1. FAST: A precision measurement (1ppm) of muon lifetime tm and GF
Chiara Casella
University of Geneva - PSI
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
9th Topical Seminar on Innovative Particle and Radiation Detectors

2. 9th Topical Seminar on Innovative Particle and Radiation Detectors
Outline
Why do we want to measure tm ?
How do we want to measure tm ?
detection principle
beam
target
readout chain
DAQ and trigger
Test beam (Sept 2003 – Dec 2003)
Future plans and conclusions
9th Topical Seminar on Innovative Particle and Radiation Detectors

3. 9th Topical Seminar on Innovative Particle and Radiation Detectors
Motivations of FAST:
Standard Model becomes predictive only when the free parameters of its Lagrangian are fixed by experimental inputs.
SM of electroweak interactions free parameters in the bosonic sector:
a = 1/( ± ) ppm
MZ = ± GeV ppm
GF = ± x 10 –5 GeV ppm
GF is a fundamental parameter of SM
it can be derived from muon lifetime: _ nm _ nm m+ m+ W+ e+ e+
Dq include higher order
QED and QCD corrections ne ne
9th Topical Seminar on Innovative Particle and Radiation Detectors

4. Uncertainties on GF: where do they come from?
PDG 2002
Ritbergen and Stuart - PLB 437 (1998) 201
PDG 2002 (from direct measurement)
Dominant contribution from:
Balandin et al. (1974)
Bardin et al. (1984)
Giovanetti et al. (1984) …
9th Topical Seminar on Innovative Particle and Radiation Detectors

5. 9th Topical Seminar on Innovative Particle and Radiation Detectors
… near future:
Dominant contribution from:
FAST
mLAN
9th Topical Seminar on Innovative Particle and Radiation Detectors

6. How do we want to measure tm ?
active high granularity (~ 1500 pixels) scintillator target
each pixel seen by the readout and DAQ chain
to photomultiplier, preamplifier, dicriminator, TDC
20cm
12.8 cm
19.2 cm
beam degrader to horizontally spread the stopping p+ position
p+  m+  e+
tp = ns PDG2002
tm = ms PDG2002 p+
DC pion beam
p = 170 MeV/c
at PSI - pM1 area
QUITE SIMPLE DETECTION PRINCIPLE
MAIN CHALLENGE : 1 ppm REQUIREMENT
9th Topical Seminar on Innovative Particle and Radiation Detectors

7. 1 ppm precision measurement:
Large data sample of 1012 events (=>1ppm statistical uncertainty)
Suppress Systematics effects
1012 x 9tm / 0.5 ~ 1.5 y
estimated
efficiency
~ 20 ms
PARALLELISATION NEEDED !!!
more p+  m+  e+ at the same time in the target
reasonable data taking period
(2 months ca.)
technical challenge: DEAL WITH A HUGE
AMOUNT OF DATA
ability to disentangle overlapping events
high beam rate
(1 MHz beam rate)
high data rate ~ 80 MB/sec
+ ONLINE ANALYSIS
tracking capabilities, high
granularity of the target
Remove time dependent effects (to the ppm level) that, together with asymmetries in the target,
could compromise the measurement.
 As an example, mSR EFFECTS ( why p+  m+  e+ ?
9th Topical Seminar on Innovative Particle and Radiation Detectors

8. 9th Topical Seminar on Innovative Particle and Radiation Detectors
mSR effects B
local magnetic field at
the detector (Earth)
Positrons are preferably emitted along Sm
Precession movement of Sm around B Sm
e+ direction emission
IF: polarised muon source AND asymmetries in positron detection
time dependent detection efficiency
Solution : ISOTROPIC m SOURCE
p+  m+
To suppress RESIDUAL mSR effects :
force the muons to precess on a time scale known and detectable
Magnetic field + proper fit procedure  mSR effects under control (0.2 ppm)
BEarth ~ 0.5 G  nm ~ 6.8 KHz  T ~ 150 ms
B ~ 80 G  nm ~ 1.1 MHz  T ~ 0.9 ms
PERMANENT MAGNETIC FIELD (80G) IN THE TARGET REGION !
9th Topical Seminar on Innovative Particle and Radiation Detectors

9. 9th Topical Seminar on Innovative Particle and Radiation Detectors
Beam : pM1 area at PSI
- pM1 at PSI
momentum
- RF frequency
- size
- intensity
- purity
: p+ DC beam
: 170 MeV/c (±3%)
: MHz ( T=19.75 ns )
: variable
: variable (~ MHz)
: ~ 50%
separation of the beam components:
positrons  for calibration purposes
pions  for normal running
sc1
sc2
( sc3 )
 4 m p+
LEVEL 1 trigger
RF signal of the machine in coincidence with 3 beam counters
Possibility to use narrow (selective) or wide trigger
9th Topical Seminar on Innovative Particle and Radiation Detectors

10. Target Fiber Active Scintillator Target active scintillator target
high granularity
ORIGINAL IDEA:
Fiber Active Scintillator Target
1 pixel =
“baguette” +
2 wave lengh shifter fibers
painted with white reflective paint
NOW:
Scintillating FIBERS replaced by scintillator BARS
4mm
16 pixels grouped (4x4) to fit in one photomultiplier
Scintillator
WLSF
Paint
Bicron
BC 400
BCF
BC 620
testbeam /10 target
1 pixel = bunch of 64 fibers
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Target
preamplifier
photomultiplier
12.8 x 19.2 x 20.0 cm3
32 x 48 pixels = 1536 pixels
8 x 12 = 96 groups of 16 bars
LED mask at the bottom of the target (1 LED for 1 pixel) to check the mapping
beam counter
9th Topical Seminar on Innovative Particle and Radiation Detectors

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Readout chain
4 x 4 channels PHOTOMULTIPLIERS ( Hamamatsu H ) rise time < 2 ns
16 channels custom made PREAMPLIFIERS ( PSI )
gain ~ 10
16 channels custom made DISCRIMINATORS ( PSI )
double threshold discriminators (remote controlled):
- low threshold (MIPS)  to TDCs
- high threshold (stopping p / m)  to LV2 trigger
128 channels continuous mode TDC’s ( CAEN V767 )
dead-time free
read out window : -10 ms < ttrig < +20 ms
- driven by 2 external Rb clocks [stability = : ~ 1sec / 300y ]
PSPM
Preamp
LV2 board
TDC
Discri
LL - data
HL - data
x 96
x 16
9th Topical Seminar on Innovative Particle and Radiation Detectors

13. DAQ system: from TDC’s to PC’s
challenging task: despite the small dimensions of the detector, the DAQ system must sustain a very high event rate
baseline DAQ architecture design:
DAQ goal in the present design (4 nodes) = 80 MB/sec
PVIC solution = PCI to VME intercrate connection (CPU-less solution for the VME)
4 VME crates ;
4 TDCs for each VME ;
2 parallel PVIC buses
(nominally: 20 MB/sec for each node)
DAQ PC:
- from raw data from TDCs to time and position data
event builder
analysis “farm”: ONLINE analyses the data and stores
the histograms (2003 run: 1 single PC)
9th Topical Seminar on Innovative Particle and Radiation Detectors

14. Date rate & LV2 trigger if : DATA REDUCTION NEEDED ! ! !
Raw data rate: ~ 2300 MB/sec
(from simulations)
if :
1 MHz beam rate
Entire target (16 TDC’s always read)
DAQ sustainable data rate :
80 MB/sec
DATA REDUCTION NEEDED ! ! !
LEVEL 2 TRIGGER
LV2 trigger idea:
all the relevant time informations are contained in the “superpixel” (5x5 pixels) centered on the stopping p
no need to read 16 TDC’s always at the same time – but only the TDC’s containing at least one pixel of the superpixel  DATA REDUCTION p m
What LV2 trigger does:
define (x,y) for stopping pion
look for the muon in a neighbouring pixel
send trigger only to the TDC’s containing the superpixel (4 at max)
9th Topical Seminar on Innovative Particle and Radiation Detectors

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LV2 trigger
TDC time window
t0+15ns
1. Calculate p stopping point coordinates:
- HL hits of the incoming pion track accumulated by rows
in the beam direction
- projection of the track in the 2 axes p
t0+100ns m
2. Look for the muon
3. If muon is found, send LV2 signal to the interested TDC’s
Flag events overlapping in time and space (very important when the trigger rate increases) t0
-10ms
+20ms
LV1
3 stages for LV2 operation:
HL-hits data
LV2
LV2 signals (x16) – one for each TDC
p stopping point coordinates, muon flag
LV1 trigger
pion overlapping flag
RF (50MHz)
9th Topical Seminar on Innovative Particle and Radiation Detectors

16. LV2 trigger architecture (CIEMAT Madrid)
17 FPGA based VME boards
(FPGA XILINX 2S 200):
16 BOX PROCESSORS:
each one processing data from 1 TDC
in charge of stages 1 and 2
MAIN CONTROLLER:
sends the triggers
manages the list of overlapped events
accomodated in a single VME crate, and interconnected via a backplane designed ‘ad hoc’
LV2 schedule:
testbeam 2003 : single board prototype test
June 2004 : prototype (1/4) test
End 2004 : final commissioning
9th Topical Seminar on Innovative Particle and Radiation Detectors

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2003 testbeam (Sept-Dec 2003)
discriminators
preamps
phomultipliers
completed target (1536 pixels)
80/96 PSPM’s available
20 from 2000 commissioning commissioned in 2003
new tubes are MUCH BETTER tubes !!!
complete readout chain
DAQ matches the requirements (75 MB/sec)
no LV2 trigger (single board prototype test performed)
trigger (LV1) rate: from 35 to 100 KHz max
TB2003 mainly devoted to target understanding and tuning
9th Topical Seminar on Innovative Particle and Radiation Detectors

18. TB2003: online event display (typical event)
9th Topical Seminar on Innovative Particle and Radiation Detectors

19. TB2003: muon lifetime plot PDG  st ~ 40 ps TB2003  st ~ 350 ps
‘Long’ run (few days run)
Total statistics : events from online analysis
(10% written to disk for offline studies)
RUN CONDITIONS:
narrow p trigger
no LV2 trigger ( all 16 TDC’s)
low LV1 trigger rate
Good exponential fit,
in agreement with PDG value
PDG  st ~ 40 ps
TB2003  st ~ 350 ps
Good background suppression
It will be a blind experiment in the future !!!
1 tick = ns
= 1/30(*) x 32(**) (*) = TDC clock (MHz)
(**) = TDC bits
9th Topical Seminar on Innovative Particle and Radiation Detectors

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TB2003: pion lifetime plot
Unexpected result from HL run
pion lifetime: not primary FAST goal
very short run with:
- narrow LV1 p trigger,
- threshold = 400 mV
(not sensitive to electrons,
only pions and muons)
Good (PRELIMINARY!) exponential fit,
in agreement with PDG value
PDG  st ~ ns
TB2003  st ~ ns
1 tick = ns
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Conclusions
FAST will measure the Fermi coupling constant GF with a precision 10 times better than the present world average.
FAST major challenges:
HIGH DATA RATE : from a m3 detector, a LHC detector equivalent throughput has to be handled
SYSTEMATICS
Feasibility of the experiment checked, NOT ONLY from simulations, BUT ALSO from real data (2003 testbeam)
Many expectations from 2004 run ! ! !
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Acknowledgments:
We wish to thank the following people who have contributed to early phases of the FAST experiment:
F. Cavallo (Bologna), P. de Jong (NIKHEF), P. Kamel (Berkeley), A. Konig (Nijmegen), R. Nahnhauer (DESY), F. Navarria (Bologna), G Passaleva (Florence), A. Perrotta (Bologna), W.Schoeps (PSI), R.Stuart (Michigan), G. Valenti (Bologna), D. Vitè (CERN), D. Della Volpe (Naples), S. Waldmeier-Wicky
9th Topical Seminar on Innovative Particle and Radiation Detectors