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