THE GRAVITATIONAL WAVE ANTENNAS EXPLORER AND NAUTILUS: A PROGRESS REPORT

Presented by Massimo BASSAN for the ROG Collaboration •

We report the most recent results obtained wi�h the two cryogenic gravitational wave (g. w.) detectors of the Rome group. A new cross correlation measurement of stochastic background is described, as well as a test of Newton inverse square law at laboratory distances. We then give a preview of sensitivity enhancement expected with the new setup of Explorer and with possible, future upgrades of Nautilus

1 Introduction

Two years have elapsed since the last time gravitational wave (g.w.) detectors were discussed at a Moriond meeting 1 . The expe�imental search for g. w. is traditionally a slowly but steadily progressing field , so this is a good opportunity to summarize the advances that we have achieved with our resonant detectors, both in the field of physics measurements and in that of technological improvements. A comprehensive review of the status and perspectives of the entire network of five resonant antennas active in the world can be found in M.Cerdonio' s paper 2 : therefore we can . focus here on the advances of the two cryogenic antennas of "the Rome group: Explorer, located in CERN and the ultra.cryogenic one Nautilus, located in the Frascati National Labs. of INFN and operating at the record temperature of 0 . 1 K. Although we shall not mention it here, an R&D program is also pursued, within the ROG group, for the so called fourth generation antennas, i .e. multimodal detectors of large mass (lumped or distributed) that represent the medium term future of this field . The development of a prototype, a TIGA resonator of 80 cm in diameter, is currently underway in the LNF lab for studies on suspension, cooling and multimodal transducer systems.

We recall that antenna sensitivity is expressed , following the latest community standard, by means of a function Sh (/) describing the minimum energy spectrum detectable by the antenna

"the ROG Collaboration is at present composed of the following scientists and institutions:

P.Astone, C.Cosmelli, S.Frasca, G. V.Pallottino Uni�er•ita La Sapienza and INFN Sezione Roma

M.Bassan, E.Coccia, Y.Minenkov, I .Modena, A.Moleti, G.Pizzella, Univer•ita Tor Vergata and INFN Sezione Roma 2

P.Bonifazi, R.Terenzi , M.Visco l•tituto Fi•ica Spazio lnterplanetario de/ CNR

P.Carelli Univer•itii dell 'Aquila and INFN

V.Fafone, A.Marini, E.Mauceli, G.Modestino, M.A.Papa, F.Ronga, L.Votano INFN - Laboratori Nazionali di Fra•cati

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Figure l : Upper limit on Yukawa-like dev1at1on from Newton's law at the 1-10 meter length scale, as measured with the Explorer detector and a spinning rotor gravitational excitation. The experimental error {at 2-<T level)

on the parameter a is alsr: shown

at each frequency (or its amplitude analogous, tne spectral g .w. amplitude h(f) = JSh(J) ) . Assuming Sh is non zero only over a finite band 6.f, we can relate it to other sensitivity figures of merit:

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Sh (/o) 2v: 6.Emin :: kB · T,1 f = -;s:j tr MaL2

where Ma , v., L are the antenna mass, speed of sound and length and r9 is the pulse duration. 2 Explorer

The cryogenic detector Explorer has reliably been taking data for the last several years , with a very high duty cycle: it stays off the air one day per fortnight for cryogenic operations (refi ll of Hquid Helium and Nitrogen) and two weeks at year's end for maintenance of CERN utilities. Data from Explorer, gathered with a sampling time of 4.5 ms, are recorded and filtered to produce an effective noise temperature for burst detection between 10 and 20 mK, depending on some details of the experimental set-up. We have recently completed the analysis of a careful calibration experiment 3 by means of an a.c. gravitational field: we used, as field generator, a mass quadrupole rotating in the range of 460 Hz and we detected the acceleration field produced on either normal mode of vibration of the antenna. As the antenna response to induction (or near field excitation) scales as r-2 with distance, an accurate measurement of the antenna signal vs distance has provided an upper limit on possible deviations from Newton's law of gravitation at distances between 1 and 6 meters. The results of this measurement are shown, in fig. 1 , as a plot in the usual (a, A) parameter plane; they can be compared with other results presented at this conference.

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Figure 2: Noise temperature of Nautilus as a function of time: the noisy periods (less than three hours per day ) correspond to daily refills of the refrigerator lK pot: besides these the antenna sensitivity remains below 5 mK

Explorer has been taken off the air last December, and is undergoing in these days the first major · upgrade in its life: we are carrying out a thorough revision of the suspension system and installing a more sensitive readout. This new readout consists of a transducer of the new -"rosette" design 4 , assembled with a gap of lOµm (five times narrower, and therefore five times more sensitive, than previous devices) and the new, ultrasensitive d.c. SQUID preampiifief developed at IESS-CNR, that has an energy sensitivity ( i .e . , minimum measurable energy per unit bandwidth , conventionally measured in units of Planck's constant h) lower than lOh . Explorer will resume operation before the summer, and with these upgrades it should perform with a noise temperature of 2-3 mK, with an improved strain sensitivity as discussed in sect. 5.

3 Nautilus

The ultracryogenic detector Nautilus had operated in the past at a sensitivity similar to that of Explorer, despite its lower thermal noise (it operates at a temperature of 100 mK): this was due to excessive SQUID noise and to some mechanical noise reaching the antenna, probably due to non linear processes in the suspensions. Nautilus has undergone, in the first half of 1998, a complete overhaul of its vibration isotation and of the SQUID wiring: this operation has struck on most of the mechanisms that were responsible for the non stationary background noise that had affected the antenna in the past. Nautilus has been producing, since J uly 1998 a consistent stream of data with duty cycle interrupted only for cryogenic operations (see fig. 2) -and a noise temperature varying between 2 and 5 mK. Nautilus is equipped with a veto system for cosmic ray showers, consisting of a telescope of counters located above and below the cryostat. An analysis now being completed shows, with a statistical significance exceeding lOcr" that, with this advanced sensitivity, some of the antenna "events" are· correlated with extensive air showers hitting the antenna. This finding underlines the need of coincidence experiments to reject spurious events, as well as the usefulness of a cosmic ray veto apparatus for antennas with noise temperature below 5 mK.

4 Explorer and Nautilus

Stochastic g.w background has raised growing interest in recent years 6• At the last Moriond meeting we presented a direct measurement of an upper limit to the g.w. energy spectrum at 915 Hz 7 , derived from the noise spectra of either detector . That measurement, that mapped into

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Figure 3: .Spectra of the Explorer and Nautilus antennas. In order to make a cross spectrum analysis , the Nautilus setup has been modified so that one of the resonant modes coincides with one of Nautilus: the two spectra overlap,

in their sensitive region, for about 0. 1 Hz arund 907. 20 Hz

a cosmological closure parameter 09w � 300, induced to smile several experts in the audience, familiar with estimates several orders of magnitude smaller. We have now improved on those measurements by performing a correlation analysis between the noise output of the two detectors. In order to tune the resonances of both detectors to the same frequency, we had to reduce the bias field in the transducer of Nautilus, thus slightly decreasing its sensitivity. The Sh spectra of both antennas are shown in fig. 3. A cross correlation of the two spectra, performed for a period of 12.5 hours and over a bandwidth of 0.lHz centered around 907.20Hz, has allowed us to reduce the cross spectrum well below the 10-43 H z- 1 level. Unfortunately the two antennas are farther away than a fraction of the g.w. wavelength (>..9w/2rr , roughly 50 km at 1 kHz) and the crosscorrelation decreases due to the phase shift between the waves acting on the two detectors. Therefore we must take int.o account a reduction (by a factor 6) in the efficiency of detection : when this correction is folded in the measurement , the resulting new experimental upper limit is 119.w � 60. Although still far from values of cosmological relevance, this measurement shows the power and significance of correlation experiments among different detectors. Also completed in the last few months is a search for simultaneous impulsive excitations in pairs of detectors 9 : the output data from Explore� were correlated with those of both Niobe (at UWA, in Perth) and Nautilus (in different periods) . The analysis 'carries a null result, despite a small (statistically not significant ) excess of coincidences at zero time delay between Explorer anct Nautilus. S The future of Explorer and Nautilus

As we mentioned earlier, the future for Explorer has already begun : should the newly equipped detector, that is being cooled as we write, behave as expected , we shall soon have a burst sensitivity in the 3 mK range, and a strain spectral amplitude h as shown in fig. 4 (a) . The new SQUID has an intrinsic noise well below 0 . 1µ¢>0/v'J[Z, but its performance inside a complex apparatus is likely to be somehow lower, due to the resonant input circuit and long service cables . Therefore we show possible spectral sensitivities (and bandwidths) depending on the amplifier noise we will achieve. If the upgrade of Explorer proves successful , we can foresee similar changes also for Nautilus: as these would not take place before the next century, it is likely that we shall be able to use a third generation , further improved transducer, now in the testing stage. This device is based on a double electrode (push-pull) design that, beside doubling the coupling with respect to present ones, can be connected to the SQUID in a fully floating

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Figure 4: Expected performance of Explorer (left) and. Nautilus detectors with an improved read-out. Explorer and Nautilus have almost identical apparata: the different sensitivity is due to the lower temperature of Nautilus

and to assuming the use of a transducer with still higher performance.

(no grounded points') configuration, that should improve shielding from e .m. disturbances. I n fig . 4 (b) we show sensitivity forecasts for Nautilus: the sensitivity is predicted to be so much better than that of Explorer because of this new transducer and of the lower temperature. Such a detector will approach a strain sensitivity h ::; 1 · 10-22 over a bandwidth exceeding 15 Hz. This is close to the expected spectral sensitivity of upcoming interferometers1°, although , obviously, on a much smaller (but no longer negligible) bandwidth. Fig. 4 shows that the spectral amplitude sensitivity is best at the normal mode frequencies for large (with respect to resonant noise) values of the wide-band SQUID noise: then, as the latter is decreased , the best sensitivity peaks in between the modes, as first shown in 1 1 . It is interesting to verify that the values of h(J± ) at the resonances are independent of the level of amplifier noise12 •

References

1 . M.Bassan in Proceedings of the XXXI/nd Rencontre de Moriond " Very High Energy Phe-nomena in the Universe " Edition Frontieres 1997, pag. 297-312 .

2 . M.Cerdonio These proceedings

3 . P.Astone et al, Eur. Phys. J . C 5, 651-664 ( 1998) . 4 . M .Bassan , Y.Minenkov, G.Zaccarian in Proc. of the jiT"st Edoardo A maldi Conference

on Gravitational Wave Experiments, Frascati 1994 E.Coccia, G.Pizzella, F .Ronga editors, World Scientific 1995

5 . P. Carelli, M.G. Castellano, R. Leoni and G. Torrioli: Appl. Phys. Let t . 72, 1 15 ( 1998) 6. R. Brustein, M. Gasperini, M. Giovannini, G. Veneziano: Phys, Lett . B 361 , 4.S ( 199.j) 7. P.Astone et al Phys. Lett. B .385, 421 ( 1996) 8. P.Astone e t al Astronomy and Astrophysics , 1 999 ( in press)

9. P.Astone et al Astroparticle Physics 10 , 83 ( 1 999) 10. See these Proceedings or, e.g. Proc. of the International Conference on Gra vitational

Waves - Sources and Detectors, l .Ciufolini and F .Fidecaro editors, World Scientific 1997 1 1 . P.F .Michelson and R.C.Taber Journal of Appl. Phys. 52, 4313 ( 1981 ) 1 2 . P.Astone,G.V.Pallottino, G.Pizzella Classical and Quantum Gravity 14 , 20 19 ( 1997)

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