTB (SLAC E-166)
Achim W. Weidemann
University of South Carolina, Columbia (_at_SLAC)

• Introduction (What, who)
• Motivation (Why)
• Experiment and Polarimetry (How)
• Outlook

2
E-166 Experiment
E-166 is a demonstration of undulator-based
polarized positron production for linear colliders

• E-166 uses the 50 GeV SLAC beam in conjunction
  with 1 m-long, helical undulator to make
  polarized photons in the FFTB.
• These photons are converted in a 0.5 rad. len.
  thick target into polarized positrons (and
  electrons).
• The polarization of the positrons and photons
  will be measured.

3
E-166 Collaborators
4
Physics Motivation for Polarized Positrons

• Polarized e in addition to polarized e- is
  recognized as a highly desirable option by the WW
  LC community (studies in Asia, Europe, and the
  US)
• Having polarized e offers (next slides)
• Higher effective polarization -gt enhancement of
  effective luminosity for many SM and non-SM
  processes
• Ability to selectively enhance (reduce)
  contribution from SM processes (better
  sensitivity to non-SM processes)
• Access to many non-SM couplings (larger reach for
  non-SM physics searches)
• Access to physics using transversely polarized
  beams (only works if both beams are polarized)
• Improved accuracy in measuring polarization.

5
Physics Motivation for Polarized
Positrons

• Electroweak processes ee- -gt WW, Z, ZH couple
  only to e-LeR or e-ReL (and not e-LeL or
  e-ReR).
  
• Can double or suppress rate using polarized
  positrons
• (in addition to pol. e-).
• Effective polarization enhanced,
  and error
  decreased, in electroweak
  asymmetry measurements,
  
  (NL NR) / (NL NR) Peff ALR,
  
  
  
• Peff (P- - P) / (1 P-P).
• - Improved accuracy in polarization
  
  
• measurement (Blondel scheme)
• ?Must have both e and e-
  
• polarization for Giga-Z project
• (sin2?W )

6
(SUSY)Physics Motivation for Polarized Positrons

• Slepton and squark produced dominantly via
• (and not or ).
• Separation of the (LL, LR) selectron pair
• with longitudinally polarized beams to test
  association of chiral quantum numbers to scalar
  fermions in SUSY
• With P(e-) -80 and
• P(e) 0 gt no separation!
• P(e) -40 gt 163fb vs 66 fb
• Cant do without positron polarization!

7
Physics Motivation for Polarized Positrons

• Transverse polarization of both beams
  
  
• ..allows separation of new physics, e.g. extra
  dimensions
• More examples in JLC, TESLA TDRs, Reviews, e.g.
  by G. Moortgat-Pick, (POWER Polarization at Work
  in Energetic Reactions collaboration
  http//www.ippp.dur.ac.uk/gudrid/power/)
• Next question How to make polarized positrons?

8
Polarized Positrons at LC
2 Target assemblies for redundancy ( polarized
e- source)
9
Polarized Positrons at FFTB

• 50 GeV, low emittance electron beam
• 2.4 mm period, K0.17 helical undulator
• 10 MeV polarized photons
• 0.5 r.l. converter target
• 51-54 positron polarization

10
E-166 vs LC

• E-166 is a demonstration of undulator-based
  production of polarized positrons for linear
  colliders (next slide)
• Photons are produced in the same energy range and
  polarization characteristics as in LC
• Same target thickness and material
• Polarization of the produced positrons is in the
  same range as at LC
• Simulation tools, diagnostics same as those
  being used for LC polarized positron source
• But the intensity per pulse is low by a factor
  of 2000.

11
LC / E-166 Parameter Comparison
12
Helical Undulator l2.4 mm, K0.17
Energy
Polarization
Alexander A. Mikhailichenko, Pulsed
Helical Undulator.CBN 02-10, LCC-106
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Circ. ? -gt long. e polarization
N(e)
P(e)
P(e)
Olsen Maximon, 1959
0.5 r.l. Ti Alloy target 0.5 yield, P(e)54
averaged over full spectrum
14
Polarimeter Overview

4 x 109 ? ? 4 x 107 ?
1 x 1010 e- ? 4 x 109 ?
4 x 109 ? ? 2 x 107 e
2 x 107 e ? 4 x 105 e
4 x 105 e ? 1 x 103 ?
15
Photon Transmission Polarimetry
M. Goldhaber et al. Phys. Rev. 106 (1957) 826.

For photons of undulator spectrum, use number-
or energy-weighted integral.
16
Expected Photon Polarimeter Performance

Si-W Calorimeter
Expected measured energy asymmetry d
(E-E-)/(EE-) and energy-weighted analyzing
power
by analytic integration and, with good
agreement, from polarized GEANT simulation
Energy-weighted Mean
Aerogel Cerenkov
will measure P? for E? gt 5 MeV
1 stat. measurements very fast ( minutes),
main syst. error of ?P? /P? 0.05 from Pe

17
Polarimetry of Positrons

• 2-step Process
• re-convert e ? ? via brems/annihilation
  process
• polarization transfer from e to ? well-known
  
• measure polarization of re-converted photons with
  photon transmission
• infer P(e) from measured photon polarization
• Experimental Challenges
• large angular distribution of the positrons at
  production target
• e collection transport efficiency -
  background rejection issues
• angular distribution of the re-converted photons
• detected signal includes large fraction of
  Compton scattered photons
• requires simulations to determine effective
  Analyzing Power 14-20
• Formal Procedure

Stat. Error (108 photons /15 minutes) d(P) 2
4 Expected systematic Error of d(P)/P 5
dominated by eff. Magnetization of iron
18
Polarimetry Summary

• Transmission polarimetry is well-suited for
  photon and positron beam measurements in E166
• Analyzing power determined from simulations
• is sufficiently large and robust
• Measurements will be very fast with negligible
  statistical errors
• Expect systematic errors of ?P/P 0.05
• from magnetization of iron

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E-166 Outlook

• Experiment approved mid-June 2003
• with proviso should study backgrounds first
• Installation under way now (Aug.2004)
• Will run Oct.2004, Jan 2005 (.before end of
  2005, after which FFTB will become LCLS)
• Hope to blaze the way for polarised positrons at
  a future LC!

For References, details see http//www.slac.sta
nford.edu/exp/e166
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Backup Slides
21
Positron Polarimeter Layout

22
Photon Detectors
For Photons
Si-W Calorimeter
Threshold Cerenkov (AeroGel)
23
Positron Transport System

e transmission () through spectrometer
photon background fraction reaching CsI-detector
24
CsI Calorimeter Detector

Crystals from
BaBar Experiment Number of crystals
4 x 4 16 Typical front face of one
crystal 4.7 cm x 4.7 cm Typical backface of
one crystal 6 cm x 6 cm Typical length
30 cm Density
4.53 g/cm³ Rad.
Length 8.39 g/cm²
1.85 cm Mean free path (5 MeV)
27.6 g/cm² 6.1 cm No. of interaction
lengths (5 MeV) 4.92 Long. Leakage (5 MeV)
0.73 Photodiode Readout (2
per crystal) Hamamatsu S2744-08 with
preamps
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Expected Positron Polarimeter Performance

Expected systematic Error of d(P)/P 5 dominated
by eff. Magnetization of iron