FAST : A precision measurement of muon lifetime υν
and Fermi coupling constant GF
FAST COLLABORATION:
A.Barczyk (1) , J. Berdugo (2) , J. Casaus (2) , C. Casella (3,4) , K. Deiters (4) , P. Dick (4) , A. Dijksman (4) , J. Kirkby (1) , L. Malgeri (1) ,
C. Mana (2) , J. Marin (2) , G. Martinez (2) , C. Petitjean (4) , M. Pohl (3,5) , E. Sanchez (2) , C. Willmott (2)
CERN 1 ,
CIEMAT 2 ,
UNIGE 3 ,
PSI 4 ,
NIJMEGEN 5
GOAL OF THE EXPERIMENT:
The goal of the FAST experiment is to measure the muon lifetime υν to a precision
of 1ppm (~ 2 ps), one order of magnitude improvement on the present world average.
A more precise value on υν will provide also a more precise estimate of the Fermi
coupling constant GF, one of the fundamental parameters of the Standard Model.
DESCRIPTION OF THE EXPERIMENT:
present scenario :
•
•
•
The detector is a small imaging target of dimensions about 20 x 13 x 20 cm3, made of 1536 bars of
ευν
> 18 ppm
υν
Balandin et al. (1974)
Bardin et al. (1984)
Giovanetti et al. (1984)
plastic scintillator, connected to position sensitive photomultipliers via wavelength shifting fibers.
9 ppm
• the uncertainty on υν is the dominant contribution to the uncertainty on GF:
2
υν . 1 >
GF mν
5
192θ 3
)1 ,
0.07 ppm
(PDG 2002)
FAST goal:
0.5 ppm
(Van Ritbergen
and Stuart - 1998)
plastic scintillator
(Bicron BC400)
≤12 ppm
(PDG 2002)
wave length
shifting fibers
ε GF
> 0.5 ppm
GF
ευν
> 1 ppm
υν
4 mm
•
single FAST pixel
 mο 2 
εGF
5 εmν
1 ευ ν
ν

> .
.
, th ,  4
 m 2 
2 mν
2 υν
GF
ν


Εq ∗
(BCF-91A)
4 mm
BEAM : θM1 area at PSI
A direct current θ + beam of momentum 170 MeV/c from PSI
(Paul Scherrer Institute) accelerator facility is stopped in the
• DC θ+ beam
FAST high granularity scintillator target , and the subsequent
• momentum
decay time (in the target) of each pion:
• RF frequency = 50.633 MHz (T = 19.75 ns)
θ,
ν , ον
υθ = 26.03 ns [PDG ’02]
- pion at rest
- kinetic energy of muon = 4.1 MeV range ~ 1.5 mm
θ+
= 170 MeV/c
• beam size
~ 16 x 16 cm2
• intensity
~ 1 MHz
θ+
and muon:
ν,
e, ον οe
υν = 2.197 νs [PDG ’02]
target dimensions:
is recorded by the entire readout chain:
- Preamplifiers
(custom made units),
- Discriminators
(custom made units),
- TDC’s
(CAEN V767).
12.8 x 19.2 x 20.0 cm3
32 x 48 pixels = 1536 pixels
8 x 12 = 96 groups of 16 bars
preamplifiers
In order to reach the 1 ppm requirement, a data sample of
discriminators
1012 ν , e+ ον!οe decay has to be collected.
photomultipliers
target
To stay in a reasonable amount of time (few months) of data
beam counter
taking, the detector and its readout have to operate at a very
from TDC’s to PC : DAQ system
high beam rate ( ~ 1 MHz) .
Such a high beam rate leads to a huge data rate to the DAQ
system ( ~ 80 MB/sec) and imposes constraints on the DAQ
architecture: event reconstruction has to be performed in real
time, and data have to be analysed ONLINE, because of the
impossibility to store such a big amount of data.
FAST setup @ PSI - Oct 2003
challenging task for the DAQ: despite the
small dimensions of the detector, the
DAQ has to sustain a very high data rate
DAQ and DATA RATE:
PRELIMINARY RESULTS (from 2003 testbeam)
testbeam)
• high beam rate ~ 1 MHz
• raw data rate without LV2 data reduction
~ 2300 MB/sec
• data rate after LV2 data reduction
2003 data taking run:
Typical event (from online event display)
at low beam rate (few kHz)
3 different decays in the target in the same TDC
time window [-10 νs, +20 νs] (time t = 0 is LV1 trigger)
~ 80 MB/sec
• mandatory ONLINE analysis
Few days of data taking
Complete readout and DAQ chain
No LV2 trigger available
low LV1 trigger (beam) rate
Total collected statistics
from online analysis: 7.4 107 events
Data reduction is performed by the LEVEL 2 TRIGGER,
whose task is to identify good events and their location in
the target, and to reduce the data bandwidth as much as
possible at an early stage of the DAQ system.
LV2 selective trigger is based on the fact that all the
beam
relevant time information for each event is confined in a
small 5x5 pixel region centered around the stopping point
(the so-called superpixel), and only the TDC’s containing
the superpixel have to be read by the DAQ system.
superpixel
stopping pion
… more info ?
http://www.cern.ch/fast ; Chiara.Casella@cern.ch
Muon lifetime plot - Good background suppression
- Good exponential fit, in agreement
with PDG value, within a statistical error
(~ 350 ps) about 7 times bigger than the
current error on the world average
1 tick = 1.041667 ns