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Various astrophysical and cosmological observations point to the existence of an unknown type of matter in the universe called “dark matter”; a type of matter that has so far eluded conventional detection techniques. For a well-motivated range of “ultralight dark matter” (UDM) candidates of peV-scale particle masses, superfluid helium represents an ideal, low-noise acoustic resonator that can provide unprecedented sensitivity to UDM, notably outperforming long-running experiments (e.g., Eot-Wash and LIGO-Virgo) after just an hour of operation. A novel detector concept using superfluid helium

as a mechanical resonator will be presented.  The current “Superfluid Helium Ultralight Dark Matter Detector” (HeLIOS) prototype relies on helium’s high-Q mechanical resonance to achieve world-leading sensitivity over a needle-thin (sweepable) range of frequencies.  Plans to create a pressure transducer capable of achieving broadband sensitivity will be discussed. The broadband readout capability will make it possible to cover previously inaccessible masses while turning years-long frequency sweeps into hours-long single acquisitions that are also sensitive to transient signals.  Moreover, multimode interference in this system leads to narrowband regions of insensitivity that can be pressure-swept for in-situ signal validation. The long-term goal is to develop a geographically distributed network of these new detectors operating as a global observatory to reject local noise sources and provide new sensitivity to time variability and directionality of a light dark matter signal.