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

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

How to detect gravitational waves? 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 arms L = arm length Suspended mirror Beam splitter LASER () Light Detection

The shot noise and the VIRGO optical design 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? Gravitational wave signal 1/ Fabry-Perot cavities to increase the effective length: ( F = finesse ) => L’ = 100km for L=3km and F=50 2/ Recycling mirror to increase the effective power: P’ = R P (R = recycling gain) => P’ = 1kW with P=20W and R=50

Noise sources in interferometers Laser Noises Shot Noise Detection Index fluctuation Seismic Acoustic Thermal

Noise sources: seismic noise 1014 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 !

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

VIRGO design sensitivity Main sources of noise limiting the VIRGO design sensitivity Shot noise 1 Seismic noise Thermal noise Shot noise

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

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: - recombined (Michelson) ITF: Feb 2004 control of the north FP cavity: Oct 2003 - control of the west FP cavity: Dec 2003 - recycled (full VIRGO) ITF: Oct 2004 output mode cleaner input mode cleaner laser recycling mirror beam splitter L=3km l=150m l=6m Fabry-Perot cavities West arm North arm Gravitational wave signal

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 Laser + - Lock acquisition Power stored in the recycling cavity (Watts) With recycling Without recycling Recycling gain ~ 30

Sensitivity summary Single arm, P=7 W Recombined, P=7 W Recycled, P=0.7 W P = 10W h ~3. 10-21/Hz

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

Present sensitivity and perspectives P=0.7 W - local angular controls P = 10W Improvements since C5: - longitudinal controls - low noise actuators Shot noise for P=0.7 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 …

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

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?

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

Comparison with LIGO first science run (S1) Virgo May 2005

Example of lock acquisition Example of the lock acquisition of a Fabry-Perot cavity Photodiode used for lock acquisition 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 keep the error signal at zero 4 seconds Coil Mirror Magnet

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!

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

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 => improve signal to noise ratio Signal detection: - InGaAs photodiodes, high efficiency

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 Shot noise

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