introduction (what, who) motivation (why) experiment and polarimetry (how) outlook

5th Rencontres du Vietnam - Aug. 7, 2004 Polarized Positrons…E166 A.W.Weidemann 1

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

Achim W. Weidemann

University of South Carolina, Columbia (@SLAC)

Polarized Positrons at a Linear Collider and FFTB (SLAC E-166)


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.


E-166 Collaborators


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 -> 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.


– Electroweak processes e+e- -> WW, Z, ZH couple only to e-Le+

R or e-

Re+L (and not e-

Le+L or e-

Re+R).

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 (sin2θW )



(SUSY)Physics Motivation for Polarized Positrons

L Le e

L Le e

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% => no separation!

•P(e+)= -40% => 163fb vs 66 fb

Can’t do without positron polarization!

L Re e

R Le e R Re e


• 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?



Polarized Positrons at LC

2 Target assemblies for redundancy (+ polarized e- source)


• 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

Polarized Positrons at FFTB


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.

E-166 vs LC


LC / E-166 Parameter Comparison

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.


Helical Undulator l=2.4 mm, K=0.17

Alexander A. Mikhailichenko, Pulsed Helical Undulator….CBN 02-10, LCC-106

2

2

30.6/ / 0.37 /

1u

dN Kphotons m e photons e

dL mm K

2

10 2

5024 9.6

1

e

c

u

E GeVE MeV MeV

mm K

Energy Polarization


Circ. γ -> long. e+ polarization

P(e+) N(e+)

P(e+)

0.5 r.l. Ti Alloy target; 0.5% yield, P(e+)=54% averaged over full spectrum

Olsen & Maximon, 1959


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


Photon Transmission Polarimetry

Pecomp

paircompphot

PP

0

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

For photons of undulator spectrum, use number- or energy-weighted integral.

05.0/

07.0

ee

e

PP

P


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 by analytic integration and, with good agreement, from polarized GEANT simulation:

will measure P for E > 5 MeV;

Aerogel Cerenkov


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) δ(P) ~ 2 – 4 %

Expected systematic Error of δ(P)/P ~5% dominated by eff. Magnetization of iron


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


• 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!

E-166 Outlook

For References, details see:

http://www.slac.stanford.edu/exp/e166


Backup Slides


Positron Polarimeter Layout


Photon Detectors

For Photons:

Threshold Cerenkov (AeroGel) Si-W Calorimeter


Positron Transport System

e+ transmission (%) through spectrometer

photon backgroundfraction reaching CsI-detector


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


Expected Positron Polarimeter Performance

Expected systematic Error of δ(P)/P ~5% dominated by eff. Magnetization of iron

introduction (what, who) motivation (why) experiment and polarimetry (how) outlook

Documents

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polarized positronspolarized

ushaving polarized e

polarized beams

electroweak processes

polarized positronsslepton

linear colliders e

polarized positron production