Powerpoint, 2.7 MB

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

Achim W. Weidemann

University of South Carolina, Columbia (@SLAC)

• Introduction (What, who)

•

Motivation (Why)

• Experiment and Polarimetry (How)

•

Outlook

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

1

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.


2

E-166 Collaborators


3

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.


4



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

-

R

+

L

-

L

+

L

-

R

+

R

L e +

R or e e (and not e e or e e ).



Can double or suppress rate using polarized positrons

(in addition to pol. e-).

– Effective polarization enhanced, and error decreased, in electroweak asymmetry measurements,

(N

L

– N

R

) / (N

L

+ N

R

) = P eff

A

LR

,

P eff

= (P

-

- P

+

) / (1 – P

-

P

+

).

- Improved accuracy in polarization measurement (Blondel scheme)

►Must have both e + and e polarization for Giga-Z project

(sin 2 θ

W

)


5

(SUSY)Physics Motivation for Polarized Positrons

Slepton and squark produced e e

R R

  e e

R e e

L L

Separation of the (LL, LR) selectron pair

  e e

L L

  e e

L R 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!


6



• 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?


7

Polarized Positrons at LC

2 Target assemblies for redundancy

(+ polarized e- source)


8

Polarized Positrons at FFTB

• 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


9

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.


10

LC / E-166 Parameter Comparison

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

Parameter

Beam Energy, E e

N e

/bunch

N bunch

/pulse

Pulses/s

Units

GeV

-

-

Hz

TESLA*

150-250

3x10

10

2820

5

NLC

150

8x10

190

120

9

E-166

50

1x10

10

1

30

Undulator Type

Undulator Parameter, K

Undulator Period

 u

1 st

Harmonic Cutoff, E c10

-

- cm planar

1

1.4 helical

1

1.0 helical

0.17

0.24 dN  /dL

Undulator Length, L

Target Material

Target Thickness

MeV

photons/m/e m

- r.l.

%

-

-

9-25

1

135

11

2.6

132

9.6

0.37

1

Ti-alloy Ti-alloy Ti-alloy, W

0.4

1-5

0.5

1.8†

0.5

0.5 Yield

Capture Efficiency

N

+

/pulse

N

+

/bunch

Positron Polarization

%

-

25

8.5x10

12

3x10

10

20

1.5x10

8x10

9

12

-

2x10

2x10

7

7

% - 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.


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Helical Undulator l=2.4 mm,

K=0.17

Table 3: FFTB Helical Undulator System Parameters

Parameter

Number of Undulators

Length

Units

- m

Value

1

1.0

Inner Diameter

Period

Field

Undulator Parameter, K

Current

Peak Voltage

Pulse Width

Inductance

Wire Type

Wire Diameter

Resistance

Repetition Rate

Power Dissipation



T/pulse mm mm kG

-

Amps

Volts

 s

H

- mm ohms

Hz

W

0

C

0.89

2.4

7.6

0.17

2300

540

30

0.9x10

-6

Cu

0.6

0.110

30

260

2.7

Energy Polarization dN

 dL



 u

30.6

 

1



K

2

K

2

/ /

 

0.37

E c 10



24



MeV



 u



E e

50

 mm





GeV



1



K

2



 

2



9.6

MeV



Alexander A. Mikhailichenko, Pulsed Helical

Undulator….CBN 02-10, LCC-106


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P(e + )

Circ. γ -> long. e + polarization

N(e + )

P(e + )

Olsen & Maximon, 1959

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


13

Polarimeter Overview

4 x 10 9 

 4 x 10 7 

1 x 10 10 e -

 4 x 10 9 

4 x 10 9 

 2 x 10 7 e +

2 x 10 7 e +

 4 x 10 5 e + 4 x 10 5 e +

 1 x 10 3 


14

Photon Transmission Polarimetry

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



 comp





 phot



0



  comp

P



P e



P

  pair

P e



P e

/ P e





0 .

07

0 .

05

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


15

Expected Photon

Polarimeter Performance

Si-W Calorimeter

Expected measured energy asymmetry δ = (E

+

A



E

-E

-

)/(E

+

+E

-

) by analytic integration and, with good agreement, from polarized

GEANT simulation:



P e





0 .

0266

0 .

07

A



E



0 .

62 Energy-weighted Mean

Aerogel Cerenkov

will measure P



for E



> 5 MeV;

1% stat. measurements very fast (~ minutes), main syst. error of ΔP



/P



~ 0.05 from P e


16

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 (~10 8 photons /15 minutes) δ(P) ~ 2 – 4 %

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


17

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


18

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.stanford.edu/exp/e166


19

Backup Slides


20

Positron Polarimeter Layout


21

For Photons:

Photon Detectors

Threshold Cerenkov (AeroGel) Si-W Calorimeter


22

Positron Transport System e+ transmission

(%) through spectrometer photon background fraction reaching

CsIdetector


23

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:

Density:

30 cm

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


24

Expected Positron

Polarimeter Performance

Expected systematic Error of

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


25

Powerpoint, 2.7 MB

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

Achim W. Weidemann

E-166 Experiment

•

•

•

E-166 Collaborators

E-166 Outlook

Backup Slides

Photon Detectors

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Powerpoint, 2.7 MB

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

Achim W. Weidemann

E-166 Experiment

•

•

•

E-166 Collaborators

E-166 Outlook

Backup Slides

Photon Detectors

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