Projected Sensitivity of DMRadio-m${}^3$: A Search for the QCD Axion Below $1\mu$eV

The QCD axion is one of the most compelling candidates to explain the dark matter abundance of the universe. With its extremely small mass ($\ll 1,$\mathrm{e}\mathrm{V}$/c^2$), axion dark matter interacts as a classical field rather than a particle. Its coupling to photons leads to a modification of Maxwell’s equations that can be measured with extremely sensitive readout circuits. DMRadio-m${}^3$ is a next-generation search for axion dark matter below $1\mu$eV using a $>4$ T static magnetic field, a coaxial inductive pickup, a tunable LC resonator, and a DC-SQUID readout. It is designed to search for QCD axion dark matter over the range $20,$\mathrm{n}\mathrm{e}\mathrm{V}$\lesssim m_ac^2\lesssim 800,$\mathrm{n}\mathrm{e}\mathrm{V}$$($5,$\mathrm{M}\mathrm{H}\mathrm{z}$<\nu<200,$\mathrm{M}\mathrm{H}\mathrm{z}$$). The primary science goal aims to achieve DFSZ sensitivity above $m_a{c}^2\approx 120$ neV (30 MHz), with a secondary science goal of probing KSVZ axions down to $m_a{c}^2\approx 40\mathrm{n}\mathrm{e}\mathrm{V}$ (10 MHz).