Probing low-mass dark matter from sub-MeV to sub-GeV with germanium-based quantum phononic spectroscopy

Abstract

We present a germanium phonon-to-charge transducer that integrates a slow-phonon phononic-crystal (PnC) region with radio-frequency quantum point-contact (RF-QPC) readout at 4 K, and we evaluate its dark-sector reach. A calibrated signal-collection model, which combines geometric guiding, propagation survival, and multiplicity-assisted primary-phonon detection, provides selection-corrected thresholds in the $10^{-3}$-$10^{-2}$~eV range and a background model informed by nanosecond timing gates and GHz-band power-spectral-density windows. Under standard halo assumptions, a 100 g module achieves projected sensitivity to DM-electron and DM-nucleon scattering at recoil energies below $10^{-2}$~eV, probing cross sections below $10^{-43}{\mathrm{cm}}^2$ for $m_{\ddot{I}}‡\in [0.01,100]\mathrm{MeV}/c^2$ (with efficiencies and thresholds folded in). We present sensitivities for both heavy-mediator ($F_{\mathrm{DM}}=1$) and light-mediator ($F_{\mathrm{DM}}\propto 1/q^2$) benchmarks, quantify dominant systematics (threshold, phonon quality factor $Q$, and bulk defect densities), and outline a staged program toward kg$\cdot$yr exposures that begins to test models approaching the solar CE$\hat{I}½$NS background.

19 pages, 12 figures