Sonic search for monopoles, gravitational waves and newtorites

Abstract

Hopes for the detection of gravitational waves and of very massive magnetic monopoles rely on cosmological sources - the big bang itself in the monopole's case. Some gravitational wave antennas are “acoustic” resonant detectors, that have reached a sensitivity not far from the “quantum limit”. We investigate in detail the response of these sensitive detectors to the passage of “relic” monopoles. We compute signal to noise ratios for a variety of target materials, and we find them to be favourable for the very cold, high quality resonators that are presently contemplated. A monopole traversing a metal produces a “thermo-acoustic” pulse, whose amplitude is linear in the monopole's velocity, β. If the metal is superconducting, there is a novel additional “magneto-acoustic” source, whose amplitude is β-independent. Monopole detectors that rely on ionization have a sensitivity threshold in β, and may conceivably be blind to relic monopoles. The response of superconducting loop detectors is β-independent, but their collection areas are limited by the requirement of a sophisticated magnetic shielding. Neither of the above limitations would be shared by an acoustic monopole detector. We sketch a “sonic antenna” that would respond directionally to coventional cosmic rays, gravitational radiation, monopoles, and even to more exotic signals, like newtorites (elementary or composite “meteorites” that interact with ordinary matter only gravitationally).