HAYSTAC

HAYSTAC, the Haloscope At Yale Sensitive To Axions, is a physics experiment hosted at Yale University that aims to detect dark matter axions, a leading hypothetical particle in particle physics and cosmology. The experiment uses the haloscope method, in which axions passing through a strong magnetic field may convert into photons, producing a detectable signal in a resonant microwave cavity.

The setup centers on a tunable microwave cavity placed inside a high magnetic field. If axions constitute a component of dark matter and have masses corresponding to the cavity’s resonance frequency, they could convert to photons at that frequency and lead to an excess of microwave power when the cavity is tuned into resonance. Scanning across frequencies allows the experiment to search a range of possible axion masses, typically in the microwave regime corresponding to tens of microelectronvolts.

To maximize sensitivity, the cavity and its detectors are cooled to millikelvin temperatures using a dilution refrigerator. The readout chain employs low-noise amplifiers, such as near-quantum-limited Josephson parametric amplifiers or SQUID-based devices, to approach the quantum limit of noise and improve the signal-to-noise ratio for potential axion signals.

HAYSTAC began taking data in the late 2010s and has produced results that place constraints on the axion-photon coupling in portions of the targeted mass range. The experiment has pursued upgrades (including Phase II efforts) to extend coverage to higher frequencies and to enhance overall sensitivity toward the parameter space expected for the QCD axion.

As part of the broader axion search program, HAYSTAC complements other haloscope efforts worldwide and contributes to the broader goal of identifying the particle nature of dark matter.