EPAC June 2003

E-166 Undulator-Based Production Undulator-Based Production

of Polarized Positronsof Polarized PositronsA proposal for the 50 GeV Beam in the FFTBA proposal for the 50 GeV Beam in the FFTB

Thursday, June 12, 2003K-P. Schüler and J. C. Sheppard

2 EPAC June 2003

Undulator-Based Production of Polarized Undulator-Based Production of Polarized PositronsPositrons

E-166 Collaboration

(45 Collaborators)

3 EPAC June 2003

Undulator-Based Production of Polarized Undulator-Based Production of Polarized PositronsPositronsE-166 Collaborating Institutions

(15 Institutions)

4 EPAC June 2003

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.

5 EPAC June 2003

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

6 EPAC June 2003

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 10% is sufficient to prove the principle of undulator based polarized positron production for linear colliders

7 EPAC June 2003

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.

8 EPAC June 2003

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

Physics Motivation: An Example

L Le e , ,L R L Re e e e

9 EPAC June 2003

NLC/USLCSG Polarized Positron System Layout

2 Target assembles for redundancy

10 EPAC June 2003

TESLA, NLC/USLCSG, and E-166 Positron Production Table 1: TESLA, NLC/USLCSG, E-166 Polarized Positron Parameters

Parameter Units TESLA* NLC E-166 Beam Energy, Ee GeV 150-250 150 50 Ne/bunch - 3x1010 8x109 1x1010

Nbunch/pulse - 2820 190 1 Pulses/s Hz 5 120 30 Undulator Type - planar helical helical Undulator Parameter, K - 1 1 0.17 Undulator Periodu cm 1.4 1.0 0.24 1st Harmonic Cutoff, Ec10 MeV 9-25 11 9.6 dN/dL photons/m/e- 1 2.6 0.37 Undulator Length, L m 135 132 1 Target Material - Ti-alloy Ti-alloy Ti-alloy, W Target Thickness r.l. 0.4 0.5 0.5 Yield % 1-5 1.8† 0.5 Capture Efficiency % 25 20 - N+/pulse - 8.5x1012 1.5x1012 2x107

N+/bunch - 3x1010 8x109 2x107

Positron Polarization % - 40-70 40-70 *TESLA baseline design; TESLA polarized e+ parameters (undulator and polarization) are the same as for the NLC/USLCSG † Including the effect of photon collimation at = 1.414.

11 EPAC June 2003

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.

12 EPAC June 2003

E-166 Beamline Schematic

50 GeV, low emittance electron beam

2.4 mm period, K=0.17 helical undulator

10 MeV polarized photons

0.5 r.l. converter target

51%-54% positron polarization

13 EPAC June 2003

E-166 Helical Undulator Design, =2.4 mm, K=0.17PULSED HELICAL UNDULATOR FOR TEST AT

SLAC THE POLARIZED POSITRON PRODUCTION SCHEME. BASIC DESCRIPTION.

Alexander A. Mikhailichenko

CBN 02-10, LCC-106

14 EPAC June 2003

Helical Undulator Radiation

2

2

30.6/ / 0.37 /

1u

dN Kphotons m e photons e

dL mm K

Circularly Polarized Photons

2

10 2

5024 9.6

1

e

c

u

E GeVE MeV MeV

mm K

15 EPAC June 2003

Photon Int., Angular Dist., Number Spctr., Polarization

16 EPAC June 2003

Polarized Positrons from Polarized ’s

(Olsen & Maximon, 1959)

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

Positron polarization varies with the energy transferred to the positron.

17 EPAC June 2003

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%.

18 EPAC June 2003

Polarimetry

K-Peter Schüler Presentation

19 EPAC June 2003

Polarimeter Overview

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

4 x 109 4 x 107

20 EPAC June 2003

Transmission Polarimetry of (monochromatic)

Photons

Pecomp

paircompphot

PP

0

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

all unpolarized contributions cancel in the transmission

asymmetry (monochromatic case)

21 EPAC June 2003

Transmission Polarimetry of Photons

Monochromatic Case

But, undulator photons are not monochromatic: Must use number or energy weighted integrals

Analyzing Power:

22 EPAC June 2003

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

23 EPAC June 2003

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

24 EPAC June 2003

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: 05.0/

07.0

ee

e

PP

P

active volumePhoton Analyzer Magnet: 50 mm dia. x 150 mm longPositron Analyzer Magnet: 50 mm dia. x 75 mm long

25 EPAC June 2003

Photon Polarimeter Detectors

Si-W Calorimeter Threshold Cerenkov (Aerogel)

E-144 Designs:

26 EPAC June 2003

Positron Polarimeter Layout

27 EPAC June 2003

Positron Transport System

e+ transmission (%) through spectromete

r

photon backgroundfraction reaching CsI-detector

28 EPAC June 2003

CsI Calorimeter Detector

Crystals: from BaBar ExperimentNumber of crystals: 4 x 4 = 16Typical front face of one crystal: 4.7 cm x 4.7 cmTypical backface of one crystal: 6 cm x 6 cmTypical length: 30 cmDensity: 4.53 g/cm³Rad. Length 8.39 g/cm² = 1.85 cmMean free path (5 MeV): 27.6 g/cm² = 6.1 cmNo. of interaction lengths (5 MeV): 4.92Long. Leakage (5 MeV): 0.73 %

Photodiode Readout (2 per crystal): Hamamatsu S2744-08with preamps

29 EPAC June 2003

1% stat. measurements very fast (~ minutes), main syst. error of ΔP /P ~ 0.05 from Pe

Expected Photon Polarimeter Performance

62.0

07.0

0266.0

E

e

A

P

Si-W Calorimeter

Energy-weighted Mean

EA

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

will measure P for E > 5 MeV (see

Table 12)

Aerogel Cerenkov

30 EPAC June 2003

Expected Positron Polarimeter Performance I

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

Photon Spectrum & Angular Distr.

Number- & Energy-WeightedAnalyzing Power vs. Energy

10 Million simulated e+ per point & polarity on the re-conversion target

31 EPAC June 2003

Expected Positron Polarimeter Performance II

Analyzing Powervs. Target Thickness

Analyzing Powervs. Energy Spread

32 EPAC June 2003

Expected Positron Polarimeter Performance III

Table 13

33 EPAC June 2003

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

34 EPAC June 2003

Beam Request

J. C. Sheppard Presentation II

35 EPAC June 2003

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.

36 EPAC June 2003

E-166 Beam Measurements

•Photon flux and polarization as a function of K.

•Positron flux and polarization for K=0.17, 0.5 r.l. of Ti vs. energy.

•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 10% is sufficient to validate the polarized positron production processes

37 EPAC June 2003

E-166 Institutional Responsibilities

Electron Beamline SLACUndulator CornellPositron Beamline Princeton/SLACPhoton Beamline SLACPolarimetry:

Overall DESYMagnetized Fe Absorbers DESY

Cerenkov Detectors PrincetonSi-W Calorimeter Tenn./ S. Carolina

CsI Calorimeter DESY/HumboldtDAQ Humboldt/Tenn./S. Car.

38 EPAC June 2003

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.

39 EPAC June 2003

E-166 Costs

Sub-system EFD

Labor

SLAC

Labor

SLAC

M&S

Coll.

Contr.Exist.

FFTB

Exist. non-FFTB

Total

Elec. BT 50 63 49 99 67 55 382

Gam. BT 54 18 58 50 29 35 244

Posi. BT 60 47 75 60 41 33 315

Gen/Infrstr 10 15 25 50 20 10 130

Grand Total 172 143 207 259 157 133 1071

(All entries in k$)

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

40 EPAC June 2003

E-166 Institutional Responsibilities

(All entries in k$)

SLAC Beamline, infrastructure 207Cornell Undulator, pulser 85Princeton Spctr. Magnets, Aerogel cntr 60DESY Fe absorber magnets 20U. Tenn/U. S. Carolina

Si-W cal., DAQ 15

Humboldt U. CsI cal, DAQ 15Simulations All 50Erroneous HSB PS 14

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

41 EPAC June 2003

Undulator Photon Beam I

Undulator basics (1st harmonic shown only)

E166 undulator parameters

(polarimetry extra)

42 EPAC June 2003

Undulator Photon Beam II

photon spectrum, angular distribution and polarization

(polarimetry extra)

43 EPAC June 2003

Positron Beam Simulation

distributions behind the converter target (0.5 r.l. Ti)based on polarized EGS shower simulations by K. Flöttmann

(polarimetry extra)

44 EPAC June 2003

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

(polarimetry extra)

45 EPAC June 2003

Trade-offs

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

(polarimetry extra)

46 EPAC June 2003

Compton Cross Section

(polarimetry extra)

47 EPAC June 2003

e+ polarimeter: typical GEANT output (example) I

(polarimetry extra)

48 EPAC June 2003

e+ polarimeter: typical GEANT output (example) II

*

* Assuming 2 x 105 e+ per pulse (1% e+ spectrometer transmission)

(polarimetry extra)