Measurement of the Positive Muon Lifetime to 1 ppm
David WebberPreliminary Examination
March 31, 2005
Outline
• Basics and theory
• How is the muon lifetime measured?
• MuLan experiment’s main components
• Systematics and design considerations
• Analysis cross-checks
• Personal contribution
• Conclusion
Basics
ee
e
e
W
s )4(19703.2Log(counts)
time
Why is the muon lifetime important?
1192
13
52mGF e
e
rM
gG
W
F 182 2
2
e
eW
Why is the muon lifetime important?
1192
13
52mGF e
e
rM
gG
W
F 182 2
2
e
eW
0Z
+ …
+ …
The theoretical uncertainty on GF as extracted from the muon lifetime is < 0.3 ppm.
…it is extremely difficult to predict, even in the relatively short term, the accuracy to which fundamental parameters will be determined and it is important that these be extracted to the limits that the current theoretical and experimental technology allows. (Ritbergen and Stuart, hep-ph/9904240)
Why is the Muon Lifetime Important?
0.06 ppm9 ppm16
2
610
10105
7.142.0
The goal of the Muon Lifetime Analysis (MuLan) experiment is to reduce the experimental uncertainty on GF to 0.5 ppm by measuring the muon lifetime to 1 ppm.
How is the muon lifetime measured?
N=1
How is the muon lifetime measured?
N=10
How is the muon lifetime measured?
N=100
How is the muon lifetime measured?
N=100
How is the muon lifetime measured?
N=104
How is the muon lifetime measured?
N=106
How is the muon lifetime measured?
N=1012
-3102.207
How long will it take?
• ~1012 events necessary for 1 ppm measurement
~ 3 weeks beam time
(usable)
Pulsed beam
~1.6 years
beam time
20 kHzContinuous beam
~104 years1 / 50 cm2 s
1 / hand s
Cosmic rays
Time to 1012Muon rateSource
s50/1
s32/22
Scint.
PMTPMT
Water
+
e+
+
+
e+PMT
The MuLan Experiment
Quads
+10 kV 0-10 kV
Kicker Plates
Kicker HV supply; 50 kHz cycle time
PSI
E
3 4.
2 M
eV
muo
n so
urce
Subsystem #1
500 MHz WFD Bank
inner
outer
N
S
Subsystem #2
Online DAQ and Analysis Farm Subsystem #3
t
Subsystem #3#4
BEAM
DETECTORS
DAQDIGITIZERS
+
The MuLan Experiment - Beamline
Key Beamline Elements•Production target•Separator•Kicker•Entrance muon chamber•Depolarizing target
The MuLan Experiment - Detector
The Mulan Experiment – Readout
2 Analog PulsesWaveform Digitizers
Plan for 2005-2006 runs
x2
Systematics
Early-to-late systematics • Clock stability• Pileup
– 2 pulses appear as one
• Muon spin precession• Others
– Sneaky muons– Instrumental changes– Kicker noise
The most dangerous systematic effects occur “early-to-late”
“early”
“late”
Clock Stability• A single clock drives the
waveform digitizers
• The clock is tunable, and the analyzers only know the 4 most significant digits (500 ppm)
Rubidium Atomic Clock
MuLan
Agilent Clock
Error
60 MHz 59.99999878 MHz 20 ppb
30 MHz 29.99999939 MHz 20 ppb
Pileup Reduction
• • Highly segmented detector
(170 detector pairs)• Analog readout by
waveform digitizers• Depolarize the collected
muons
/2~ te
The importance of waveform digitizers
• Identify false pulses
• No missed pulses from pileup
• Pileup identification
Good Pulse
vs.
Phototube Breakdown
2 pulses become 1 large pulse
Pulse A
rea (outer)
Pulse Area (inner)
Muon Spin Precession
• Muons are highly polarized and can remain so when they stop in the target
• Muon decay violates parity
• Muons precess in a magnetic field.
• Example: the Earth’s magnetic field will change the preferred decay direction by one detector in one muon lifetime
Muon Spin Precession - Fixes
• Point-like symmetric detector ball
• Depolarizing target – Sulfur has ~8% residual
polarization– Arnokrome-3 (30%
chromium, 10% cobalt, 60% iron) has 0.5 T internal field
• Ring magnet on sulfur dephases ensemble during accumulation
FrontBack
Silver Target
Cross-Checks
• Multiple identical detectors• LED system
– Test-fire the detector– Check for timing shifts
• Stable clock system• Blind analysis• Analysis checks
– Start-time scan– Stop-time scan
Plots courtesy: D. Chitwood
Personal Contribution
Ball Discriminators TDCs
Gigabit Switch
TDC frontend
Backend
LED Drivers Flight Simulators Flight Simulator frontend
High Voltage frontend High Voltage
Entrance Muon Chamber
TDCs EMC frontend
Beamline Beamline frontend
CAMAC frontend
RAID and TAPE: Data Storage and Offline Analysis
Ethernet
Online Monitor
PSI Archive
Kicker Programmable gate generator
Programmable gate generator frontend
Marker Pulses
MULAN Continuous Data Acquisition
ADC/SCALER # fills protons hits in detector
infinite persistence scope
serial port
enhanced parallel port
network
camac
Muon Production Target
Waveform Digitizers Waveform Digitizer frontends
Discriminators
Note:Yellow – Frontend programsGreen – Frontend computers
fiberoptic
fiberoptic
fiberoptic
fiberoptic
The MuLan Experiment - Software
“Good” “2 AM Phone Call”
High-rate, Entrance Muon Chamber
I helped design the green boards (above), commissioned the chamber and readout electronics (above right), and wrote the real-time online beam profiler to the right.
Conclusion
• Basics of muon decay
• The MuLan experiment
• Systematics
• Personal contribution
• Last muon lifetime measurements 1984– Muon decay gives best determination of GF
– Muon lifetime gives the dominant error on GF
– It is time to measure the muon lifetime again
Thank you!
References
• Ritbergen and Stuart, hep-ph/9904240.• Chitwood, Dan. “Measuring the Positive Muon Lifetime
to 1 ppm.” Preliminary Exam Paper. September 23, 2002.
• R. M. Carey et al. MuLan Proposal. http://www.npl.uiuc.edu/exp/mulan/proposal/MuLan.ps
History of the Muon Lifetime
-50
0
50
100
150
Bardin Giovanetti Balandin Duclos
Valu
e -
PD
G A
vera
ge
(p
pm
)
PDG Average = 2197.03 +/- 0.04 ns
Lan goal 5
Positron Michel Sprectrum
Positron energy
Rel
ativ
e In
ten
sity
Michel Spectrum
53 MeV
Why is the Muon Lifetime Important?
Model-independent extraction of GF
General Analysis
Restricted Analysis
Other “Early-to-Late” Effects
• Sneaky Muons– Fix: Entrance Muon chamber
• Instrumental Changes– Fix: LED test-firing system
• Kicker Noise– Recently reduced by 103
– Under investigation
The Mulan Experiment – Readout
2 Analog Pulses
2 Analog PulsesTime to Digital
Converter
Waveform Digitizer
Discriminator20-bit
time word
Now
Planned for 2005-2006 Runs
What is the Muon Lifetime?
/0)( teNtN
s )4(19703.2