an experiment for a high precision muon lifetime (and F ... · BC620 white dif f use reflective...

FAST

FAST:

an experiment for a high

precision muon lifetime (and GF )

measurement

LucaMalgeriUniversity of Geneva

July 3, 2002 - νFact02, London Run 7591 Ev. 9704

-20 usec

20 usec

FA

ST O

utlin

e:

Why

do

you

want

todo

it?

What

isit

really?

How

do

you

want

do

toit?

When

do

you

pla

nto

do

it?

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alg

eri

FA

ST

:an

experim

entfo

ra

hig

hpre

cisio

nm

uon

life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

2)

FAST

Why?

Because GF is one of the three free parameters of the bosonicsector of the Standard Model. They are constrained by:

α(µ2 = 0) = 1/(137.035990 ± 0.000006) → (0.045 ppm)MZ = 91.1867 ± 0.0021 GeV → (23 ppm)GF = 1.16639 ± 0.00001 × 10−5 GeV−2 → (9 ppm)

† Note: the three parameters fully describe the strength of theelectroweak fields: γ, W and Z

LucaMalgeri

FAST:an experiment for a high

precision muon lifetime (and GF )measurement

(page 3)

FAST

Parameter n. 1: α

- Josephson effect- Rydberg constant and de Broglie

wavelength of neutrons- Quantum hall effect- Anomalous magnetic moment of the

electron

(α-1-137.0360)x104

-0.40 -0.10 0.20

CODATA 1986 -.105 ± 0.061

h/Mn 0.113 ± 0.052

acJ -.230 ± 0.077

q-Hall 0.037 ± 0.027

ae -.001 ± 0.005

-0.40 -0.10 0.20

....but at nowadays energies the vacuumpolarization effects degrade this accuracy:

� � ��

� ��

�

� � � �

� � � �

� � � � � ��

LucaMalgeri



(page 4)

FA

ST P

ara

mete

rn.

1:

α

Had

ronic

vacu

um

pol

ariz

atio

nm

easu

rem

ents

:

are

use

dto

extr

apol

ate

αat

hig

her

scal

es:

α−

1(M

2 Z)

=12

8.97

8±

0.02

7→

(209

ppm

)

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ST

:an

experim

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hig

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nm

uon

life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

5)

FA

ST P

ara

mete

rn.

2:

MZ

Nee

dle

ssto

say: 19

90-9

2

1993

1994

1995

√s [G

eV]

σ [nb]

L3

e+e− →

had

rons

(γ)

ratio

10203040

8890

9294

0.991

1.01

MZ

=91

.186

7±

0.00

21G

eV→

(23

ppm

)

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:an

experim

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life

tim

e(a

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GF

)m

easu

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ent

(page

6)

FA

ST P

ara

mete

rn.

3:

GF

Just

alitt

lere

min

d:

GF

Ã2

W+

g

g

ν e

µ+

ν µ

e+

µ+

ν µ

e+

ν e τ−

1µ

=G

2 FM

5 µ

192π

3(1

+∆

q)

∆q

incl

ude

the

hig

her

order

QE

Dan

dQ

CD

corr

ecti

onknow

nup

totw

o-lo

ople

vel(0

.5ppm

)

→T

.va

nRitbe

rgen

and

R.G

.Stu

art

Phys

.Let

t.B

437

(1998)

201

Unlike

α,G

Fdoes

n’t

suffer

had

ronic

unce

rtai

nti

es

whic

har

esu

ppre

ssed

by

m2 f/M

2 W:

mea

suri

ng

the

muon

life

tim

eis

aso

rtof

hig

h

ener

gyphysi

csex

per

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t!

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ST

:an

experim

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hig

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cisio

nm

uon

life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

7)

FA

ST P

ara

mete

rn.

3:

GF

Goi

ng

from

Fer

mim

odel

(con

tact

inte

ract

ion)

toSta

ndar

dM

odel

:

GF

=

√2g

2

8M2 W

(1+

∆r)

∆r

are

the

Ele

ctro

-Wea

kco

rrec

tion

s:

∆r

=f(∆

α,m

t,m

H)

=∆

α−

cot2

θ W∆

ρ+

...

Wpro

pag

ator

effec

ts(∝

O(m

2 µ/m

2 W)

∼0.

52ppm

)ar

etr

adit

ional

lyin

cluded

inth

edefi

nit

ion

ofG

F

Oth

erco

ntr

ibuti

ons

(∝m

2 tan

dlo

gm

H)

are

use

dto

der

ive

indir

ect

lim

its

onunm

easu

red

par

amet

ers.

The

pre

sent

valu

eof

δGF/G

Faff

ect

the

pre

dic

tion

for

MH

atth

eper

cent

leve

l.

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life

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e(a

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GF

)m

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(page

8)

FA

ST P

ara

mete

rn.

3:

GF

Wher

eth

eunce

rtai

nty

onG

Fco

mes

from

?

δGF

GF

=−

5 2

δmµ

mµ−

1 2

δτµ

τ µ+

th.(

+4m

2 ν

m2 µ

)

5 2

δmµ

mµ

=0.

38ppm

PD

G2000

theo

ry=

0.50

ppm

Ritbe

rgen

and

Stu

art

4m

2 ν

m2 µ

=10

ppm

PD

G2000

(ifyo

uas

sum

eneu

trin

osto

be

mas

sive

)

δτµ

τ µ=

18ppm

PD

G20

00

Dom

inan

tco

ntr

ibution

sfrom

:Bal

andi

net

al.

(197

4)Bar

din

etal

.(1

984)

Gio

vanet

tiet

al.

(198

4)

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ST

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hig

hpre

cisio

nm

uon

life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

9)

FAST

What is it really?

An experiment aiming to measure the muonlifetime with an accuracy of 2 ps, i.e., GF to 1 ppm

including all errors.

Requirements:

Large data sample of 1012 events over at least 9 τµ periods

⇓1012 × 9τµ = 1.5 years of data taking at 50 % efficiency if only one

event per cycle is accepted: parallelization needed!

⇓Tracking capabilities in order to disentangle overlapping events

⇓In order to stay in a reasonable data taking period ( ∼2 months), a

high data rate is needed: 1 MHz.

LucaMalgeri



(page 10)

FA

ST W

hat

isit

really?

LucaM

alg

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FA

ST

:an

experim

entfo

ra

hig

hpre

cisio

nm

uon

life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

11)

FAST

Systematics: learn from the past

Saclay experiment (Bardin et al.):A scintillator telescope surrounding a sulphur target in a pulsed pionbeam.

• The polarized µ component of the beam introduced time dependentinefficiencies due to the limited coverage of the detector

• The low granularity of the detector, together with the beam timestructure, produced time dependent effects

• Pile-up free background

TRIUMF experiment (Giovanetti et al.):

A water C̆erenkov system coupled to two PM in a DC muon beam.

• Electronic pile-up suppression: low statistics.

LucaMalgeri



(page 12)

FAST

Systematics: how to afford them

FAST wanted features:

• Capability to recognize the π → µ signature avoiding the“polarization effect”

• Controlled pile-up thanks to a high granularity

• Full solid angle coverage

• High detection efficiency over the complete positron energyspectrum

• Cross checked time measurements and stability

• Something else ?????

LucaMalgeri



(page 13)

FA

ST D

esi

gn

chara

cteri

stic

s

Beam

:

✔πM

1at

PSI:

DC

curr

ent

✔M

omen

tum

:17

0M

eV/c

(±3

%)

✔Siz

e:10×

16cm

2

✔In

tensi

ty:

106

s−1

✔P

uri

ty:

90%

Targ

et:

✔4*

4*16

00m

m3

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tillat

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uet

tes

form

ing

the

read

out

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×40

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:

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itio

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PM

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seti

me

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ns

✔1.

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me-

less

TD

Cch

annel

s

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hig

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nm

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life

tim

e(a

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GF

)m

easu

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ent

(page

14)

FA

ST C

once

pt:

Asc

inti

llat

ing

pix

eldet

ecto

r,on

aD

Cpio

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ains

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dow

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tic S

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µ→eν

ν

µ→eν

ν

π→µν

π→µν

π+

π+

µ+ µ+

e+

e+

16 cm 10 cm

read

out p

ixel

(P

SP

M, 4

x4 m

m2 )

170

MeV

/c π

+

(1 M

Hz)

170

MeV

/c π

+

(1 M

Hz)

a) y

z vi

ew (

elev

atio

n)

b) x

z vi

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plan

)

20 c

m

20 cm 20 cm

to P

SP

M v

ia

clea

r lig

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beam

deg

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beam

deg

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r (w

edge

)

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eam

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y (v

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z (b

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tical

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x (h

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FAS

T

beam

co

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r

beam

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:an

experim

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hig

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nm

uon

life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

15)

FAST

Concept: from beam to decay time

☞ The pion stopping point is modulated by the plastic degrader suchthat the occupancy is kept uniform

☞ The pulse height in the stopping pixel is ∼ 5 times higher than am.i.p. ⇒ a double threshold discriminator (developed at PSI)is used for tagging purposes

☞ Every event is a good event and the readout system is almostdead-time free. So, why do we need a trigger?

✧ t0 for the whole system;✧ definition of a ”region of interest” where to look for decaying

muons and decrease the data rate.

LucaMalgeri



(page 16)

FA

ST Tri

gger

syst

em

1st

level

:th

ree-

fold

coin

ciden

cedefi

nin

gth

et0

2n

dle

vel

:tw

osn

apsh

ots

ofth

eev

ent

(15

ns

and

100

ns)

are

use

dto

find,re

spec

tivel

y,th

est

oppin

gpoin

tofth

epio

nand

muon.

Only

the

5×

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per

pix

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und

the

stoppin

gpoin

tw

illbe

analy

sed.

For

each

row

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hfo

r“st

oppin

gpatt

erns”

:

stop

and

dec

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µde

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20

s µ

0 (t

rigg

er)

15 n

s1s

t

5 ro

w tr

igge

r ro

ads

rese

t

100

ns2n

d tr

igge

r sn

apsh

ot

y sy sy 0

z s

z s

colu

mn

OR

a)

b)

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hig

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nm

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life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

17)

FA

ST R

eadout

syst

em

:P

SP

M

The

sign

alis

tran

spor

ted

from

the

scin

tillat

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Ham

amat

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itio

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gree

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aves

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s:

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x1

60

mm

3 b

ag

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tte

(Bic

ron

BC

40

0)

BC

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ptica

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each

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m (

17 fi

bres

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17.5 x 17.5 mm2

30 x

30

mm

2

17.3 mm (20 fibres)

π+ dire

ctio

n

PS

PM

en

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4 x

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hig

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life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

18)

FAST

Readout system: TDC

The stopping point coordinates are used to mask the hits digitizationin the heart of the system:

✔ 64 TDC chips developed by CERN/ECP-MIC group andengineered by CAEN on 16 128-channel boards (v767)

✔ “0” conversion time

✔ up to 520 ps of time resolution

✔ up to 1 MHz rate capability

The TDC system is driven by an external Rubidium clock with a baseoutput frequency of 30 MHz and a stability of ∆f/f = 2.2 × 10−10,

much beyond the requirements.

A second Rubidium clock is used as an external time reference andcalibration system.

LucaMalgeri



(page 19)

FAST

DAQ system: a challenge

The Data AcQuisition system must sustain a continuous event rate of 1 MHz.

From physics to tape:

☞ 60 hits/event + control words → 420 Mbytes/s☞ using LV2 info (+ tricks) → 80 Mbytes/s

Note: only a 5 × 5 superpixel region is used. We are forced to drop the rest.

An unfair comparison:

LHC detector FAST

n. channels 100,000,000 2304event size 1 MByte 500 BytesPhysics data throughput 100 MBytes/s 420 MBytes/s

The data are sent to a PC farm which perform the analysis and stores histograms.Only a fraction of the event is recorded for systematics checks.

The full (LV2 info) saving would require a huge amount of mass storage(∼ 80 Tbytes) filled with W.O.R.N.’s†.

The reading+processing time would require much longer than a newdata taking period.

†W.O.R.N.: Write Once Read Never

LucaMalgeri



(page 20)

FA

ST D

AQ

syst

em

:a

challenge

Aco

mm

erci

ally

available

CP

U-les

sso

luti

on

has

bee

nadopte

dfo

rth

eV

ME

syst

em:

✦P

VIC

=P

CI

toV

ME

Inte

rcra

teC

onnec

tion:

ades

kto

pP

Cis

use

das

are

mote

CP

Ufo

racc

essi

ng

VM

Eaddre

sses

✦H

igh

PC

Idata

rate

→80

MB

yte

s/s

✦H

igh

VM

Edata

rate

(tes

ted

inla

b.)

→18M

Byte

s/s

✦4

VM

Ecr

ate

s+

4co

ntr

oller

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’sare

suffi

cien

t

T D C

V M E P C I

T D C

T D C

T D C

T D C

VM

E b

us

T D C

V M E P C I

T D C

T D C

T D C

T D C

VM

E b

us

T D C

V M E P C I

T D C

T D C

T D C

T D C

VM

E b

us

T D C

V M E P C I

T D C

T D C

T D C

T D C

VM

E b

us

PC

PC

PC

Con

trol

−PC

Ana

lysi

sF

arm

Ana

lysi

sF

arm

Ana

lysi

sF

arm

PC

Ana

lysi

sF

arm

PV

IC b

us

Fast

−E

ther

net

Fast−Ethernet

direct connection

LucaM

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FA

ST

:an

experim

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hig

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nm

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life

tim

e(a

nd

GF

)m

easu

rem

ent

(page

21)

FAST

How do you want do to it?

Simulations have been performed to study efficiencies and systematics:

1) Trigger configuration and efficiency51 % of the incoming pions are triggered within the two snapshotstructure. The losses are mainly due to early decaying pions, beforethe second snapshot takes place. Muons are identified by means ofthe pulse height signal.

2) Selection algorithm85 % of the events have one or more particles originating fromother decay chains and traversing the same 5 × 5 pixels region. Inorder to reduce this accidental background component, positrontopology cuts have to be applied

=⇒

LucaMalgeri



(page 22)

FAST

How: selection and efficiency

✦ Hits in the 5×5 pixels region areclustered in time

✦ A positron track is defined as atrack with hits in both the inner(3× 3) and outer (5× 5) region

✦ Only one cluster of less than 4pixels is required in the outerlayer

✦ Events with more than onepositron track are rejected

Total efficiency: 25 %Accidentals: 3 × 10−4

– Total a) all events– Muons b) positrons tracks– Accidentals c) only one positron track

LucaMalgeri



(page 23)

FAST

How: systematics - muon spin rotation

A polarized muon source (from beam or from asymmetric muondetection efficiency) precesses around a magnetic field (Earth). IfALSO the positron detection efficiency is asymmetric this can cause atime dependent effect:

µ spin direction

spin t=0

spin

t=t1

positron

positron

B field

cos θ (e+)

cos

θ (e

+)

even

ts

φ (e+)

φ (e+)

LucaMalgeri



(page 24)

FAST


✔ Avoid polarized beam muons requiring a detectable π → µtransition.

✔ Make the detector as uniform as possible:

- residual muon detection anisotropy along the beam direction fromdouble hit time resolution and high threshold discriminator (10 %)

- positron detection inefficiency from local effects (unstablethresholds/gain → 1 %)

Precession frequencies:

νµ = 13.55KHz

G× B νmuonium = 1355

KHz

G× B

LucaMalgeri



(page 25)

FAST


Residual of time distributions after applying a weak

magnetic field (50 G):

Time [µs]

Res

pons

e (p

pm) a) B field parallel to fibres

- generator

a) B field parallel to fibres - experiment

b) B field perpendicular to fibres - generator

b) B field perpendicular to fibres - experiment

0

0 2 4 6 8 10

Time [µs]0 2 4 6 8 10

Time [µs]0 2 4 6 8 10

Time [µs]0 2 4 6 8 10

255075

100

-100-75-50-25

Res

pons

e (p

pm)

0255075

100

-100-75-50-25

Eve

nts

(ppm

)

0255075

100

-100-75-50-25

Eve

nts

(ppm

)

0255075

100

-100-75-50-25

With a suitable choice of the magnetic field and a proper fit procedurethis effect can be kept at the level of (0.2 ppm).

LucaMalgeri



(page 26)

FAST

How: systematics - time dependent detection efficiencies

✦ After-pulses, in a 10 ns scale, could fake a muon pulse and affectthe requirement of a fully detected π → µ → e chain.

Prob. → 10−4 with double threshold discriminator

✦ On a longer time scale (0.5 µs), after-pulses, gain changes andbaseline shifts may affect the electron detection efficiency in pixelsalready visited by a pion or muon.

Low Threshold probability

≥ 1 pe 10 %≥ 2 pe 1 %≥ 3 pe 0.1 %

The decay time distribution gets two additional terms:

1) ∝ exp(

−t 2

τµ

)

: prob. for an electron to be just under threshold

2) ∝ exp[

−t(

1

τµ+ 1

τβ

)]

: τβ is the slow after-pulse time component

Simulations based on lab. tests give a total effect of less than 0.3 ppm.

LucaMalgeri



(page 27)

FAST

How: systematics - tracks overlap and pile-up

Given the high occupancy of the detector, event and single trackoverlap have to be taken into account.

Two separate decay chains (independent on each other):

(π1, µ1, e1) ⇐⇒ (π2, µ2, e2)

may “interact” and give rise to fake decay chains:

(π1, µ1, π2), (π2, µ1, e1), (π1, µ2, π1),. . .

Most of these backgrounds have either flat or well behaved

∝ exp(

− tτµ

)

time distributions.

→ no or little effect on final measurement

but for some of them .......

LucaMalgeri



(page 28)

FAST


(π1, µ1, π2): the positron e1 is lost and an incoming π2 fakes it. If the following decayappear in the predefined time window [−10 µs; 20 µs] the event is not accepted(→ multiple-track): late pions have better chance to be accepted

(π1, µ1, e2): mirror component, with the positron coming from a (previous) very late decayin the same super-pixel

Decay time [ns]

t max

τµ

− t− )-(exp

τµ

t minexp(- )+ t

Flat acc.

Using tmin = −10 µs, tmax = 20 µs, and the probability forthe pion track to be reconstructed as a positron track(Montecarlo), this effect is found to be negligible andunder control studying the negative time distribution region.

LucaMalgeri



(page 29)

FAST


A little bit more complicated case: double kill

The presence of two events with same (or somehow related) space and timestructure may give rise to time dependent inefficiencies:

prob(ekill) ∝ 1 + af(te2 − te1) prob(πkill) ∝ 1 + a′f ′(tπ2− tπ1

)

eff. = 1 − af(te2 − te1) − a′f ′(tπ2− tπ1

) + aa′f(te2 − te1)f′(tπ2

− tπ1)

The first two “single kill” terms are NOT dependent on (te1 − tπ1), while the third

gives an additional contribution ∝ exp (−t/τµ).

π2 ↔ π1 correlations:

- π2 enter the detector just after π1 during the trigger dead-time

- π2 enter the detector just after π1 and TDC cannot resolve the two hits

- π2 enter much before π1 when pile-up rejection doesn’t apply (< −10 µs)

e2 ↔ e1 correlations:

- e2 overlaps in time and space e1

under control using the beam rate → 0.2 ppm

LucaMalgeri



(page 30)

FAST

How: systematics - physics effects

Muonium formation in scintillator is highly probable (66 %). Is themuon lifetime bound in Muonium state different?

✔ Early studies claimed YES

✔ A more recent paper from Czarneki, Lepage and Marciano(hep/ph-9908439) show that the correction is at the level of 10−9,in agreement with phase space arguments

✔ Negligible effect on FAST measurement

Rare decays:

✔ Radiative decays may alter the topology of the event and hence thedetection efficiency

✔ Full GEANT simulation show negligible effects

✔ Additional electrons from γ conversions increase the number ofpositron tracks in the detector by 0.25 % → negligible effect

✔ Internal conversions µ+ → e+ν̄µνee+e− produce an efficiency loss of

< 3.4 × 10−5 but same time structure as the signal

LucaMalgeri



(page 31)

FAST

How: fit and summary

Including all considered systematic effects the final fit function willlook like:

N(t) =N0

τµ

δt

[

P0 + exp

(

− t

τµ

)

+ P1 exp

(

− 2t

τµ

)

+ P2 exp

(

+t

τµ

)]

⊗ µSR

P0 =flat accidentalP1 =double killP2 =late pions

P0 and P2 can be left free in the fit but P1 wouldincrease the systematic error by 2-3 ppm.Need to get it from data: dedicated runs for effi-ciency studies, beam rate, off-line extrapolation.

Systematics summary tables

Source Syst. error (ppm)Before After corr.

Muon spin rotation 1.7 0.2Time dep. efficien. 0.9 0.3Track overlap 1.1 0.2Physics effects - 0.001

Total systematics 0.41

Source Error (ppm)

Statist. + acc. 1.2Systematics 0.4

Error on τµ 1.3

Radiative corr. 0.2

Error on GF 0.66

LucaMalgeri



(page 32)

FAST

When do you plan to do it

First milestone met at the end of year 1998: test-beam at PSI

✦ Check feasibility studies

✦ 1/10 of the detector with several configurations

✦ Low intensity

✦ Check fibers, PSPM and TDC’s behaviors

Run 7591 Ev. 9704

-20 usec

20 usec

LucaMalgeri



(page 33)

FAST

Test beam

The dynamic range has been studied, together with double hitresolution, discriminator requirements, TDC performances, etc.

0

2000

4000

6000

8000

0 250 500 750 1000 1250 1500 1750 2000

Mean 170.7

Energy loss for positrons (arb. unit)

N/1

8

a)

0

100

200

300

0 250 500 750 1000 1250 1500 1750 2000

Mean 456.1

Energy loss for incoming pions(arb. unit)

N/1

6

b)

0

200

400

600

800

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Mean 948.5

Energy loss for stopping pions(arb. unit)

N/2

0

c)

193.4 / 191

P1 8.738 0.4386E-02P2 -0.4543E-03 0.1320E-05

τµ = (2201.0 +- 6.4 ) ns

χ2/d.o.f. = 1.01

τµ(PDG) = (2197.03 +- 0.04) ns

10-1

1

10

102

103

104

0 4000 8000 12000 16000 20000Time (ns)

No.

of µ

→e

deca

ys

LucaMalgeri



(page 34)

FAST

When do you plan to do it: cont’d

Date Activity

✔ 2000 (1st) Experiment approved at PSI✔ 2001 (1st) Prototype tests, re-design of DAQ

2002 (3rd) System integration tests at PSI2003 (2nd) Installation2003 (3rd) Check-out2003-2004 Data taking

Collaboration: CERN, University of Geneva, NIKHEF, PSI

http://www.cern.ch/fast/

LucaMalgeri



(page 35)

FAST

Conclusions

✦ FAST aims to measure with 10 times better accuracy than thepresent world average the Fermi coupling constant GF

✦ Major challenges:

- stability and data rate: from a 0.008 m3 detector, an LHCdetector equivalent throughput has to be handled → a test benchfor new generation experiments

- very subtle systematic effects have to be sorted out

✦ Feasibility established in simulation and test beams

✦ Stay tuned for results in 1-2 years

Credits for the material:

A. Barczyk, F. Cavallo, P. de Jong, P. Kammel, J. Kirkby, R. Nahnhauer, F.

Navarria, G. Passaleva, A. Perrotta, C. Petitjean, M. Pohl, R. Stuart, G. Valenti,

D. della Volpe

LucaMalgeri



(page 36)

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