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Sensitivity Studies for Third-Generation Gravitational Wave Observatories

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Advanced gravitational wave detectors, currently under construction, are expected to directly observe gravitational wave signals of astrophysical origin. The Einstein Telescope, a third-generation gravitational wave detector, has been proposed in
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  Sensitivity Studies for Third-GenerationGravitational Wave Observatories SHild 3 , MAbernathy 3 , FAcernese 4 , 5 , PAmaro-Seoane 33 , 46 ,NAndersson 7 , KArun 8 , FBarone 4 , 5 , BBarr 3 , MBarsuglia 9 ,MBeker 45 , NBeveridge 3 , SBirindelli 11 , SBose 12 , LBosi 1 ,SBraccini 13 , CBradaschia 13 , TBulik 14 , ECalloni 4 , 15 ,GCella 13 , EChassandeMottin 9 , SChelkowski 16 ,AChincarini 17 , JClark 18 , ECoccia 19 , 20 , CColacino 13 ,JColas 2 , ACumming 3 , LCunningham 3 , ECuoco 2 ,SDanilishin 21 , KDanzmann 6 , RDeSalvo 23 , TDent 18 ,RDeRosa 4 , 15 , LDiFiore 4 , 15 , ADiVirgilio 13 , MDoets 10 ,VFafone 19 , 20 , PFalferi 24 , RFlaminio 25 , JFranc 25 ,FFrasconi 13 , AFreise 16 , DFriedrich 6 , PFulda 16 , JGair 26 ,GGemme 17 , EGenin 2 , AGennai 16 , AGiazotto 2 , 13 ,KGlampedakis 27 , CGr¨af  6 MGranata 9 , HGrote 6 ,GGuidi 28 , 29 , AGurkovsky 21 , GHammond 3 , MHannam 18 ,JHarms 23 , DHeinert 32 , MHendry 3 , IHeng 3 , EHennes 45 ,JHough 4 , SHusa 44 , SHuttner 3 , GJones 18 , FKhalili 21 ,KKokeyama 16 , KKokkotas 27 , BKrishnan 33 , T.G.F.Li 45 ,MLorenzini 28 , HL¨uck 6 , EMajorana 34 , IMandel 35 , 36 ,VMandic 31 , MMantovani 13 , IMartin 3 , CMichel 25 ,YMinenkov 19 , 20 , NMorgado 25 , SMosca 4 , 15 , BMours 37 ,HM¨uller–Ebhardt 6 , PMurray 3 , RNawrodt 3 , 32 , JNelson 3 ,ROshaughnessy 38 , CDOtt 39 , CPalomba 34 , APaoli 2 ,GParguez 2 , APasqualetti 2 , RPassaquieti 13 , 40 ,DPassuello 13 , LPinard 25 , WPlastino 42 , RPoggiani 13 , 40 ,PPopolizio 2 , MPrato 17 , MPunturo 1 , 2 , PPuppo 34 ,DRabeling 10 , 45 , PRapagnani 34 , 41 , JRead 33 , TRegimbau 11 ,HRehbein 6 , SReid 3 , FRicci 34 , 41 , FRichard 2 , ARocchi 19 ,SRowan 3 , AR¨udiger 6 , LSantamar´ıa 23 , BSassolas 25 ,BSathyaprakash 18 , RSchnabel 6 , CSchwarz 32 , PSeidel 32 ,ASintes 44 , KSomiya 39 , FSpeirits 3 , KStrain 3 , SStrigin 21 ,PSutton 18 , STarabrin 6 , ATh¨uring 6 , JvandenBrand 10 , 45 ,MvanVeggel 3 , CvandenBroeck 45 , AVecchio 16 , JVeitch 18 ,FVetrano 28 , 29 , AVicere 28 , 29 , SVyatchanin 21 , BWillke 6 ,GWoan 3 , KYamamoto 30 E-mail:  stefan.hild@glasgow.ac.uk 1 INFN, Sezione di Perugia, I-6123 Perugia, Italy 2 European Gravitational Observatory (EGO), I-56021 Cascina (Pi), Italy 3 SUPA, School of Physics and Astronomy, The University of Glasgow, Glasgow,G128QQ, UK 4 INFN, Sezione di Napoli, Italy 5 Universit`a di Salerno, Fisciano, I-84084 Salerno, Italy   a  r   X   i  v  :   1   0   1   2 .   0   9   0   8  v   1   [  g  r  -  q  c   ]   4   D  e  c   2   0   1   0  Sensitivity Studies for Third-Generation Gravitational Wave Observatories   2 6 Max–Planck–Institut f¨ur Gravitationsphysik and Leibniz Universit¨atHannover, D-30167 Hannover, Germany 7 University of Southampton, Southampton SO171BJ, UK 8 LAL, Universit´e Paris-Sud, IN2P3/CNRS, F-91898 Orsay, France 9 AstroParticule et Cosmologie (APC), CNRS; Observatoire de Paris, Universit´eDenis Diderot, Paris VII, France 10 VU University Amsterdam, De Boelelaan 1081, 1081 HV, Amsterdam, TheNetherlands 11 Universit´e Nice ‘Sophia–Antipolis’, CNRS, Observatoire de la Cˆote d’Azur,F-06304 Nice, France 12 Washington State University, Pullman, WA 99164, USA 13 INFN, Sezione di Pisa, Italy 14 Astronomical Observatory, University of warsaw, Al Ujazdowskie 4, 00-478Warsaw, Poland 15 Universit`a di Napoli ‘Federico II’, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy 16 University of Birmingham, Birmingham, B15 2TT, UK 17 INFN, Sezione di Genova, I-16146 Genova, Italy 18 Cardiff University, Cardiff, CF24 3AA, UK 19 INFN, Sezione di Roma Tor Vergata I-00133 Roma, Italy 20 Universit`a di Roma Tor Vergata, I-00133, Roma, Italy 21 Moscow State University, Moscow, 119992, Russia 22 INFN, Laboratori Nazionali del Gran Sasso, Assergi l’Aquila, Italy 23 LIGO, California Institute of Technology, Pasadena, CA 91125, USA 24 INFN, Gruppo Collegato di Trento, Sezione di Padova; Istituto di Fotonica eNanotecnologie, CNR-Fondazione Bruno Kessler, I-38123 Povo, Trento, Italy 25 Laboratoire des Mat´eriaux Avanc´es (LMA), IN2P3/CNRS, F-69622Villeurbanne, Lyon, France 26 University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK 27 Theoretical Astrophysics (TAT) Eberhard-Karls-Universit¨at T¨ubingen, Auf der Morgenstelle 10, D-72076 T¨ubingen, Germany 28 INFN, Sezione di Firenze, I-50019 Sesto Fiorentino, Italy 29 Universit`a degli Studi di Urbino ‘Carlo Bo’, I-61029 Urbino, Italy 30 INFN, sezione di Padova, via Marzolo 8, 35131 Padova, Italy 31 University of Minnesota, Minneapolis, MN 55455, USA 32 Friedrich–Schiller–Universit¨at Jena PF, D-07737 Jena, Germany 33 Max Planck Institute for Gravitational Physics (Albert Einstein Institute)Am M¨uhlenberg 1, D-14476 Potsdam, Germany 34 INFN, Sezione di Roma 1, I-00185 Roma, Italy 35 Department of Physics and Astronomy, Northwestern University, Evanston,IL 60208, USA 36 NSF Astronomy and Astrophysics Postdoctoral Fellow 37 LAPP-IN2P3/CNRS, Universit´e de Savoie, F-74941 Annecy-le-Vieux, France 38 The Pennsylvania State University, University Park, PA 16802, USA 39 Caltech–CaRT, Pasadena, CA 91125, USA 40 Universit`a di Pisa, I-56127 Pisa, Italy 41 Universit`a ‘La Sapienza’, I-00185 Roma, Italy 42 INFN, Sezione di Roma Tre and Universit`a di Roma Tre, Dipartimento diFisica, I-00146 Roma, Italy 43 Universit`a degli Studi di Firenze, I-50121, Firenze, Italy 44 Departament de Fisica, Universitat de les Illes Balears, Cra. ValldemossaKm. 7.5, E-07122 Palma de Mallorca, Spain 45 Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands 46 Institut de Ci`encies de l’Espai (CSIC-IEEC), Campus UAB, Torre C-5,parells, 2 na planta, ES-08193, Bellaterra, Barcelona, Spain Abstract.  Advanced gravitational wave detectors, currently under construc-tion, are expected to directly observe gravitational wave signals of astrophysicalsrcin. The Einstein Telescope, a third-generation gravitational wave detector,has been proposed in order to fully open up the emerging field of gravitationalwave astronomy. In this article we describe sensitivity models for the Einstein  Sensitivity Studies for Third-Generation Gravitational Wave Observatories   3 Telescope and investigate potential limits imposed by fundamental noise sources.A special focus is set on evaluating the frequency band below 10Hz where a com-plex mixture of seismic, gravity gradient, suspension thermal and radiation pres-sure noise dominates. We develop the most accurate sensitivity model, referredto as ET-D, for a third-generation detector so far, including the most relevantfundamental noise contributions.PACS numbers: 04.80.Nn, 95.75.Kk 1. Introduction The currently operating Gravitational Wave (GW) detectors LIGO [1], Virgo [2], GEO600 [3] and TAMA [4] are based on extremely sensitive Michelson interferometers. While the sensitivity achieved by these first generation detectors is mainly limitedby shot noise, mirror thermal noise and seismic noise, for the second generationof instruments, such as Advanced LIGO [5], Advanced Virgo [6], GEO-HF [7] and LCGT [8], additional fundamental noise sources will start to play a role towards thelow-frequency end of the detection band: Thermal noise of the test mass suspension,photon radiation pressure noise and seismically driven gravity gradients acting onthe test masses. These three sources of noise will become even more important forthird-generation GW observatories such as the Einstein Telescope (ET) [9], [10], as these detectors aim to significantly increase the detection band towards frequenciesas low as a few Hz [11], [13]. Therefore, major parts of the ET design are driven by exactly these noise sources. An overview of the importance of the sub-10Hz band forastrophysical and cosmological analyses can be found in [9].In this article we will give an overview of the currently ongoing ET designactivities with a special focus on the modelling of the achievable sensitivity takingthe most important fundamental noise sources into account. The first sensitivityestimate for a third-generation interferometer was described in [11], [12] and was based on a single interferometer covering the full frequency range from about 1Hz to10kHz. In the following we will refer to this sensitivity curve as  ET-B  . Subsequentlywe developed a more realistic design, taking cross-compatibility aspects of the variousinvolved technologies into account. This led to the so-called xylophone-design, inwhich one GW detector is composed of two individual interferometers: A low-power,cryogenic low-frequency interferometer and a high-power, room-temperature high-frequency interferometer. A detailed description of this xylophone detector sensitivity,in the following referred to as  ET-C  , can be found in [13]. The ET-C configuration willserve as a starting point for the investigations described in this article. We improvedthe sensitivity models for ET by including additional new noise sources as well asby amending and updating noise contributions already previously included. Theseimprovements, which led to a new sensitivity estimate, referred to as  ET-D  , will bepresented and discussed in this article.In section 2 we discuss seismic and gravity gradient noise, followed by the quantumnoise contribution in section 3. Thermal noise of the suspensions and test masses willbe presented in section 4. An improved noise budget for the Einstein Telescope isthen given in section 5. We conclude with a brief overview of the configuration of afull third-generation observatory, consisting of several GW detectors.  Sensitivity Studies for Third-Generation Gravitational Wave Observatories   4 10 −1 10 0 10 1 10 −12 10 −11 10 −10 10 −9 10 −8 10 −7 10 −6 Frequency [Hz]    S  e   i  s  m   i  c   d   i  s  p   l  a  c  e  m  e  n   t   [  m   /  s  q  r   t   (   H  z   )   ]   Black Forest (BFO),12.9.201010 −1 10 0 10 1 10 −20 10 −15 10 −10 10 −5 10 0 Frequency [Hz]    T  r  a  n   f  e  r   f  u  n  c   t   i  o  n   Superattenuator,17m, 6 stages10 0 10 1 10 −25 10 −24 10 −23 10 −22 10 −21 Frequency [Hz]    S   t  r  a   i  n   [   1   /  s  q  r   t   (   H  z   )   ]   ET−B total noiseET−C total noiseSeismic noise(50m suspension)Seismic noise(17m suspension +Blackforest seismic) Figure 1.  Seismic noise spectrum from an underground location in the BlackForest, Germany (left hand panel). Transfer function of a superattenuatorconsisting of 6 stages with an overall height of 17m (center panel). The right handpanel shows the resulting seismic noise contribution for the 17m superattenuatorfor the seismic excitation at the Black Forest site (green dashed line). Forcomparison also ET-B and ET-C are plotted. Their seismic noise contribution isbased on the assumption of a generic 5-stage 50m suspension. 2. Seismic Isolation and Gravity Gradient Noise Seismic noise couples into the differential arm length of a GW detector via two mainpaths. First of all, seismic excitation can mechanically couple through the suspensionand seismic isolation systems. Secondly, seismic noise excites density fluctuations inthe environment of the GW detector, which couple via gravitational attraction to thetest mass position. In the following we will refer to these two noise sources as  seismic noise   and  gravity gradient noise  , respectively. The main difference between thesetwo noise sources is that while seismic noise can be reduced by application of complexseismic isolation systems, the only guaranteed way to reduce the gravity gradient noiseis to reduce the initial seismic excitation. ‡  Therefore, third-generation GW detectorsare proposed to be built in quiet underground locations.The seismic noise contribution of the low-frequency interferometer of ET-C wasbased on a seismic excitation of 5 · 10 − 9 m / √  Hz /f  2 (where  f   is the frequency in Hz) anda generic 50m tall seismic isolation system consisting of 5 passive pendulum stages,each of 10m height. A more realistic seismic isolation design, based on the Virgo superattenuator concept [14], [15], has been developed recently [16]. To achieve a lower cut- off frequency the height of the individual pendulum stages of the super attenuator willbe extended to 2m per stage. The overall isolation of the proposed modified superattenuator, consisting of 6 pendulum stages (each stage providing horizontal as wellas vertical isolation) and a total height of 17m, is shown in the center panel of Figure1. Using the seismic excitation level, measured in an underground facility of the BlackForest Observatory (BFO) [17], [18], shown in the left panel of Figure 1, we can derive the expected seismic noise contribution to the ET noise budget. The result is shown inthe right hand panel of Figure 1. Reducing the height of the seismic isolation system ‡  Many promising gravity gradient noise subtraction schemes have been suggested in the literature[19]. However, as none of these schemes has been demonstrated so far, we do not consider them inthis article.  Sensitivity Studies for Third-Generation Gravitational Wave Observatories   5 10 0 10 1 10 −25 10 −24 10 −23 10 −22 10 −21 Frequency [Hz]    S   t  r  a   i  n   [   1   /  s  q  r   t   (   H  z   )   ]   ET−B total noiseET−C total noiseGravity Gradients β =1.2Gravity Gradients β =0.6Gravity Gradients β =0.3Gravity Gradients β =0.15 10 0 10 1 10 2 10 −27 10 −26 10 −25 10 −24 10 −23 10 −22 10 −21 Frequency [Hz]    S   t  r  a   i  n   [   1   /  s  q  r   t   (   H  z   )   ]   ET−C total noise Suspension thermal noise Figure 2.  Left panel: Gravity gradient noise contribution to ET, for various  β values, assuming the BFO spectrum shown in Figure 1 as seismic excitation level.Right panel: Suspension thermal noise of the low frequency interferometer of ETas described in [34] from 50 to 17m increases the cut-off frequency only slightly from about 1.2 to 1.7Hz.Gravity gradient noise has been described in detail [20, 21, 22]. In our simulationswe estimate the power spectral density of the gravity gradient noise contribution as: N  GG ( f  ) 2 = 4  · β  2 · G 2 · ρ 2 r L 2 · f  4  · X  2seis ,  (1)where  G  is the gravitational constant,  ρ r  the density of the rock around the GWdetector,  L  the arm length of the interferometer,  f   the frequency and  X  2seis  the powerspectral density of the ground motion.  β   accounts for the actual coupling transferfunction from seismic excitation to the differential arm length noise and depends forinstance on the wave type of the seismic excitation (e.g. ratio of P and S waves) andsoil characteristics.Within the ET design study we carried out a campaign of measuring the seismicnoise in various underground locations across Europe. These measurements haveindicated that a couple of the test locations show a seismic excitation level similar to orbelow the BFO measurement [23]. Therefore, we assumed the BFO seismic excitationas a conservative estimate of a potential ET site. The left panel of Figure 2 showsthe corresponding gravity gradient noise contribution at the BFO site for different β  . Since the detailed evaluation of a realistic  β   for potential ET sites is an ongoingactivity, we will use  β   = 0 . 58, as given in the literature [20, 21], in the following for the ET-D sensitivity. Please note that our models do not take atmospheric newtoniannoise into account. 3. Shaping of Quantum Noise Quantum noise, composed of photon shot noise at high-frequencies and photonradiation pressure noise at low frequencies, contributes significantly to the overallsensitivity of ET’s high frequency and low-frequency detectors. The high-frequencyinterferometers will feature a light power stored in the arm cavities of about 3MW toreduce shot noise, while the low-frequency interferometers make use of only 18kW of light power in the arms, in order to reduce the radiation pressure noise. For ET-C the
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