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Exploring Muon Decay
with
TWIST
Carl A. Gagliardi
Texas A&M University
for the TWIST Collaboration
Outline
• Physics of muon decay
• TWIST experiment
• TWIST results to date
• How will we do better?
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 1
Muon decay matrix element
• Most general local, derivative-free, lepton-number conserving muon
decay matrix element:
4GF
M
2
g e | | ( e ) n ( ) m | |
S ,V ,T
, R ,L
• In the Standard Model, gVLL = 1, all others are zero
• Pre-TWIST global fit results (all 90% c.l.):
• Theoretical constraints have recently been derived on the LR and
RL terms from neutrino mass limits (hep-ph/0608163)
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 2
Muon decay spectrum
• The energy and angle distributions of positrons following
polarized muon decay obey the Michel spectrum:
d 2 2 x
(3 3x ) ( 4 x 3) 3 0 (1 x )
x 2 dxd (cos ) 3 x
2
P cos (1 x ) ( 4 x 3)
3
Ee
where x
E e ,m ax
• Pre-TWIST accepted values for the Michel parameters:
SM
= 0.7518 ± 0.0026 3/4
= -0.007 ± 0.013 0
P = 1.0027 ± 0.0079 ± 0.0030 1
= 0.7486 ± 0.0026 ± 0.0028 3/4
P(/) > 0.99682 (90% c.l.) 1
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 3
Goal of TWIST
• Search for new physics that can be revealed by order-
of-magnitude improvements in our knowledge of ρ, δ,
and Pμξ
Two examples
• Model-independent limit on muon handedness
1 1 16
QR 1
2 3 9
• Left-right symmetric models
3 3 2 ML ML
2 4
1 P 4
2
M M
4 2 R R
• …..
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 4
What is required?
Must:
• Understand sources of muon depolarization
-- Pμ and ξ come as a product
• Determine spectrum shape
-- All three parameters
• Measure forward-backward asymmetry
-- For Pμξ and δ
to within a few parts in 104
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 5
Surface muon beam
TRIUMF
M13 beam line
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 6
TWIST spectrometer
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 7
Typical events
• Use pattern recognition (in
position and time) to sort hits
into tracks, then fit to helix
• Must also recognize beam
positrons, delta tracks,
backscattering tracks
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 8
2-d momentum-angle spectrum
In angular
fiducial
In momentum
fiducial
Acceptance of the TWIST spectrometer
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 9
αMC hidden
Fitting the data distributions blind analysis
• Fit data to sum of a MC
base spectrum plus MC-
generated derivative
distributions.
• Decay distribution is
linear in the Michel
parameters, so this is
exact, no matter what
values (αMC) are used in
the MC base spectrum.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 10
Physics data sets
• Fall 2002
– Test data-taking procedures and develop analysis techniques
– First physics results – ρ and δ
– Graphite-coated Mylar target not suitable for Pμξ
• Fall 2004
– Al target and Time Expansion Chamber enabled first Pμξ
measurement
– Improved determinations of ρ and δ are underway
• 2006-07
– Achieve ultimate TWIST precision for ρ, δ, and Pμξ
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 11
Fitting the 2002 data to determine ρ and δ
Normalized residuals
[(Data-Fit)/sigma] of the 2-d momentum-angle fit
Fit describes the data well, even when
extrapolated far outside the fiducial region
Angle-integrated results
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 12
Fitting the 2004 data to determine Pμξ
Separating the asymmetry
into components
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 13
Results to date
• From Fall, 2002 run:
– ρ = 0.75080 ± 0.00032 (stat) ± 0.00097 (syst) ± 0.00023 (η)
PRL 94, 101805
– δ = 0.74964 ± 0.00066 (stat) ± 0.00112 (syst)
PRD 71, 071101
• New global analysis (PRD 72, 073002) using the ρ and δ
results, together with previous measurements and recent
e+ transverse polarization measurements (PRL 94, 021802):
– Significant improvements in the limits for gS,V,TLR
– η = -0.0036 ± 0.0069
• From Fall, 2004 run (so far):
– Pμξ = 1.0003 ± 0.0006 (stat) ± 0.0038 (syst)
PRD 74, 072007
• Factors of 2-3 improvements on pre-TWIST precisions
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 14
New limits in left-right symmetric models
Restricted (“manifest”) LRS model General LRS model
Initial TWIST measurements already provide significant new limits
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 15
Results to date
• From Fall, 2002 run:
– ρ = 0.75080 ± 0.00032 (stat) ± 0.00097 (syst) ± 0.00023 (η)
PRL 94, 101805
– δ = 0.74964 ± 0.00066 (stat) ± 0.00112 (syst)
PRD 71, 071101
• New global analysis (PRD 72, 073002) using the ρ and δ
results, together with previous measurements and recent
e+ transverse polarization measurements (PRL 94, 021802):
– Significant improvements in the limits for gS,V,TLR
– η = -0.0036 ± 0.0069
• From Fall, 2004 run (so far):
– Pμξ = 1.0003 ± 0.0006 (stat) ± 0.0038 (syst)
PRD 74, 072007
• Factors of 2-3 improvements on pre-TWIST precisions
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 16
Systematics in the previous measurements
The same effects tend to dominate
the systematic uncertainties for all
three parameters.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 17
Reducing the leading systematics
• Issues that were unique to 2002 data
– Stopping target thickness uncertainty
– Chamber orientation uncertainty with respect to magnetic field
• For all three Michel parameters
– Chamber response
• Improved gas system regulation and monitoring
• Improved determination of foil geometry
• Improved treatment of drift chamber behavior
– Positron interactions
• Specific to Pμξ
– Muon depolarization when crossing fringe field
– Muon depolarization in the stopping target
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 18
Controlling positron interaction uncertainties –
“upstream stops”
• Our Monte Carlo must simulate positron interactions properly
• Need a well-understood test beam to validate the Monte Carlo
• We use the Michel spectrum!
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 19
Validating the Monte Carlo with “upstream stops”
(Downstream fit result) – (upstream fit result)
Bremsstrahlung Scattering
• The TWIST Monte Carlo provides an excellent description of the
hard interaction physics
• Took ~50 times more upstream stop events in 2004 than in 2002
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 20
Muon depolarization across the fringe field
Use Time Expansion Chamber (TEC) to
measure and optimize the muon beam
• First installed for 2004 run
• Found vertical beam offset – now corrected
• Now take frequent beam characterizations
• Have techniques to identify when the beam
2004 muon beam spot changes between TEC measurements
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 21
Muon depolarization after stopping
• Observed significant depolarization in 2004 data
• New techniques allow us to veto muons that stop outside the
metal stopping target
• More sensitive analysis procedures to determine the residual
depolarization rate
• Now taking data with a Ag target to explore material dependence
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 22
Conclusions
• The initial TWIST measurements have improved our
knowledge of the Michel parameters ρ, δ, and Pμξ by
factors of 2-3
• Improvements by additional factors of 3-5 are anticipated
• Stay tuned!
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 23
TWIST Participants
TRIUMF Alberta Kurchatov Institute
Ryan Bayes ¤y Andrei Gaponenko ¤¤
Yuri Davydov Vladimir Selivanov
Peter Kitching Vladimir Torokhov
Jaap Doornbos Robert MacDonald ¤
Wayne Faszer Maher Quraan
Makoto Fujiwara Nate Rodning x Texas A&M
David Gill John Schaapman
Alex Grossheim Carl Gagliardi
Glen Stinson Jim Musser ¤¤
Peter Gumplinger
Anthony Hillairet ¤y Bob Tribble
British Columbia
Robert Henderson Maxim Vasiliev
Jingliang Hu James Bueno ¤
John A. Macdonald x Mike Hasinoff Valparaiso
Glen Marshall Blair Jamieson ¤¤
Dick Mischke Don Koetke
Mina Nozar Montréal Paul Nord
Konstantin Olchanski Pierre Depommier Shirvel Stanislaus
Art Olin y
Robert Openshaw Regina ¤ Graduate student
Tracy Porcelli z ¤¤ Graduated
Jean-Michel Poutissou Ted Mathie
Roman Tacik y also U Vic
Renée Poutissou
z also Manitoba
Grant Sheffer
Bill Shin zz zz also Saskatchewan
x deceased
Supported under grants from NSERC (Canada) and DOE (USA).
Additional support from TRIUMF, NRC (Canada), and the Russian Ministry of Science.
Computing facilities of WestGrid are gratefully acknowledged.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 24
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 25
Coupling constants and Michel parameters
• The Michel parameters are bilinear combinations of the
coupling constants:
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 26
Detector array
• 56 low-mass high-precision planar chambers symmetrically placed around thin
target foil
• Measurement initiated by single thin scintillation counter at entrance to detector
• Beam stop position controlled by variable He/CO2 gas degrader
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 27
Analysis method
• Extract energy and angle distributions for data:
– Apply (unbiased) cuts on muon variables.
– Reject fast decays and backgrounds.
– Calibrate e+ energy to kinematic end point at 52.83 MeV.
• Fit to identically derived distributions from simulation:
– GEANT3 geometry contains virtually all detector components.
– Simulate chamber response in detail.
– Realistic, measured beam profile and divergence.
– Extra muon and beam positron contamination included.
– Output in digitized format, identical to real data.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 28
Data set by data set 2-d fit results
2002
ρ
2004
Pμξ
δ
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 29
Recent muon decay global analysis
PRD 72, 073002
These improvements arise
from the TWIST ρ and δ
measurements.
• Fit also finds η = -0.0036 ± 0.0069, a factor of ~2 more precise than
the previously accepted value, -0.007 ± 0.013.
• Significant improvement in η comes from new measurements of the
transverse polarization of the e+ in muon decay (PRL 94, 021802), but
the TWIST ρ and δ measurements also play an important part.
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 30
Validating the soft physics simulation
with “upstream stops”
dE/dx Multiple Scattering
• Multiple scattering is well reproduced
• Small differences in dE/dx are seen between data and MC
– Exploring differences between GEANT3 and GEANT4
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 31
Tracking (in)efficiency
• Measured in upstream stop events
• Reconstruct track upstream, then ask if downstream also reconstructs
• Overestimates
Difference between true inefficiency – not all tracks reach the downstream half
inefficiencies in data and
Monte Carlo events
Carl Gagliardi – Exploring Muon Decay with TWIST – DPF06 32
