2. SIGRA6 - Villa Mondragone 11 Sept.2002 BREAKING THE BANDWIDTH BARRIER IN RESONANT G.W. DETECTORS IN RESONANT G.W. DETECTORS MASSIMO BASSAN Università di Roma Tor Vergata and INFN - Sezione Roma2 For the ROG Collaboration or Recipes for a broadband and sensitive antenna

3. SIGRAV 15 ABSTRACT BAR DETECTORS: A TUTORIAL AND SOME TECHNICAL TERMS –Crucial components that make an antenna work –Sensitivity: h, S h (f), T eff, f and all that –A historical perspective BANDWIDTH: WHERE WE STAND and what we can expect –Status of the existing detectors –The two antennas of the ROG group –Present sensitivity : is it meaningful ? WHAT TO DO NEXT: or –Is there a future for bars in the age of interferometers ? PREEMPTIED

4. SIGRAV 15 ROG A collaboration of: INFN, Univ. Roma1, Univ. Roma2 and CNR EXPLORER (CERN) 2300 kg Al antenna Resonances at 888,919 Hz Cooled to 2.6 K Readout: Capacitive resonant transducer with d.c. SQUID amplifier Operational since 1990 Upgrade 1999 New run since 2000 Cosmic Ray telescope starting 2002 NAUTILUS (LNF) 2300 kg Al antenna Resonances at 906, 922 Hz Cooled to 0.14 K Readout: Capacitive resonant transducer with d.c. SQUID amplifier Operational since1995 New run since 1998 Cosmic Ray Telescope Veto for events due to EAS or hadrons

5. SIGRAV 15 A DICTIONARY OF ANTENNA TERMS The mechanical oscillator Mass M Speed of sound v s Temperature T Quality factor Q Res. frequency f r The transducer Efficiency The amplifier Noise temperature T n L0L0 LiLi Thermal noise S F = MkT r /Q Amplifier noise V n ; I n T n =V n 2 I n 2 /k

6. SIGRAV 15 Minimum detectable energy change A low effective temperature makes the sensitivity higher and the bandwidth larger Bandwidth NOISE TEMPERATURE, WAVE AMPLITUDE AND SPECTRAL SENSITIVITY strain sensitivity

7. SIGRAV 15 BANDWIDTH IN A RESONANT DETECTOR Why are we sensitive only around resonance ? Why can we be sensitive in a region f >>f/Q ?

8. SIGRAV 15 SENSITIVITY AND BANDWIDTH: A Quick History Of Our Mistakes Pre-history (60s), naive approach: focus is on burst detection, bandwidth is not an issue –Sample as fast as you can (i.e. to beat slowly varying thermal noise: can be made small at will : Obviously wrong ! Gibbons & Hawking (PRD 1971): sampling time limited by detector noise

9. SIGRAV 15 Giffard (PRD 1976) introduces back action (the amplifier shakes the antenna). First rigorous, although unpractical, derivation of minimum detectable energy: coupling coefficient -more in a moment back action: amplif. noise, but t

10. SIGRAV 15 Pallottino Pizzella 1981 T/ Q Thermal noise negligible wrt Amplifier >>1 => Amplifier noise dominated by back action As of today, the challenge of meeting these 2 conditions is still open 2 requirements for best sensitivity:

11. SIGRAV 15 1984 : we begin talking bandwidth sensitivity implies and requires bandwidth. There is no trade off : is there a free lunch after all ?

12. SIGRAV 15 So, to increase sensitivity and bandwidth, we need a large What is this energy coupling coefficient ? It is the figure of merit of the antenna transduction system: transduction constant (V/m) circuit impedance resonator mass Need large M to capture g.w. (M cross section) Need small M to efficiently couple to the amplifier => light mass resonant transducer (Paik 74)

13. SIGRAV 15 TWO MODE DETECTOR : A resonant transducer with a mass m=µM allows us to gain a factor µ -1 in. => make a tiny transducer mass : Stanford 1980, m=20 g, µ~10 -5 Badly penalized by thermal noise in the small resonator ! (In modern terms, transducer motion noise grows intolerably outside f )

14. SIGRAV 15 TWO MODE DETECTOR (2) Indeed, the bandwidth is limited by transfer time between the oscillators (beat frequency) to f = f beat = f µ An optimum does exist for m: the value for which f beat = f single mode This limited the useful bandwidth to ~ 1 Hz Is there a way out ? Beats in Explorer -Aug 2002

15. SIGRAV 15 MULTIMODE DETECTORS ? Iterate many (N) times the light mass oscillator trick Then µ= M j /M j+1 can grow up to ~10% (µ = f / f ) and final mass (m = M N ~ 0.1 g) makes very large Would it work ?

16. SIGRAV 15 MULTIMODE DETECTORS (2) Hidden catch : N modes bring N k B T noise in the detection bandwidth ! Multimode detectors have not been pursued in recent years

17. SIGRAV 15 So, we are back to the problem: how to increase in order to improve sensitivity and bandwidth Lets give another look at our energy coupling coefficient : transduction constant (V/m) circuit impedance resonator mass Only surviving handle is. It depends on the density of e.m. field stored in the transducer What is the best transducer for the job ? (touchy question!)

18. SIGRAV 15 TRANSDUCERS A transducer (Trx) works converting a mechanical signal in an electric one, by modulating a stored e.m. field, that can be –Electrostatic => Capacitive devices –Magnetic (usually superconductive) => Inductive Trx –a.c. electromagnetic (r.f through optical) => Parametric Trx

19. SIGRAV 15 A fair (?) comparison of transducers Parametric Trx Best in principle ( >1 ) but beware of pump noise (both amplitude and phase) Inductive Trx Direct coupling to a Squid amplifier High field density Capacitive trx Large active surface, small gap Test @RT, no diff. contractions a.c. coupling cuts off slow (and large !) antenna motion It is a tie. We chose Capacitive because it is convenient.

20. SIGRAV 15 TRANSDUCERS (3) A careful analysis of two mode antennas w/ passive transducers [Bassan, Pizzella 1997] shows that To a good approx. it works also for our 3mode antennas. By writing it out in terms of parameters, we find: – gap We need a small gap device, that holds high field

21. SIGRAV 15 THE SIMPLEST DEVICE: PARALLEL PLATE, D.C. BIASED CAPACITOR e.d.m. machining to carve the rosette Diamond tool machining for flatness tolerance < 5 µm Hand lapping for final finishing Painstaking attention to dust and parallelism in assembling Main credit to dr. Yu F.Minenkov for developing these techniques

22. SIGRAV 15 ROME GROUP TRANSDUCERS OLD MUSHROOM SHAPED NEW ROSETTE SHAPED Resonating disk Pb washers Teflon insulators Gap 10 m Diam. 140 mm 170 mm Resonating diskTeflon insulators Antenna Gap 50 m

23. SIGRAV 15 PRECISION MACHINING: The rosette capacitive transducer; gap=9 m

24. SIGRAV 15 THE LATEST CHALLENGE: A TRANSDUCER FOR MINIGRAIL

25. SIGRAV 15 dc-SQUID Flux quantization + Josephson effect (2 JJ) in a superconducting loop of inductance L Requires nanofabrication processes Yields the world most sensitive magnetometer ( fA, µ o /rt(Hz) ) Now available commercial devices of good performance. Josephson junction Resistors Quantum at work

26. SIGRAV 15 Carelli et al. 98 Experimental flux noise spectral density

27. SIGRAV 15 CORRECT APPROACH TO SENSITIVITY: All our noise sources are white, but some appear colored due to filtering of the resonators If the detector parameters are well known, compute the transfer functions and sum the noise voltages at the antenna output, or better still the noise displacements at the input (where a g.w. has white spectrum) Compute the SNR(f) = const/ S h (f). Plot S h (f) to observe the bandwidth. S h (f) provides info on sensitivity to all kinds of source

28. SIGRAV 15 NOISE PLOTS FOR NAUTILUS 1998+

29. SIGRAV 15 S h (f) per Nautilus 1998+

30. SIGRAV 15 Calibration peak The bandwidth depends mainly on the transducer and amplifier The sensitivity of a detector is usually given in terms of the noise spectral density referred to the input of the antenna To increase the overall sensitivity a larger bandwidth is required. This can be obtained decreasing the amplifier noise contribution and/or by increasing the transducer coupling The peak sensitivity depends on T/MQ

31. SIGRAV 15 ER WIDENING THE BAND IN EXPLORER The readout chain has been changed in 1999. After a tune-up period, EXPLORER has been on the air since May 2000 The noise temperature is very stable, at values < 5 mK for 84% of the time. Bandwidth: the detector has a sensitivity better than 10 -20 Hz -1/2 on a band larger than 40 Hz EXPLORER 1999

32. SIGRAV 15 NOISE PLOTS FOR EXPLORER 2000+

33. SIGRAV 15 July 2001 h = 5 · 10 -19 Calibration peak December 2001 h = 2 · 10 -19 EXPLORER PERFORMANCES

34. SIGRAV 15 Which situation is to be preferred ? However, the blue line has a larger bandwidth, if we use the current definition ! While waiting for a better definition, we define as useful bandwidth the region where S h (f) <10 -40 /Hz 40 Hz

35. SIGRAV 15 THE ROLE OF WIDE-BAND NOISE: This is not science fiction: A SQUID with 0.1 µ o / Hz Carelli et al. Appl. Phys. Lett. 72,115 (1998) 10 -22 / Hz NAUTILUS 2002 ? Tuned to 935 HZ, the frequency of the pulsar in SN1987A 6 10 -23 /Hz

36. SIGRAV 15 12 hours of data Bandwidth =0.1 Hz gw < 6*10 A correlation between Nautilus and Auriga (or Virgo) will lower this limit to gw =1 EXPLORER & NAUTILUS 1997 Crosscorrelation of stochastic g.w. background with two resonant detectors Astr. Astroph 351,1999- Phys. Lett. B, 385, 1996

37. SIGRAV 15 THE FUTURE (in the age of interferometers) There is still ample room for improvements in sensitivity LIGO preliminary data shows IFOs might take longer to operate than expected : bars are still the only sentinels A coincident detection by two totally different instruments will be a stronger evidence Cross correlation IFO-Bar for stoch. bkgnd will be crucial (D < New, upcoming multimode resonators will exploit the technology with a sensitivity boost + omnidirectionality

38. SIGRAV 15 TARGET SENSITIVITY OF EXPLORER EXPLORER can reach a sensitivity of T eff =170 µK h = 1 · 10 -19 New transducer double gap C=20 nF Q = 2 · 10 6 New transformer low dissipation Q e = 10 5 New SQUID n = 0.5 0 /Hz

39. SIGRAV 15 WHAT CAN WE OBSERVE WITH THESE ANTENNAS ? PLEASE STAY TUNED FOR NEXT TALK (AFTER COFFEE)