1 The status of VIRGO E. Tournefier LAPP(Annecy)-IN2P3-CNRS Journées SF2A, Strasbourg 27 juin – 1er juillet 2005 The VIRGO experiment The commissioning of VIRGO Towards a global network Conclusions

2 Suspended mirror Suspended mirror Beam splitter LASER ( ) Light Detection Effect of a gravitational wave on free masses:  A Michelson interferometer is suitable: -suspend mirror with pendulum => ‘free falling masses’ -Gravitational wave => phase shift h =  L/L  L = length difference between the 2 arms L = arm length How to detect gravitational waves? 

3 Gravitational wave signal Limitation of a Michelson interferometer due to photon shot noise: the minimum measurable relative displacement is => Can reach h ~ with L=100km and P=1kW How to achieve that? The shot noise and the VIRGO optical design 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

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

5 Noise sources: seismic noise Seismic noise measured on the Virgo site m/  Hz Hz Seismic noise spectrum for f  few Hz: a ~  shot noise !  Need a very large attenuation! Solution: suspend the mirrors to a chain of pendulums  With 6 pendulums: attenuation of the seismic noise by more than 10 orders of magnitude above 4 Hz! Transfert function

6 Suspensions and control of the interferometer All mirrors are suspended to a cascade of 6 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 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 cavity length to m  Use the interferometer signals (photodiodes)

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

8 Gravitationnal waves sources and VIRGO design sensitivity (sources: see talk by N. Leroy) Distance to the Virgo cluster = 10Mpc

9 VIRGO French-italian collaboration (CNRS – INFN) Site : Cascina close to Pisa 5 french labs: Annecy (LAPP), Lyon (LMA), Nice (OCA), Paris (ESPCI), Orsay (LAL) 6 italian labs: Firenze, Frascati, Napoli, Perugia, Pisa, Roma (all INFN)

10 The commissioning of VIRGO Started in summer 2003 The steps of the VIRGO commissioning: output mode cleaner input mode cleaner laser recycling 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 stability  Sensitivity/noise studies  Data taking on ‘long’ period Gravitational wave signal North arm West arm - control of the north FP cavity: Oct control of the west FP cavity: Dec recombined (Michelson) ITF: Feb recycled (full VIRGO) ITF: Oct 2004

11 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

12 The lock of the full VIRGO Lock of the recycled interferometer (full VIRGO): –Need to control 4 degrees of freedom (3 cavities + Michelson) –The lock is acquired in several steps: Start without recycling Slowly increase the recycling gain  2 technical runs: -C5 (3 days, Dec 2004) -C6 (2 weeks) planned for this summer Laser + - POWER IN THE RECYCLING CAVITY Lock acquisition sequence Without power recycling With power recycling Recycling gain ~ 30

13 Noise studies Sensitivity measured during C4 run and identified sources of noise Noise hunting => see talk by R. Gouaty 1/ Identify the sources of noise which limit the sensitivity 2/ Perform the necessary improvements / implement new controls Attention a l’unite!

14 Typical unforseen 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: - rotate the mode cleaner mirror - reduce the incident light (/10)  We are now working with only P in = 0.7 Watts Final solution: install a Faraday isolator  A new input bench will be installed in september

15 Sensitivity summary The VIRGO sensitivity will significantly improve with: - the implementation of the automatic alignment of the mirrors (low frequency) - the full power (high frequency) Single arm, P=7 W Recombined, P=7 W Recycled, P=0.7 W P = 10W h ~ /  Hz

16 Data analysis Calibration and reconstruction of the signal: Watts -> meters - Apply a known displacement to the mirrors A lot of tests on simulated data including interferometer noise 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 gravitational waves =>decrease false alarm rate (rare events in non gaussian noise) Combined data analysis is necessary to extract the source parameters - Injected events

17 Towards a worldwide network Look for events in coincidence Combined analysis is needed to extract information on the source GEO VIRGO TAMA AIGO LIGO

18 LIGO Livingston Observatory (LLO) L1 : 4 km arms LIGO Hanford Observatory (LHO) H1 : 4 km arms H2 : 2 km arms Status of LIGO reaching the Virgo cluster ! Two sites: –Hanford (Washington): 4km and 2 km interferometer –Livingston (Louisiana): 4km interferometer Same optical configuration as Virgo Less sophisticated suspensions The commissioning started in 1999 The three interferometers are operational Long science runs have started: –S1 (Aug 2002) –S2 (March-April 2003) –S3 (Nov-Dec 2003) –S4 (Fev-March 2005) –6 month run this year The LIGO sensitivity is now very close to the design sensitivity !

19 Conclusion VIRGO commissioning is progressing –The recycled (full VIRGO) interferometer is working –Sensitivity will make big progress with Automatic alignment of the mirrors New input bench –First scientific run in 2006? LIGO is very close to its design sensitivity –Long science runs will start this year The detection with the first generation of detectors is not guaranteed –A global network is needed –A second generation of detectors is being prepared to reach h~few /  Hz => Upgraded VIRGO and LIGO ~

20 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

21 Future: How to go to lower frequencies Frequency range limited on the earth due to seismic noise => go to the space: the LISA project Much lower frequencies: – 1 Hz It is complementary to terrestrial detectors LISA: Spatial interferometer (NASA-ESA) 3 satellites, size = km start: 201?

22 GEO (UK, Germany) 600m long arms An interferometer for the development of new techniques: –Signal recycling –Monolitic suspensions (-> reduce thermal noise) Signal recycling Power recycling 600m arm (no FP) Laser

23 TAMA (Japon) Located at Tokyo Same optical configuration as VIRGO Started the commissioning in 1997 Reached first a sensitivity of h ~ Hz –1/2 But limited by the small arm length design

24 Gravitational wave detectors: resonant bars The gravitational wave excites the resonant mode of the bar  Good sensitivity for frequency = mode of the bar First detectors in 1960 Many improvements since then: –Cryogenic –New transducers for the detection of bar oscillation Several detectors in operation => perform coincidence data analysis

25 Bars events NAUTILUS (Italie) EXPLORER (CERN) Coincident analysis between Explorer and Nautilus –2001 data –Small excess of events when the detectors are optimally oriented with respect to the galactic plane –Excess not confirmed by recent data taking

26 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: <  400 mm Large mirrors (FP cavities): –  35 cm, 10 cm thick – 20 kg

27 Laser: powerful and stable -20W -Power stability: 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

28 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!

29 Data analysis Supernovae –The signal shape is not well known  several techniques are developed to detect bursts  Problem of non gaussian detector noise  Detection in coincidence with other detectors is needed Binary coalescences –Well known signal  Use a matched filtering technique  The parameters of the sources can be extracted Pulsars –Need to integrate on long periods –But the signal is distorted by Doppler effect due to the earth’s rotation  Huge parameter space  Limited by computational resources

30 output mode cleaner input mode cleaner laser recycling mirror beam splitter L=3km l=150m l=6m Fabry-Perot cavities Control of the cavity Control of the laser frequency