The status of VIRGO Edwige Tournefier (LAPP-Annecy ) for the VIRGO Collaboration HEP2005, 21st- 27th July 2005

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The status of VIRGOEdwige Tournefier
(LAPP-Annecy ) for the VIRGO CollaborationHEP20
05, 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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How to detect gravitational waves?

• Effect of a gravitational wave on free masses
• A Michelson interferometer is suitable
• suspend mirror with pendulum gt free falling
  masses
• Gravitational wave gt phase shift
• Measure h ?L/L
• ?L length difference between the 2 arms
• L arm length

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The shot noise and the VIRGO optical design

• Limitation of a Michelson interferometer due to
  photon shot noise
• the minimum measurable relative displacement is
• gt Can reach h 3.10-23 with L100km and P1kW
• How to achieve that?

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Noise sources in interferometers
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Noise sources seismic noise

• 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

Transfert function

• With a chain of 6 pendulums
• attenuation of the seismic noise by 1014 at 10
  Hz !

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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 ( gt 10
  Hz)
• Large residual motion at low frequencies lt 1mm
• Need active controls to
• maintain the interferometers 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

Seismic noise Thermal noise Shot noise
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Gravitationnal wave sources and VIRGO design
sensitivity

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

Distance to the Virgo cluster 10Mpc
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The commissioning of VIRGO

• Technical runs (3 to 5 days) at each step
• C1(Nov 2003),, C5(Dec 2004)
• Lock stability
• Sensitivity/noise studies
• Data taking on long period

• End of construction 2003
• The steps of the VIRGO commissioning

West arm
North arm
Gravitational wave signal
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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

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Sensitivity summary
Single arm, P7 W
Recombined, P7 W
Recycled, P0.7 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

Frequency noise
Recycling mirror - aligned - not aligned
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Present sensitivity and perspectives
P0.7 W
P 10W

• Improvements since C5

Shot noise for P0.7 W

• 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

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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
• -gt measure efficiencies, false alarm rate,
• Start collaboration with LIGO
• Coincident analysis will help the detection of
  GW
• gtdecrease 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 (-gt 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
  sensitivity
• 2/ 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

Photodiode used for lock acquisition
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The commissioning of the CITF

• Commissioning of the central interferometer
  09/2001 -gt 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

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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
• lt few ppm
• Good uniformity on large dimension
• lt 10-3 ? 400 mm

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

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The injection and detection systems

• Laser powerful and stable
• 20W
• Power stability 10-8
• Frequency stability ?Hz
• The input and output mode cleaners
• optical filter gt improve signal to noise
  ratio
• Signal detection
• - InGaAs photodiodes, high efficiency

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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