The virgo detector status and first experimental results
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The Virgo detector: status and first experimental results. Nicolas Arnaud CERN EP Seminar 17/03/2003. Outline. The quest for gravitational waves ( GW ): a long history Detection principles Interferometric detectors Description of the Virgo interferometer Optical scheme

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The virgo detector status and first experimental results

The Virgo detector: status and first experimental results

Nicolas Arnaud CERN EP Seminar 17/03/2003


Outline

Outline

  • The quest for gravitational waves (GW): a long history

  • Detection principles

    • Interferometric detectors

  • Description of the Virgo interferometer

    • Optical scheme

    • Main features of the instrument

    • Foreseen sensitivity

  • Experimental control of the Central Interferometer (CITF)

    • CITF description and CITF commissioning goals

    • Experimental results (spring 2001 summer 2002)

  • Main GW sources and filtering techniques

  • Virgo versus the other GW interferometric detectors

  •  The LIGO interferometers (USA) + TAMA (Japan)


Gravitational waves a brief history

Gravitational waves: a brief history

  • First «imagined» by Poincaré in 1905

«J'ai été d'abord conduit à supposer que la propagation de

la gravitation n'est pas instantanée mais se fait à la vitesse

de la lumière (…) Quand nous parlerons donc de la position ou

de la vitesse du corps attirant, il s'agira de cette position ou

de cette vitesse à l'instant où l'onde gravifique est partie de

ce corps (…)» [Italics of the author]

  • GW existence predicted by Einstein in 1918

    •  Direct consequence of the General Relativity (1915)

  • But not immediately accepted:

  •  GW appear after a linearization of the Einstein equations

  • while non-linearity is the key of their physical content

  •  Non linear correction terms cannot be calculated this way

«GW travel at the speed of mind » Sir A.S. Eddington

  • Theoretical framework developped in the 50’s and 60’s

  • (Pirani & Isaacson)

  • Indirect evidence of the GW existence:PSR 1913+16

  • Hulse & Taylor(Nobel Prize 1993) [& Damour]

 GW must exist !


Gravitational waves main characteristics

GW amplitude:

L

L + DL

Gravitational waves main characteristics

  • Perturbations of the Minkowski metric propagating at the speed of light

  • Quadrupolar emission

  • Extremely weak!!!

    Ex:Jupiter radiates 5.3 kW as GW during its orbital motion

    •  over 1010 years:EGW = 2  1021 J  Ekinetic  2  1035 J

  • A good source of GW must be: asymetric, compact, relativistic

Luminosity  G/c5  10-53 W-1

No Hertz experiment possible!

GW effect : differential modification of lengths

 Detectors:IFO, resonant bars, LISA…

Detections expected up to the Virgo cluster (~ 20 Mpc)


Interferometric detection

Interferometric detection

Suspended

Michelson

Interferometer

Mirrors used as

test masses

Variation of the

power Pdet at the

IFO output port

Optical path

modification

Incident GW

Sensitivity :


The virgo interferometer

The Virgo Interferometer

  • French-Italian collaboration

    • (CNRS/INFN)

    • ~ 50 physicists

    • ~ 50 engineers

  • Budget: ~ 75 M€

  • 55% Italy, 45% France

Site: Cascina, near Pisa

  • Planning:

  • Spring 2001 - Summer 2002: successfully

  • Central Interferometer Commissioning completed

  •  2003:Shutdown and transition to the full detector

  • From summer 2003:Full scale Virgo commissioning

  • First Physical Data foreseen for2004 … or later


The virgo detector

White

fringe

10-17

The Virgo Detector

Laser power: Pin = 20 W

Sensitivity

Gain :

3000

 30

~ 106

Laser

Sensitivity : hsens ~

3 10-21

10-23

10-22

Detection

Photodiode

 To increase the arm length : 1 m  3 km

 To add Fabry-Perot cavities (Finesse = 50  Gain = 30)

 To add a recycling mirror (P = 1 kW on the Beam Splitter)


The virgo superattenuator

The Virgo SuperAttenuator

INFN

Pisa

Length ~ 7 m; Mass ~ 1 ton

Structure in inverted pendulum

-

 fres ~ 30 mHz

  • Dual role:

  • Passive seismic isolation

  • Mirror active control

  • only 0.4 N needed

  • for a 1 cm motion

Seismic Attenuation:

~ 1014 à 10 Hz


Virgo configuration

Full Virgo

configuration

Virgo configuration


Virgo foreseen sensitivity

Thermal

noise

mirrors

Violin

modes

Thermal

noise

Tail of the

0.6 Hz marionetta/

mirror resonance

Shot noise

«Seismic Wall»

Minimum ~3 10-23between ~ 500 Hz et 1 kHz

Virgo foreseen sensitivity


Virgo central interferometer citf

  • CITF commissioning = 1rst step of Virgo commissioning

  • Recycled and suspendedMichelson Interferometer

  • Uses the main technology developped for Virgo

  • CITF commissioning goals:

    • check the different component performances

    • validate control algorithms

    • test data management (acquisition, storage…)

«West» Mirror

Arm

lengths

~ 6 m

Recycling

Mirror

«North»

Mirror

The CITF is not sensitive enough:

no hope to collect data with GW signal!!!

Virgo central interferometer (CITF)


Citf and working point

Very narrow Working Point

In addition: residual low frequency motion of mirrors (0.6 Hz)

 CITF active controls needed (local and global)

Longitudinal control

«Locking »

 Resonant cavities

dl ~ 10-10 – 10-11 m

Angular control

«Alignment »

 Aligned mirrors

dq ~10-9 – 10-7 rad

Goal :

CITF and working point

  • Best sensitivity :

  • Michelson on dark fringe control arm asymmetry:l2-l1

  • Recycling cavity resonant (maximize the stored power)

  •  control IFO mean length:l0 + (l1+l2)/2


The steps of the virgo control

The steps of the Virgo control

Control aim: to go from an initial

situation with random mirror

motions to the Virgo working point

Virgo

frame

  • Decreasing the residual motion

  • separately for each mirror

  •  Local controls

  • + First alignment of mirrors

  • Lock acquisition of the cavities

  • Check working point control stability

  • Switch on the angular control

  •  Automatic Alignment

Switching from

local controls

to

global controls


First control of the michelson

Fringe Counting

Fringe interval

~ 0.5 mm

Global

Control

Time (s)

AC Power

Error signal

Time (s)

DC Power

Interferometer

power output

Dark fringe

June 13th 2001

Time (s)

First control of the Michelson


First control of the recycled citf

Stored

Power

  • Pmax ~ 5.8 W

  •  Gain ~ 70

  • (Plaser ~ 80 mW)

  • Dark fringe

  • less «dark»

  •  unperfect contrast

  • Large fluctuations of

  • the stored power:

    • low feedback gain

    • misalignments

      December 16th 2001

IFO

output

power

D5

Recycling

correction

West

correction

  • Recycling : D5 photodiode

  • Signal reflected by the Beam Splitter 2nd face (AR coating)

 Correction applied on the Recycling Mirror

First control of the recycled CITF

  • A complex problem:

    • Two lengths to be controlled instead of one

    •  coupled error signals

    • Narrow resonance of the recycling cavity (high finesse)

    • Limited force available to act on mirrors

    • Error signal ~ to the electronic noise outside resonance

    • [weak laser power + Recycling mirror reflectivity = 98.5%]

  • Main issues:

    • To select the right resonance

    • [trigger on the stored power]

    • Simultaneous acquisition of the 2 cavity controls

    • Fast damping of the 0.6 Hz pendulum resonance excited

    • each time the locking attempt fails


Citf main steps

CITF Main steps

  • 5 ER for the CITF commissioning

  • 3 days duration (24h/24h)

  • ~ 1 TB data collected / ER

  • ~ 5 MBytes/s ~ 160 TB/an

  • The 2 first in Michelson configuration (9/01 and 12/01)

  • The 3 others Recycled configuration (4/02, 5/02 and 7/02)

All sources of control losses understood

 Improvements already done or in progress

  • Main CITF improvements:

    • Suspension hierarchical control (feedback splitting)

    • Output Mode-Cleaner locking

    • Laser frequency stabilization above few Hz

    • West mirror linear alignment


Citf sensitivity improvements

June 2001  July 2002

Factor 103

improvement

@ 10 Hz

Factor 105

improvement

@ 1 kHz

Readout electronic noise

Back scattered light

on laser bench

Alignment

Noise:

peaks = qx

resonances

Auxiliary laser

frequency noise

Room for

many more

Improvements

Laser

frequency noise

due to vibrations

of the input

Mode-Cleaner length

Laser frequency

noise + some

test masses

resonant modes

CITF sensitivity improvements


From the citf to the full virgo 1

From the CITF to the full Virgo (1)

  • Question:what does the CITF teach us on the full Virgo?

  • Answer:many (encouraging) things but the commissioning

  • of the complete Virgo remains a major challenge

  • Main results of the CITF commissioning:

    • Scientific program completed

    • The different parts of the control chain work well

      • seismic noise no more dominant above 1 Hz

      • methods for resonance acquisition developped

      • linear feedbacks (z and q) stable on large timescales

    • Dark fringe control @ 10-12 m as requested for Virgo

    • Large improvements in sensitivity in only one year

    • Gain in ‘experimental experience’ allows one to prepare

    • several upgrades for the full Virgo in various systems

    • Main concerns:input laser system

  •  CITF studies are direct benefits for Virgo


From the citf to the full virgo 2

From the CITF to the full Virgo (2)

  • CITF  Virgo will provide ‘free’ sensitivity improvements:

    • Arm length:6 m 3 km  gain of a factor 500 in h

    • Fabry-Perot cavities: factor 30 in addition

  • In reality, such gains are unfortunately not automatic:

    • some noises do not depend on the laser optical path

    • as soon as the main noise in a given bandwidth is

      lowered, other sources previously hidden appear!

  • Virgo optical scheme more complicated (4 lengths)

    • Dark Fringe (difference in the arm lengths)

    • Recycling Length (mean length of the interferometer)

    • 2 Fabry-Perot cavities

  •  Lock acquisition procedure  from CITF methods

  • more complex

    • currently under study with simulations

    • Virgo can benefit from the other detector experiences


First beam in the 3 km north arm

« As a first conclusion, the tube is straight! »

First beam in the 3 km North arm

Thursday March 13th 2003:

First beam travelling in the 3km North Arm!!!

LAST MINUTE


Preparing the gw data analysis

Preparing the GW Data Analysis

  • Activity parallel to the experimental work on detectors

  •  1 international conference / year (GWDAW)

  • Large number of potential GW sources:

    • compact binary coalescences (PSR 1913+16)

    • black holes

    • supernovae

    • pulsars

    • stochastic backgrounds

  • The corresponding signals have very different features

  •  various data analysis techniques

  •  Brief review of the main GW signals and methods


Compact binary coalescences

Chirp signal:

amplitude and frequency

increase with time until

the final coalescence

  • The signal knowledge ends

  • before the coalescence

  • when approximations used

  • for the computation are

  • no more valid.

  • large theoretical work

    to go beyond this limit!

Waveform analytically estimated by developments in v/c

 Wiener (optimal) filtering used for data analysis

Compact binary coalescences


Impulsive sources bursts

Impulsive sources (‘bursts’)

  • Examples:

  • Merging phase of binaries

  • Supernovae

  • Black hole ringdowns

  • GW main characteristics:

  • Poorly predicted waveforms

  •  model dependent

  • Short duration (~ ms)

  • Weak amplitudes

Zwerger

/ Müller

examples of

simulated

supernova

GW signals

  •  Need to develop  filters :

    • robust (efficient for a large class of signals)

    • sub-optimal (/ Wiener filtering)

    • online (first level of event selection)


Pulsars

Pulsars

  • GW signal:permanent, sinusoidal, possibly 2 harmonics

  • Weak amplitude  detection limited to the galaxy

  • Matched filtering-like algorithms using FFT periodograms

  • Idea:follow the pulsar freq. on large timescales (~ months)

  •  compensation of frequency shifts: Doppler effect

  • due to Earth motion, spindown…

  • Very large computing power needed (~ 1012 Tflops or more)

  •  Hierarchical methods are being developped  1 TFlop

  •  Need to define the better strategy:

    • search only in the Galactic plane, area rich of pulsars

    • uniform search in the sky not to miss close sources

    • focus on known pulsars

  • Permanent signal  coincident search in a single detector:

    compare candidates selected in 2 different time periods


  • Stochastic backgrounds

    Stochastic backgrounds

    • Described by an energy density per unit logarithmic

    • frequency normalized to the critical density of the universe:

    • Two main origins:

      • Cosmological

      • Emission just after the Big Bang: ~10-44 s, T~1019 GeV

      • Detection  informations on the early universe

      • Astrophysical

      • Incoherent superposition of GW of a given type emitted

      • by sources too weak to be detected separately.

    • Detection requires correlations between 2 detectors

    • After 1 year integration: h02stoch  10-7 (1rst generation)

    • 10-11 (2nd generation)

    • Theoretical predictions: ~ 10-13 10-6

    • Current best limit:stoch  60 @ 907 Hz [Explorer/Nautilus]

    with


    Coincidence detections

    Coincidence detections

    • Why ?

    • Some detectors

    • will be working

    • in the future

    • LIGO : 4 km

    • VIRGO : 3 km

    • GEO : 600 m

    • TAMA : 300 m

    • ACIGA : 500 m

    • Coincidence = only way to separate a real GW from

    • transient noises in a particular interferometer

    • Coincidences may allow to locate the source position in sky

    • Coïncidences with other emissions: g, n

    now ACIGA


    Interferometer angular response

    Interferometer angular response

    The detectable GW amplitude is a linear combinationof the

    two GW polarizations h+ et h

    h(t) = F+ h+(t) + F h(t)

    Declination d

    Beam Pattern

    statistical

    distribution

    Angular

    response

    RMS ~ 0.45

    Right ascension a

    • 2 maxima ( detector)

    • 4 minima (blind detector)

    Reduction of a factor ~ 2

    in average of the amplitude


    Example of the virgo ligo network

    Example of the Virgo-LIGO network

    • Spatial responses

    •  in a given direction

    • Similarities between

    • the maps of the two

    • LIGO interferometers

    • Complementarity

    • Virgo / LIGO

    •  Good coverage of

    • the whole sky

    • Double or triple

      coincidences

      unlikely


    Virgo versus other interferometers

    Virgo versus other interferometers

    10-7

    October-November 2002

    June-August 2002

    10-12

    LIGO

    TAMA

    10-20

    10-20

    1 Hz

    10 Hz

    10 kHz

    10 kHz

    10-7

    • All sensitivities in m/Hz

    •  Comparable plots!

    • Improvements still needed!

    • Record sensitivity:Tama

    • 10-18 m/Hz @ 1 kHz

    • @ 10 Hz, the CITF has the

    • best sensitivity: 10-13 m/Hz

    Virgo CITF

    July 2002

    10-20

    5 kHz

    1 Hz


    Summary

    Summary

    • Many interferometers are currently under developpement

    •  Worldwide network in the future

      • All instruments work already although they did

        not prove yet there can fulfill their requirements

    •  Control of complex optical schemes with suspended mirrors

      • All sensitivities need to be significally improved to

      • reach the amplitude of GW theoretical predictions

    • Many different GW sources

    •  various data analysis methods in preparation

    • In the two last years, the Virgo experiment became real

      • The different parts of the experiment work well together

      • Successful commissioning of the CITF

      • 2003: CITF  Full Virgo

      • First ‘physically interesting’ data expected for 2004 !?!?!


    Gw a never ending story

    GW: a never ending story

    The future of gravitational astronomy looks bright.

    1972

    That the quest ultimately will succeed seems almost assured.

    The only question is when, and with how much further effort.

    1983

    [I]nterferometers should detect the first

    waves in 2001 or several years thereafter (…)

    1995

    Km-scale laser interferometers are now coming on-line, and it

    seems very likely that they will detect mergers of compact

    binaries within the next 7 years, and possibly much sooner.

    2002

    Kip S. Thorne


    References about virgo and gw

    References about Virgo and GW

    • Virgo web site: www.virgo.infn.it

    • Virgo-LAL web site (burst sources): www.lal.in2p3.fr

    • Source review: (recent)

    • C. Cutler - K.S. Thorne, gr-qc/0204090

    • Supernova signal simulation:

    • H. Dimmelmeier et al.astro-ph/0204288 and 89 (2002)

    • Some other GW experiment websites:

      • LIGO: www.ligo.caltech.edu

      • GEO: www.geo600.uni-hannover.de

      • TAMA: www.tamago.mtk.nao.ac.jp/tama.html

      • IGEC (bar network): igec.lnl.infn.it

      • LISA: sci.esa.int/home/lisa


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