Progress on the final TWIST measurement of James Bueno, University of British Columbia and TRIUMF on behalf of the Triumf Weak Interaction Symmetry Test

Progress on the final TWIST measurement of James Bueno, University of British Columbia and TRIUMF

P

on behalf of theTriumf Weak Interaction Symmetry Test

Outline

• Muon production and decay

• Previous measurements of

• Sources of depolarisation,

and their simulation.– Depolarisation in fringe field of solenoid.– Depolarisation in the muon stopping target.

P

James Bueno, WNPPC 2008, 15 February

1/20

Muon production and


P

hh

Muons from stationary

pion decay are perfectly

polarised

(for massless neutrinos)

TWIST only accepts a narrowmomentum range

muon beam is highly polarised

2/20

Muon decay and


P

e

e

cosP,xF,,xF

cosddx

dASIS

2

TWIST measures

e+ energy (x) and

angle (θ) relative

to muon spin.

3/20

Muon decay and


P

e

e

cosP,xF,,xF

cosddx

dASIS

2

TWIST measures

e+ energy (x) and

angle (θ) relative

to muon spin.

determines forward-

backward asymmetry

3/20

Muon decay and


P

e

e

cosP,xF,,xF

cosddx

dASIS

2

TWIST measures

e+ energy (x) and

angle (θ) relative

to muon spin.

determines forward-

backward asymmetry appears as an

inseparable productP

3/20

Physics motivation

muon handedness

9

16

3

11

2

1QR

SM predicts LH muon decays to LH positron.

Probability of RH muon decay to a LH

or RH positron is


4/20

Physics motivation

muon handedness left-right symmetric models

9

16

3

11

2

1QR

SM predicts LH muon decays to LH positron.

Probability of RH muon decay to a LH

or RH positron is

cosWsinWW

sinWcosWW

21R

21L

4

2

1

2

2

12

m

m

m

m4P1


4/20

Previous measurements of

1987, Beltrami et al. (PSI)1.0027 ± 0.0079 (stat.)

± 0.0030 (syst.)

2006, Jamieson et al. (TWIST, TRIUMF)

1.0003 ± 0.0006 (stat.)

± 0.0038 (syst.)

P


Standard model, 1,1P

5/20



± 0.0030 (syst.)


1.0003 ± 0.0006 (stat.)

± 0.0038 (syst.)

2004, Jodidio et al. with TWIST ρ, δ (Indirect)

0.9960 < < 1.0040

(90% confidence)

P



P

5/20



± 0.0030 (syst.)


1.0003 ± 0.0006 (stat.)

± 0.0038 (syst.)


0.9960 < < 1.0040

(90% confidence)

2009, final result

from TWIST (goal)± 0.0003 (stat.)± 0.0010 (syst.)

P



P

5/20



± 0.0030 (syst.)


1.0003 ± 0.0006 (stat.)

± 0.0038 (syst.)


0.9960 < < 1.0040

(90% confidence)

2009, final result

from TWIST (goal)± 0.0003 (stat.)± 0.0010 (syst.)

P



P Previous TWIST direct measurement

limited by uncertainties in simulating P

5/20

TWIST measurement of P

1

x

= E/Emax

-1

+10cos θ

Michel spectrum

from real data

P

0

6/20

TWIST measurement of P

1

x

= E/Emax

-1

+10cos θ

Michel spectrum

from real data

Spectrum from

GEANT simulationhidden

difference in is measuredP

Spectra from decay (depolarised) μ are compared depolarisation in simulation must match data

P

0

6/20

Sources of depolarisation


symmetric tracking volume

7/20




solenoid fringe field

depends on trajectoryP

7/20



high purity metal

target to stop muons


depends on timeP

solenoid fringe field

depends on trajectoryP

7/20

Simulating the depolarisation


Beam is measuredby time expansion

chambers BEFOREthe fringe field


7/20

XY

Simulating the depolarisation


Beam is measuredby time expansion

chambers BEFOREthe fringe field

Time dependence ofe+ forward-backward

asymmetry gives in target tP


7/20

XY

Simulating muon trajectories


1. Measure each muon’s

position and angle with

time expansion chambers.

projection

in x

8/20

Simulating muon trajectories


1. Measure each muon’s

position and angle with

time expansion chambers.

projection

in x

2. Correct angles for

multiple scattering.

3. Simulate muons with

GEANT and track spin to

predict at target.P

Uncertainties due to beam instability, initial position& angle of beam, position & shape of B-field.

8/20

Beam stability (1 of 2)


During data acquisition, we needed a

stable muon beam for weeks on end.

9/20



During data acquisition, we needed a

stable muon beam for weeks on end.

And a method of detecting beam

instabilities without the time expansion

chambers in place.

(too much multiple scattering)

9/20



For 2006/7, internal beamspots are fit to a helix.

Helix fit parameters

indicate position and

momentum.

most of

data set

beam

instability

10/20

5mm

Misalignment uncertainties


For 2006/2007 data, the beam was steered

onto the solenoid axis.

2004 beam

2006/7 beam

y (cm)

11/20

Misalignment uncertainties


For 2006/2007 data, the beam was steered

onto the solenoid axis.

2004 beam

2006/7 beam

y (cm)

2006

2004

Sensitivity to misalignmentsis now reduced

11/20

Spin tracking in the B-field


To validate GEANT’s spin tracking: two weeks

of low data taken in 2006/7.

nominalsteered

at angle

P

12/20





nominalsteered

at angle

inside detector

nominal

steered

P

12/20





nominalsteered

at angle

inside detector

nominal

steered

P

Validation (in progress now) will require:

SIMULATION

steerednominal

DATA

steerednominal PPPP

12/20

Stopping target depolarisation


Ideally, the muons would not depolarise at the

stopping target

(it’s 99.999% pure aluminium, or silver).

However, an individual muon spin can interact

with local magnetic fields and reverse sign.

13/20



The simulation must include the correct model

for in the target. tP

14/20





TWIST result for silver

theoretically

preferred

atexp0PtP

also possible

2btexp0PtP

14/20





TWIST result for silver

theoretically

preferred

atexp0PtP

also possible

2btexp0PtP no data below

~ 1 μs (or above 10 μs)

A subsidiary experiment would be helpful…

14/20

Subsidiary muSR experiment

collimatortarget

trigger scintillatorsSchematic, not to scale

15/20


collimatorbackward

e+ counter

target

forward e+

counter


15/20


collimatorbackward

e+ counter

target

forward e+

counter


e

15/20


Advantages:• Higher rate (~30kHz compared to ~3kHz)• Larger time fiducial

(~5 ns 14 μs, compared to ~1 μs ~9 μs).

16/20



Advantages:• Higher rate (~30kHz compared to ~3kHz)• Larger time fiducial

(~5 ns 14 μs, compared to ~1 μs ~9 μs).

Disadvantages:• Background from beam positrons.• A fraction of muons stop in the scintillator.

16/20



Unfortunately, still no discrimination betweenand atexp0PtP 2btexp0PtP

17/20


exp(-at)Aluminium

a (x10-6ns-1)

Silver

a (x10-6ns-1)

TWIST 1.4 ± 0.1 1.1 ± 0.3

muSR 1.7 ± 0.2 1.2 ± 0.2

Unfortunately, still no discrimination betweenand atexp0PtP 2btexp0PtP

17/20


exp(-at)Aluminium

a (x10-6ns-1)

Silver

a (x10-6ns-1)

TWIST 1.4 ± 0.1 1.1 ± 0.3

muSR 1.7 ± 0.2 1.2 ± 0.2

Unfortunately, still no discrimination betweenand

Experiment confirmed no fast depolarisationbetween ~5 ns and 1 μs, and results are

consistent with TWIST.

atexp0PtP 2btexp0PtP

17/20

Using target PCs in TWIST

proportional chambers

closest to stopping target

stopping

target

μ can stop

in chamber gas

18/20

PC6PC5




stopping

target

μ can stop

in chamber gas

pulse width in final PCgreater for muons that

stop here (moreenergy deposited)

18/20

PC6PC5




stopping

target

μ can stop

in chamber gas



PC 5 (ns) PC 6 (ns)

18/20

PC6PC5




stopping

target

μ can stop

in chamber gas



deep cut selectstarget stop μ

PC 5 (ns) PC 6 (ns)

18/20

PC6PC5and rejects

these

Summary


• The final measurement is dominated by uncertainties in simulating .

• Validation of the spin tracking is well underway.• Improvements in data acquisition in 2006/7 will

reduce uncertainties.

19/20

PP

P

Summary


• The final measurement is dominated by uncertainties in simulating .

• Validation of the spin tracking is well underway.• Improvements in data acquisition in 2006/7 will

reduce uncertainties.• Subsidiary muSR experiment has confirmed

no fast depolarisation exists

(this could not be measured by TWIST).• Time dependent depolarisation uncertainties can

be minimised by selecting genuine target stops.

19/20

PP

P


Alberta

Ryan Bayes*† Glen Marshall Andrei Gaponenko**

Yuri Davydov Dick Mischke Peter Kitching

Wayne Faszer Mina Nozar Robert MacDonald*

Makoto Fujiwara Konstantin Olchanski Maher Quraan

David Gill Art Olin†

Alexander Grossheim Robert Openshaw British Columbia

Peter Gumplinger Jean-Michel Poutissou James Bueno*

Anthony Hillairet*† Renée Poutissou Mike Hasinoff

Robert Henderson Grant Sheffer Blair Jamieson**

Jingliang Hu Bill Shin‡‡

Texas A&M

Regina Montréal Carl Gagliardi

Ted Mathie Pierre Depommier Jim Musser**

Roman Tacik Bob Tribble

Valparaiso

Kurchatov Institute Don Koetke

Vladimir Selivanov Shirvel Stanislaus

TRIUMF

* graduate student, ** graduated

† also UVic, ‡‡ also Saskatchewan

TWIST is supported in part by

the Natural Sciences and

Engineering Research Council

and the National Research

Council of Canada, the Russian

Ministry of Science, and the

U.S. Department of Energy.

Computing resources for the

analysis are provided

by Westgrid.

The TWIST collaboration 20/20

Documents

Progress on the final TWIST measurement of James Bueno, University of British Columbia and TRIUMF on behalf of the Triumf Weak Interaction Symmetry Test