giovanni andrea prodi university of trento and infn italy 2 nd gwpw, penn state, nov.6 th, 2003 igec...
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Giovanni Andrea Prodi University of Trento and INFN Italy
2nd GWPW, Penn State, Nov.6th, 2003
IGEC observations in 1997-2000:• exchanged data• multiple detector analysis• background of accidental coincidences • upper limits and their interpretation
INTERNATIONAL GRAVITATIONAL EVENT COLLABORATION:
OBSERVATION SUMMARY
International Gravitational Event Collaborationhttp://igec.lnl.infn.it
ALLEGRO group: ALLEGRO (LSU) http://gravity.phys.lsu.eduLouisiana State University, Baton Rouge - Louisiana
AURIGA group: AURIGA (INFN-LNL) http://www.auriga.lnl.infn.itINFN of Padova, Trento, Ferrara, Firenze, LNLUniversities of Padova, Trento, Ferrara, FirenzeIFN- CNR, Trento – Italia
NIOBE group: NIOBE (UWA) http://www.gravity.pd.uwa.edu.auUniversity of Western Australia, Perth, Australia
ROG group: EXPLORER (CERN) http://www.roma1.infn.it/rog/rogmain.htmlNAUTILUS (INFN-LNF)
INFN of Roma and LNFUniversities of Roma, L’AquilaCNR IFSI and IESS, Roma - Italia
Resonant Bars
The planar gravitational wave impinging on the bar with an angle excites its longitudinal mechanical mode, with amplitude proportional to sin2()
Bar
pre-amplifier
electromechanical transducer tuned to the lowest longitudinal mode
low T
vibration insulation
L
The detector is sensitive in a narrow frequency range near the resonance(~900Hz)Typical 1997-2000 bandwidths ~ 1 Hz
DIRECTIONAL SENSITIVITY
The achieved sensitivity of bar detectors limits the observation range to sources in the Milky Way. The almost parallel orientation of the detectors guarantees a good coverage of the Galactic Center
ALLEGROAURIGA -EXPLORER –NAUTILUS
NIOBE
amplitude directional sensitivity factor vs sideral time (hours)
TARGET GW SIGNALS
Fourier amplitude of burst gw
0=⋅−()()htHttδ
arrival timeeach detector applies
an exchange threshold on measured H
Detectable signals:transients with flat Fourier amplitude at the detector frequencies (900 Hz)
threshold on burst gw
OBSERVATIONTIME 1997-2000(days)
EXCHANGED PERIODS of OBSERVATION 1997-2000
fraction of time in monthly bins
threshold on burst gw
211610Hz−−>⋅2113610Hz−−⋅÷211310Hz−−<⋅
ALLEGRO
AURIGA
NAUTILUS
EXPLORER
NIOBE
MULTIPLE DETECTOR ANALYSIS
efficiency of detection
fluctuations of false alarmsmaximize the chances of detection i.e. the ratio
network is needed to estimate (and reduce) the false alarms
time coincidence search among exchanged triggerstime window is set according to timing uncertainties by requiring
a conservative false dismissal2221ijijttkfalsedismissalkσσ−≤+↔≤
false alarms k
measure the false alarms:time shifts resampling the stochastic processes so that:
• gw sources are off (as well as any correlated noise)• statistical properties are preserved (max shift ~ 1 h)• independent samples (min shift > largest time window ~ few s)
by Tchebyscheff inequality
AMPLITUDE DISTRIBUTIONS of EXCHANGED EVENTS
relative counts
10-5
10-4
10-3
10-2
10-1
1
relative counts
10-5
10-4
10-3
10-2
10-1
1
NIOBENIOBEAMP/THR1 10
NAUTILUSNAUTILUSAMP/THR1 10
AURIGAAURIGAAMP/THR1 10
ALLEGROALLEGROAMP/THR1 10
EXPLOREREXPLORERAMP/THR1 10
normalized to each detector threshold for trigger search
typical trigger search thresholds:SNR 3 ALLEGRO, NIOBESNR 5 AURIGA, EXPLORER, NAUTILUS The amplitude range is much wider than expected: non modeled outliers dominate at high SNR
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0 6 12 18 24 30 36 42 48 54 60
amplitude directional sensitivity2sinGCϑ
2sinGCϑ−
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
time (hours)
ampl
itude
(H
z-1·1
0-21)
time (hours)
DIRECTIONAL SEARCH: sensitivity modulationam
plitu
de (
Hz-1
·10-2
1)
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60
time (hours)
Resampling statistics by time shiftsam
plitu
de (
Hz-1
·10-2
1)
We can approximately resample the stochastic process by time shift.
in the shifted data the gw sources are off, along with any correlated noise
Ergodicity holds at least up to timescales of the order of one hour.
The samples are independent as long as the shift is longer than the maximum time window for coincidence search (few seconds)
POISSON STATISTICS of ACCIDENTAL COINCIDENCES
Poisson fits of accidental concidences: 2 test
sample of EX-NA background
one-tail probability = 0.71
histogram of one-tail 2
probabilities for ALL two-fold observations
agreement with uniform distribution coincidence times are random
FALSE ALARM RATES
2E-21 1E-201E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
AL-AU AL-AU-NA
falsealarm rate[yr-1]
common search threshold [Hz-1]
dramatic improvement byincreasing the detector number:
3-fold or more would allow to identify the gw candidate
meanrate ofevents[ yr -1]
mean timing[ms]
Setting confidence intervalsIGEC approach is
frequentistic in that it computes the confidence level or coverage as the probability that the confidence interval contains the true value
unified in that it prescribes how to set a confidence interval automatically leading to a gw detection claim or an upper limit
based on maximum likelyhood confidence intervals (different from Feldman & Cousins)
false dismissal is under control (but detection efficiency is only lower-bounded)
estimation of the probability of false detection (many attempts made to enhance the chances of detection)
UPPER LIMIT on the RATE of BURST GW from the GALACTIC CENTER DIRECTION
• signal template = -like gw from the Galactic Center direction
Poissonrate of
detectedgw
[year –1]
search threshold
dashed region excluded with probability 90%
overcoverage
21210002~/.SHzMconvertedinburstgwatGalacticCeHnter−⋅↔esignal amplitude HS= FT[hS ] at 2 900 Hz
UPPER LIMIT on the RATE of BURST GW from the GALACTIC CENTER DIRECTION (2)
• analysis includes all the measured signal amplitudes search threshold result is cumulative for HM Ht
Poissonrate of
detectedgw
[year –1]
search threshold
• systematic search vs threshold Ht many trials (20 /decade)almost independent results
0
2
4
6
8
10
12
14
16
18
1.0 10.0 100.0
search threshold [10-21/Hz]
Ngw
many trials !all upper limits but one:
testing the null hypothesis
overall false alarm probability 33% for 0.95 coverage56% for 0.90 coverage
at least one detection in the set in case
NO GW are in the data
NULL HYPOTHESIS WELL IN AGREEMENT WITH THE
OBSERVATIONS
TESTING the NULL HYPOTHESIS
UPPER LIMIT on the RATE of BURST GW from the GALACTIC CENTER DIRECTION (3)
• no coincidences found, limited by the observation time
Poissonrate of
detectedgw
[year –1]
search threshold
dashed region excluded with probability 90%
overcoverage
• limited by accidental coincidences• observation time cuts off: sensitivity cut
1.8 yr -1
1
10
100
1,000
1E-21 1E-20 1E-19
0.60
0.80
0.90
0.95
search threshold(Hz -1 )
rate(year –1)
HOW to UNFOLD IGEC RESULTSin terms of GW FLUX at the EARTH
• Compare with IGEC results to set confidence intervals on gw flux parameters
1
10
100
1,000
1E-21 1E-20 1E-19
0.60
0.80
0.90
0.95
search threshold (Hz -1 )
rate(year –1)
• Estimate the distribution of measured coincidences HM Ht (cont.line)
Ht
• Take a model for the distribution of events impinging on the detector HS Ht (dashed line)
coverage
Case of gw flux of constant amplitude: comparison with LIGO results
Poissonrate of
detectedgw
[year –1]
search threshold
• IGEC sets an almost independent result per each tried threshold Ht
• correct each result for the detection efficiency as a function of gw amplitude HS:
at HS 2 Ht efficiency = 1 enough above the threshold
e.g. at HS Ht efficiency 0.25 due to 2-fold observations at threshold
Poissonrate of
detectedgw
[year –1]
search threshold
Case of gw flux of constant amplitude: comparison with LIGO results (2)
• the resulting interpreted upper limit
• convert from HS= FT[hS ] at 2 900 Hz to template amplitude parameter
e.g. for a sine-gaussian(850 Hz;Q=9) hrss= 10 Hz 0.5 HS
0
1
2
3
4
5
6
7
8
9
10
0 6 12 18 24 30 36 42 48 54 60time (hours)
Data selection at work
Duty time is shortened at each detector in order to have efficiency at least 50%
A major false alarm reduction is achieved by excluding low amplitude events.
ampl
itude
(H
z-1·1
0-21)
time
amplitude
time
amplitude
time
amplitude
time
amplitude
AΔFALSE ALARM REDUCTION
by amplitude selection of events
consequence:
selected events have consistent amplitudes
Auto- and cross-correlation of time series (clustering)
Auto-correlation of time of arrival on timescales ~100s
No cross-correlation
UPGRADE of the AURIGA resonant bar detector
Previous set-up during1997-1999 observations current set-up for the
upcoming II run
• beginning cool down phase• at operating temperature by November
Transducer
Electronics wiring support
LHe4 vesselAl2081 holder
Main Attenuator
Compression Spring
Thermal Shield Sensitive bar
AURIGA II run
new mechanical suspensions: attenuation > 360 dB at 1 kHz FEM modelled
new capacitive transducer: two-modes (1 mechanical+1 electrical) optimized mass
new amplifier: double stage SQUID 200 energy resolution
new data analysis: C++ object oriented code frame data format
AURIGA II run: upgrades
DUAL detectors estimated sensitivity at SQL:
• Only very few noise resonances in bandwidth.
• Sensitive to high frequency GW in a wide bandwidth.
PRD 68 (2003) 1020XX in press
PRL 87 (2001) 031101
Science with HF GW• BH and NS mergers and ringdown• NS vibrations and instabilities • EoS of superdense matter• Exp. Physics of BH
Mo Dual 16.4 ton height 2.3 m Ø 0.94m SiC Dual 62.2 ton height 3 m Ø 2.9m
T~0.1 K , Standard Quantum Limit
New concepts - new technologies:
measure differential motion of massive cylindrical resonators
• No resonant transducers:
• Mode selective readout:
• High cross section materials
(up to 100 times larger than Al5056 used in bars)
measured quantity: X = x1+x2-x3-x4
Dual detector: the concept
Intermediate frequency range:• the outer resonator is driven above resonance, • the inner resonator is driven below resonance → phase difference of
In the differential measurement: → the signals sum up → the readout back action
noise subtracts
2 nested masses: below both resonances: the masses are driven in-phase → phase difference is null
above both resonances: the masses are driven out-of-phase → phase difference is null
Differential measurement strategy
• Average the deformation of the resonant masses over a wide area:
• Readout with quadrupolar symmetry: ‘geometrically selective readout’ that rejects the non-quadrupolar modes
reduce thermal noise contribution from high frequency resonant modes which do not carry the gravitational signal
bandwidth free from acoustic modes not sensitive to gw.
Example:
- capacitive readout -
The current is proportional to:
Dual Detector with √Shh~10-23/√Hz in 1-5 kHz range
Readout:
• Selective measurement strategy
• Quantum limited
• Wide area sensor
• Displacement sensitivity
Detector:
• Massive resonators ( > 10 tons )
• Cooling
• Suspensions
• Low loss and high cross-section materials
Feasibility issues
Silicon Carbide (SiC)
• Q/T > 2x108 K-1 - Mass = 62 tons
• R = 1.44 m - height = 3 m
Molybdenum
• Q/T>2x108 K-1 - Mass = 16 tons
• R = 0.47 m - height = 2.3 m
R&D on readouts: status
• Requirement: ~ 5x10-23 m/√Hz • Present AURIGA technology: 10-19 m/√Hz
with:optomechanical readout - based on Fabry-Perot cavities
capacitive readout - based on SQUID amplifiers
Develop non-resonant devices to amplify the differential deformation of the massive bodies.
Foreseen limits of the readout sensitivity: ~ 5x10-22 m/√Hz.Critical issues:
optomechanical – push cavity finesse to current technological limit together with Watts input laser power
capacitive – push bias electric field to the current technological limit
Idea to relax requirements on readout sensitivity: mechanical amplifiers
Requirements:
GOAL:Amplify the differential deformations of the massive bodies
over a wide frequency range.
• based on the elastic deformation of monolithic devices • well known for their applications in mechanical engineering.
* Gain of at least a factor 10.* Negligible thermal noise with respect to that of the
detector.