Response of VIRGO detectors to pre-big-bang gravitons

TL;DR

This paper evaluates VIRGO detectors' sensitivity to pre-big-bang gravitational waves, introducing a semi-analytical method to compute signal-to-noise ratios for various energy spectra, and explores noise reduction strategies to enhance detection prospects.

Contribution

It presents a new semi-analytical technique for assessing VIRGO detectors' sensitivity to relic gravitons from pre-big-bang models, including non-minimal scenarios with long dilaton phases.

Findings

Thermal noise reduction significantly improves sensitivity.

Shot noise reduction has limited impact on sensitivity.

Non-minimal models with dilaton phases affect detection prospects.

Abstract

The sensitivity achievable by a pair of VIRGO detectors to stochastic and isotropic gravitational wave backgrounds produced in pre-big-bang models is discussed in view of the development of a second VIRGO interferometer. We describe a semi-analytical technique allowing to compute the signal-to-noise ratio for (monotonic or non-monotonic) logarithmic energy spectra of relic gravitons of arbitrary slope. We apply our results to the case of two correlated and coaligned VIRGO detectors and we compute their achievable sensitivities. We perform our calculations both for the usual case of minimal string cosmological scenario and in the case of a non-minimal scenario where a long dilaton dominated phase is present prior to the onset of the ordinary radiation dominated phase. In this framework, we investigate possible improvements of the achievable sensitivities by selective reduction of the thermal contributions (pendulum and pendulum's internal modes) to the noise power spectra of the detectors. Since a reduction of the shot noise does not increase significantly the expected sensitivity of a VIRGO pair (in spite of the relative spatial location of the two detectors) our findings support the experimental efforts directed towards a substantial reduction of thermal noise.