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E-166 Undulator-Based Production of Polarized Positrons A proposal for the 50 GeV Beam in the FFTB Thursday, June 12, 2003 K-P. Sch ü ler and J. C. Sheppard. Undulator-Based Production of Polarized Positrons. E-166 Collaboration. (45 Collaborators).

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Undulator based production of polarized positrons

E-166Undulator-Based Production of Polarized PositronsA proposal for the 50 GeV Beam in the FFTBThursday, June 12, 2003K-P. Schüler andJ. C. Sheppard

Undulator based production of polarized positrons
Undulator-Based Production of Polarized Positrons

E-166 Collaboration

(45 Collaborators)

Undulator based production of polarized positrons1
Undulator-Based Production of Polarized Positrons

E-166 Collaborating Institutions

(15 Institutions)

Undulator based production of polarized positrons

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.

Undulator based production of polarized positrons

The Need for a Demonstration Experiment

Production of polarized positrons depends on the fundamental process of polarization transfer in an electromagnetic cascade.

While the basic cross sections for the QED processes of polarization transfer were derived in the 1950’s, experimental verification is still missing

Undulator based production of polarized positrons

The Need for a Demonstration Experiment

Each approximation in the modeling is well justified in itself.

However,the complexity of the polarization transfer makes the comparison with experiment important so that the decision to build a linear collider w/ or w/o a polarized positron source is based on solid ground.

Polarimetry precision of 5% is sufficient to prove the principle of undulator based polarized positron production for linear colliders.

Physics motivation for polarized positrons
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:

  • Higher effective polarization -> 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.

Physics motivation an example
Physics Motivation: An Example

Separation of the selectron pair in with longitudinally polarized beams to test association of chiral quantum numbers to scalar fermions in SUSY transformations

Undulator based production of polarized positrons

NLC/USLCSG Polarized Positron System Layout

2 Target assembles for redundancy

Undulator based production of polarized positrons

E-166 Vis-à-vis a Linear Collider Source

E-166 is a demonstration of undulator-based production of polarized positrons for linear colliders:

- Photons are produced in the same energy range and polarization characteristics as for a linear collider;

-The same target thickness and material are used as in the linear collider;

-The polarization of the produced positrons is expected to be in the same range as in a linear collider.

-The simulation tools are the same as those being used to design the polarized positron system for a linear collider.

- However, the intensity per pulse is low by a factor of 2000.

Undulator based production of polarized positrons

E-166 Beamline Schematic

50 GeV, low emittance electron beam

2.4 mm period, K=0.17 helical undulator

0-10 MeV polarized photons

0.5 rad. len. converter target

51%-54% positron polarization

Undulator based production of polarized positrons

E-166 Helical Undulator Design, l=2.4 mm, K=0.17


Alexander A. Mikhailichenko

CBN 02-10, LCC-106

Helical undulator radiation
Helical Undulator Radiation

Circularly Polarized Photons

Polarized positrons from polarized g s
Polarized Positrons from Polarized g’s

Circular polarization of photon transfers to the longitudinal polarization of the positron.

Positron polarization varies with the energy transferred to the positron.

(Olsen & Maximon, 1959)

Undulator based production of polarized positrons

Polarized Positron Production in the FFTB

Polarized photons pair produce polarized positrons in a 0.5 r.l. thick target of Ti-alloy with a yield of about 0.5%.

Longitudinal polarization of the positrons is 54%, averaged over the full spectrum

Note: for 0.5 r.l. W converter, the yield is about 1% and the average polarization is 51%.

Undulator based production of polarized positrons


K-Peter Schüler Presentation

Polarimeter overview
Polarimeter Overview

4 x 109 4 x 107

1 x 1010 e-

 4 x 109

4 x 109

 2 x 107 e+

4 x 105 e+  1 x 103 

2 x 107 e+

 4 x 105 e+

Transmission polarimetry of monochromatic photons

M. Goldhaber et al. Phys. Rev. 106 (1957) 826.

Transmission Polarimetry of (monochromatic) Photons

all unpolarized contributions cancel in the transmission asymmetry  (monochromatic case)

Transmission polarimetry of photons
Transmission Polarimetry of Photons

Monochromatic Case

Analyzing Power:

But, undulator photons are not monochromatic:

 Must use number or energy weighted integrals 

Transmission polarimetry of positrons
Transmission Polarimetry of Positrons

2-step Process:

  • re-convert e+   via brems/annihilation process

    • polarization transfer from e+ to  proceeds

      in well-known manner

  • measure polarization of re-converted photons

    with the photon transmission methods

    • infer the polarization of the parent positrons

      from the measured photon polarization

      Experimental Challenges:

  • large angular distribution of the positrons

    at the production target:

    • e+ spectrometer 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 the effective Analyzing Power

      Formal Procedure:

Fronsdahl & Überall;

Olson & Maximon;

Page; McMaster

Spin dependent compton scattering
Spin-Dependent Compton Scattering

  • Simulation with modified GEANT3

  • (implemented by V. Gharibyan)

  • standard GEANT is unpolarized

  • ad-hoc solution:

  • - substitute unpolarized Compton subroutines

  • with two spin-dependent versions (+1 and -1)

  • and run these in sequence for the same

  • same beam statistics

  • - then determine analyzing power from this data

Analyzer magnets
Analyzer Magnets

g‘ = 1.919  0.002 for pure iron,

Scott (1962)

Error in e- polarization is dominated by knowledge in effective magnetization M along the photon trajectory:

active volume

Photon Analyzer Magnet: 50 mm dia. x 150 mm long

Positron Analyzer Magnet: 50 mm dia. x 75 mm long

Photon polarimeter detectors
Photon Polarimeter Detectors

E-144 Designs:

Si-W Calorimeter

Threshold Cerenkov (Aerogel)

Positron transport system
Positron Transport System

e+ transmission (%) through spectrometer

photon background

fraction reaching


Csi calorimeter detector
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

Expected photon polarimeter performance
Expected Photon Polarimeter Performance

Si-W Calorimeter

Expected measured energy asymmetry δ = (E+-E-)/(E++E-)

and energy-weighted analyzing power

determined through analytic integration and, with good agreement, through special polarized GEANT simulation

Energy-weighted Mean

Aerogel Cerenkov

will measureP for E > 5 MeV (see Table 12)

1% stat. measurements very fast (~ minutes),

main syst. error of ΔP /P ~ 0.05 from Pe

Expected positron polarimeter performance i
Expected Positron Polarimeter Performance I

Simulation based on modified GEANT code, which correctly describes the spin-dependence of the Compton process

Number- & Energy-Weighted

Analyzing Power vs. Energy

Photon Spectrum & Angular Distr.

10 Million simulated e+ per point & polarity

on the re-conversion target

Expected positron polarimeter performance ii
Expected Positron Polarimeter Performance II

Analyzing Power

vs. Energy Spread

Analyzing Power

vs. Target Thickness

Polarimetry summary
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 ΔP/P ~ 0.05

    from magnetization of iron

Undulator based production of polarized positrons

Beam Request

J. C. Sheppard Presentation II

Undulator based production of polarized positrons

E-166 Beam Request

  • 6 weeks of activity in the SLAC FFTB:

  • 2 weeks of installation and check-out

  • 1 week of check-out with beam

  • 3 weeks of data taking:

    • roughly 1/3 of time on photon measurements, 2/3 of time on positron measurements.

Undulator based production of polarized positrons

E-166 Beam Measurements

  • Photon flux and polarization as a function of K

  • (Pg~ 75% for Eg > 5 MeV).

  • Positron flux and polarization for K=0.17, 0.5 r.l. of Ti vs. energy. (Pe+~ 50%).

  • Positron flux and polarization for 0.1 r.l. and 0.25 r.l. Ti and 0.1, 0.25, and 0.5 r.l. W targets.

  • Each measurement is expected to take about 20 minutes.

  • A relative polarization measurement of 5% is sufficient to validate the polarized positron production processes.

E 166 as linear collider r d
E-166 as Linear Collider R&D

  • E-166 is a proof-of-principle demonstration of undulator based production of polarized positrons for a linear collider.

  • The hardware and software expertise developed for E-166 form a basis for the implementation of polarized positrons at a linear collider.

E 166 costs
E-166 Costs

Experiment E-166_attach1-052703.xls (J. Weisend, E-166 Impact Report)

(All entries in k$)

E 166 institutional responsibilities1
E-166 Institutional Responsibilities

Experiment E-166_attach1-052703.xls(J. Weisend, E-166 Impact Report)

(All entries in k$)

Undulator photon beam i

(polarimetry extra)

Undulator Photon Beam I

Undulator basics (1st harmonic shown only)

E166 undulator parameters

Undulator photon beam ii

(polarimetry extra)

Undulator Photon Beam II

photon spectrum, angular distribution and polarization

Positron beam simulation

(polarimetry extra)

Positron Beam Simulation

distributions behind the converter target (0.5 r.l. Ti)

based on polarized EGS shower simulations by K. Flöttmann

Low energy polarimetry

(polarimetry extra)

Low-Energy Polarimetry

Candidate Processes

  • Photons: Compton Scattering on polarized electrons

    • forward scattering (e.g. Schopper et al.)

    • backward scattering

    • transmission method (e.g. Goldhaber et al.)

  • Positrons: all on ferromagnetic = polarized e- targets

    • Annihilation polarimetry (e+e- ) (e.g. Corriveau et al.)

    • Bhabha scattering (e+e-  e+e-) (e.g. Ullmann et al.)

    • brems/annihilation (e+  ) plus -transmission (Compton) polarimetry

Trade offs

(polarimetry extra)


Principal difficulties of e+ polarimetry:

  • huge multiple-scattering at low energies even in thin targets

  • cannot employ double-arm coincidence techniques

    or single-event counting due to poor machine duty cycle

  • low energies below 10 MeV, vulnerable to backgrounds

    All of the candidate processes have been explored by us:

  • the transmission method is the most suitable

Compton cross section

(polarimetry extra)

Compton Cross Section

E polarimeter typical gea nt output example i

(polarimetry extra)

e+ polarimeter: typical GEANT output (example) I

Undulator based production of polarized positrons

e+ polarimeter: typical

GEANT output (example) II

(polarimetry extra)



Assuming 2 x 105 e+ per pulse

(1% e+ spectrometer transmission)