Transcript Polarized Positrons at a Linear Collider and FFTB (SLAC E-166) Achim W.

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
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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.
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E-166 Collaborators
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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 nonSM physics searches)
• Access to physics using transversely polarized beams
(only works if both beams are polarized)
• Improved accuracy in measuring polarization.
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Physics Motivation for
Polarized Positrons
– 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 epolarization for Giga-Z project
(sin2θW )
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(SUSY)Physics Motivation
for Polarized Positrons
Slepton and squark produced
 
dominantly via eR eL
(and not eReR or eLeL ).
Separation of the (LL, LR) selectron pair
eLeL
eLeR
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!
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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?
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Polarized Positrons at LC
2 Target
assemblies
for
redundancy
(+ polarized
e- source)
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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
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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.
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LC / E-166 Parameter Comparison
Table 1: TESLA, NLC/USLCSG, E-166 Polarized Positron Parameters
Parameter
Units
TESLA*
NLC
E-166
GeV
150-250
150
50
Beam Energy, Ee
10
9
3x10
8x10
1x1010
Ne/bunch
2820
190
1
Nbunch/pulse
Hz
5
120
30
Pulses/s
planar
helical
helical
Undulator Type
1
1
0.17
Undulator Parameter, K
cm
1.4
1.0
0.24
Undulator Period u
st
MeV
9-25
11
9.6
1 Harmonic Cutoff, Ec10
photons/m/e
1
2.6
0.37
dN /dL
m
135
132
1
Undulator Length, L
Ti-alloy Ti-alloy Ti-alloy, W
Target Material
r.l.
0.4
0.5
0.5
Target Thickness
%
1-5
1.8†
0.5
Yield
%
25
20
Capture Efficiency
12
12
8.5x10
1.5x10
2x107
N+/pulse
3x1010
8x109
2x107
N+/bunch
%
40-70
40-70
Positron Polarization
*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
Inner Diameter
Period
Field
Undulator Parameter, K
Current
Peak Voltage
Pulse Width
Inductance
Wire Type
Wire Diameter
Resistance
Repetition Rate
Power Dissipation
T/pulse
Energy
Units
m
mm
mm
kG
Amps
Volts
s
H
mm
ohms
Hz
W
0
C
Value
1
1.0
0.89
2.4
7.6
0.17
2300
540
30
0.9x10-6
Cu
0.6
0.110
30
260
2.7
Polarization
30.6 K 2



photons
/
m
/
e

0.37
photons
/
e
dL u  mm 1  K 2
dN
Ec10  24  MeV 
E
e
50 GeV 
2
u  mm 1  K 2 
 9.6 MeV
Alexander A. Mikhailichenko,
Pulsed Helical
Undulator….CBN 02-10, LCC-106
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Circ. γ -> 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
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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+
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4 x 105 e+
 1 x 103 
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Photon Transmission Polarimetry
M. Goldhaber et al. Phys. Rev. 106 (1957) 826.


 comp 
 phot   comp   pair
 0  P Pe  P
Pe  0.07
Pe / Pe  0.05
For photons of undulator spectrum, use number- or energy-weighted integral.
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Expected Photon
Polarimeter Performance
Si-W Calorimeter
Expected measured energy asymmetry δ = (E+-E-)/(E++E-)
and energy-weighted analyzing power AE
by analytic integration and, with good agreement, from polarized
GEANT simulation:
  0.0266
Pe  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 Pe
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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
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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
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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.stanford.edu/exp/e166
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Backup Slides
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Positron Polarimeter Layout
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Photon Detectors
For Photons:
Threshold Cerenkov (AeroGel)
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Si-W Calorimeter
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Positron Transport System
e+
transmission
(%) through
spectrometer
photon
background
fraction
reaching
CsIdetector
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CsI Calorimeter Detector
Crystals:
Number of crystals:
Typical front face of one crystal:
Typical backface of one crystal:
Typical length:
Density:
Rad. Length
Mean free path (5 MeV):
No. of interaction lengths (5 MeV):
Long. Leakage (5 MeV):
from BaBar Experiment
4 x 4 = 16
4.7 cm x 4.7 cm
6 cm x 6 cm
30 cm
4.53 g/cm³
8.39 g/cm² = 1.85 cm
27.6 g/cm² = 6.1 cm
4.92
0.73 %
Photodiode Readout (2 per crystal):
Hamamatsu S2744-08
with preamps
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Expected Positron
Polarimeter Performance
Expected systematic Error of δ(P)/P ~5% dominated by eff. Magnetization of iron
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