Title
The Invisible Drummer: Optomechanical Accelerometry for an Ultralight Dark Matter Search
Abstract
The nature of dark matter is an enduring scientific mystery. Compelling astrophysical evidence suggests ~85% of the Universe’s matter content is dark. Yet, no direct detection has been made despite decades of searching for leading candidates such as WIMPs, axions, and sterile neutrinos. Ultralight dark matter (UDM), which behave like coherent, classical fields, are a class of viable candidates. UDM could produce continuous, narrow-band accelerations on ordinary matter, representing a natural target for accelerometers and torsion balances. Searching for UDM, however, is a challenging proposition: the signal is probably very weak, and its frequency is unknown.
In this dissertation, we develop a membrane-based cavity optomechanical accelerometry platform and apply it as an UDM detector. Nanomechanical membranes with ultralow dissipation are used as acceleration test masses to resonantly amplify signals via extremely high mechanical quality factors. Two silicon nitride membranes, vertically integrated from the same chip, form an optical cavity, enabling a panoply of metrological tools developed in the field of quantum optomechanics: quantum-limited displacement readout, high detection bandwidth, calibration traceable to fundamental constants, and radiation pressure feedback-enhanced sensing.
Cryogenic operation and long signal averaging times are essential to achieve the requisite sensitivities set by UDM searches using state-of-the-art torsion balances (the Eöt-Wash experiment). To this end, we perform a proof-of-principle UDM search in a 4 K closed-cycle cryostat. Vibrations from cryostat operation are addressed using custom vibration isolation, resulting in continuous measurements at the thermal noise limit at 4 K base temperature. A combination of photothermal frequency scanning and quantum-limited readout allows a resonant-bandwidth of several hundred UDM linewidths. A Bayesian search based on matched-filter statistics is employed to search for UDM signals. No signal is observed, and an upper bound for UDM coupling is obtained.
While not currently competitive with leading constraints, our experiments demonstrate stable cryogenic, quantum-limited operation as a first step in a staged approach to resonant UDM detection. With optimized test masses, millikelvin temperatures, longer integration times, and quantum-enhanced, multiplexed arrays, our platform offers a path towards competitive searches.
Please email Jini at jini@optics.arizona.edu or Mitul at mituldc@arizona.edu for a Zoom link.
