A Near-Cutoff Waveguide Haloscope for sub-meV Dark Matter

We propose a near-cutoff parallel-plate waveguide haloscope for sub-meV dark matter. The concept retains the large-area openness of a dish antenna while providing cavity-like field enhancement through slow-wave response and coherent accumulation, without relying on a closed standing-wave resonance. For a copper waveguide, the projected dark photon sensitivity reaches $\epsilon \simeq 2.1\times {10}^{-15}$ near $m_{A^{\mathrm{\prime }}}\simeq 0.1\mathrm{m}\mathrm{e}\mathrm{V}$. With an external magnetic field, the same transducer can approach QCD axion parameter space. The waveguide haloscope highlights a sensitive and scalable route toward future sub-meV bosonic dark matter searches.