Listening to the Sound of Dark Matter with Superfluid Helium

Recently, we studied a prototype gravitational wave detector based on high-Q acoustic modes of superfluid ⁴He inside a centimeter-scale cross-shaped cavity.¹ A sensitive parametric transducer allowed for the observation of thermally excited modes at millikelvin temperatures, while the kHz mechanical frequencies could be tuned through pressurizing the helium. Now, we are developing a new experiment aimed at proving that the high-Q acoustic modes of superfluid helium can also facilitate an efficient search for ultralight scalar dark matter (DM) candidates, when combined with sensitive optomechanical readout. Such particles with a mass < 0.1 eV would appear as a classical field periodically modulating the Bohr radius and thus continuously causing a uniform harmonic stress on a resonant-mass antenna.² Featuring an improved re-entrant microwave optomechanical transducer and a significantly larger helium mass in a cylindrical cavity, this prototype DM detector could beat current constraints on scalar DM candidates after a few hours of integration time.<sup>3

1</sup>Physical Review D 104, 082001

²Physical Review Letters 116, 031102

³Physical Review Letters 124, 151301