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The status of VIRGO

Edwige Tournefier (LAPP-Annecy ) for the VIRGO Collaboration

HEP2005, 21st- 27th July 2005

• The VIRGO experiment and detection of gravitational waves

• The commissioning of VIRGO

• Conclusions

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VIRGO

French-italian collaboration (CNRS – INFN)Annecy (LAPP), Firenze, Frascati, Lyon (LMA), Napoli, Nice (OCA), Paris (ESPCI), Perugia, Pisa, Roma, Orsay (LAL)

Virgo site : Cascina close to Pisa

Virgo goal: detection of gravitational waves

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2

hLL 2

hLL

Suspended mirror

Suspended mirror

Beam splitter

LASER ()

LightDetection

• Effect of a gravitational wave on free masses:

A Michelson interferometer is suitable:

- suspend mirror with pendulum => ‘free falling masses’

- Gravitational wave => phase shift

Measure: h = L/L

L = length difference between the 2 armsL = arm length

L4

How to detect gravitational waves?

4Gravitational wave signal

Limitation of a Michelson interferometer due to photon shot noise:the minimum measurable relative displacement is

=> Can reach h ~ 3.10-23 with L=100km and P=1kW How to achieve that?

The shot noise and the VIRGO optical design

PLh

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4

~

2/ Recycling mirror to increase the effective power: P’ = R P (R = recycling gain)

=> P’ = 1kW with P=20W and R=50

1/ Fabry-Perot cavities to increase the effective length: ( F = finesse )

=> L’ = 100km for L=3km and F=50

LFL 2

'

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Noise sources in interferometers

Laser Noises

ShotNoise Detection

Noise

Index fluctuation

SeismicNoise

Acoustic NoiseThermal

Noise

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Noise sources: seismic noise

Hz

m

f

axs 2~

With a chain of 6 pendulums:

attenuation of the seismic noise by ~1014 at 10 Hz !

Transfert function

Seismic noise spectrum for f few Hz:

a ~ 10-6 - 10-7

shot noise !

Need a very large attenuation!

Solution: suspend the mirrors to a chain of pendulums

1014

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Suspensions and control of the interferometer

All mirrors are suspended to a cascade of pendulums: Large attenuation in the detection band ( > 10 Hz) Large residual motion at low frequencies: < ~1mm

Need active controls to:- maintain the interferometer’s alignment- maintain the required interference conditions

The control is done in 2 steps:

1/ Local control of the suspensions: Residual motion ~2 m/sec Obtain interference fringes

2/ To keep the interferometer at interference conditions:– Need to control the length of the cavities to 10-12 m– Need to keep the interferometer aligned Use the interferometer signals: photodiodes

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VIRGO design sensitivity

Main sources of noise limiting the VIRGO design sensitivity

Shot noise1

Seismic noiseThermal noiseShot noise

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Gravitationnal wave sources and VIRGO design sensitivity

Distance to the Virgo cluster = 10Mpc

Coalescing binaries (1.4 Mo) Pulsars: upper limit (1 year) Supernovae at 15Mpc

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The commissioning of VIRGO

• End of construction: 2003

• The steps of the VIRGO commissioning:

output mode cleaner

input mode cleaner

laserrecycling

mirror

beam splitter

L=3km

l=150m

l=6m

Fabry-Perot cavities

• Technical runs (3 to 5 days) at each step C1(Nov 2003),…, C5(Dec 2004)

Lock stabilitySensitivity/noise studiesData taking on ‘long’ period

Gravitational wave signal

North armW

est

arm

- control of the north FP cavity: Oct 2003- control of the west FP cavity: Dec 2003

- recombined (Michelson) ITF: Feb 2004

- recycled (full VIRGO) ITF: Oct 2004

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Laser

+

-

The lock of the full VIRGO• Lock of the recycled interferometer (full VIRGO):

– Need to control 4 degrees of freedom (3 cavities + Michelson on dark fringe)

– The lock is acquired in several steps (‘variable Finesse’ strategy):

• Start without recycling• Slowly increase the recycling gain and move to the dark fringe

Lock acquisition

Power stored in the recycling cavity (Watts)

With recycling

Without recycling

Recycling gain ~ 30

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Sensitivity summary

Single arm, P=7 W

Recombined, P=7 W

Recycled, P=0.7 W

P = 10W

h ~3. 10-21/Hz

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Typical unforeseen difficulties• Injection bench:

– A small fraction (bigger than expected) of the light reflected by the

interferometer is retro-diffused by the input mode cleaner mirror spurious interferences

Temporary solutions:- tried to rotate the mode cleaner mirror- reduce the incident light (/10)

We are now working with only Pin = 0.7 Watts

Final solution: install a Faraday isolator A new input bench will be installed in September 2005

Frequency noise Recycling mirror: - aligned- not aligned

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Present sensitivity and perspectives

• Futur: the VIRGO sensitivity will significantly improve with– full power (new input bench)– the automatic alignment of the interferometer (global angular control) – the improvement of the longitudinal controls– lower noise actuators– …

• Improvements since C5:

P=0.7 W

P = 10W

Shot noise for P=0.7 W

- local angular controls - longitudinal controls - low noise actuators

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Data analysis: some examples

- Injected events• Test of the data analysis on real data from the technical runs:

– Test the full chain of data analysis– Learn how to put vetoes– Inject events in the real data: software and hardware injections-> measure efficiencies, false alarm rate,…

• Start collaboration with LIGO: Coincident analysis will help the detection of GW

=>decrease false alarm rate (rare events in a non gaussian noise)

Combined data analysis is necessary to extract the source parameters

Event amplitude

Quiet period

Event amplitude

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Conclusion

– The recycled (full VIRGO) interferometer is working

– Next engineering run (C6), 29/07-12/08: 2 weeks of data taking with the best sensitivity

– The sensitivity will make big progress with• New input bench (-> full input power) • Automatic alignment of the mirrors

– The data analysis is been prepared and tested on real data Collaboration with LIGO is starting

– First scientific run in 2006/7?

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Noise studies

Sensitivity measured during C4 run and identified sources of noise

Noise hunting:

1/ Identify the sources of noise which limit the sensitivity2/ Perform the necessary improvements / implement new controls

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Comparison with LIGO first science run (S1)

Virgo May 2005

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Example of lock acquisition

Example of the lock acquisition of a Fabry-Perot cavity

Coil

Mirror

Magnet

Power stored inside the Fabry-Perot cavity

Error signal of the cavity

Correction sent to the actuators of the mirror

/2

Lock acquisition: Apply force on the mirror to keepthe error signal at zero

4 seconds

Photodiode used for lock acquisition

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The commissioning of the CITF

• Commissioning of the central interferometer: 09/2001 -> 07/2002– CITF = Recycled Michelson interferometer (no Fabry-Perot cavities)- a lot of common points with VIRGO

• The evolution: configuration and sensitivity: 4 runs of 3 days each

- E0/E1: Michelson - E2: Recycled Michelson

- E3: + automatic angular alignment - E4: + final injection system

• Results:– Viability of the controls– Sensitivity curve understood– And gain experience for the VIRGO commissioning- Improvements triggered by the CITF experience

unit = meters!

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The mirrors

• Fused silica mirrors

• Coated in a class 1 clean room at SMA-Lyon (unique in the world).

– Low scattering and absorption: < few ppm– Good uniformity on large dimension: < 10-3 400 mm

• Large mirrors (FP cavities):– 35 cm, 10 cm thick– 20 kg

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• Laser: powerful and stable- 20W- Power stability: 10-8

- Frequency stability: Hz• The input and output mode cleaners:

- optical filter => improve signal to noise ratio

• Signal detection: - InGaAs photodiodes, high efficiency

The injection and detection systems

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Shot noise

Future: how to improve the sensitivity?

The first generation of detectors might not be able to see gravitational waves

Need to push the sensitivity further down:

• Seismic noise:– The VIRGO suspensions already meet the requirements for next

generation interferometers

• The main limit: thermal noise– Monolitic suspensions (silica)– Better mirrors (material, geometry, coating)

• Shot noise– More powerful lasers– Signal recycling technique

• And the technical noises– Better sensors– Better electronics– Better control systems

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Recombined interferometer

• Recombined interferometer: keep the two Fabry-Perot cavities on resonance + the Michelson on the dark

fringe

Power ‘stored’ inside the FP cavities

Power at the interferometer output

Lock on the dark fringe

Example of lock acquisition