Probing KSVZ Axion Dark Matter near 5.9 GHz Using a 8-Cell Cavity Haloscope

We report on a search for axion dark matter in the frequency range near 5.9 GHz, conducted using the haloscope technique. The experiment employed an 8-cell microwave resonator designed to extend the accessible frequency range by a multi-fold factor relative to conventional single-cell configurations, while maintaining a large detection volume. To enhance sensitivity, a flux-driven Josephson parametric amplifier (JPA) operating near the quantum noise limit was utilized, together with a sideband-summing method that coherently combines mirrored spectral components generated by the JPA. Data were acquired over the frequency range 5.83-5.94 GHz. With no statistically significant excess observed, we exclude axion-photon couplings $g_{a\gamma\gamma}$ down to $1.2 \times 10^{-14}$ GeV$^{-1}$ at a 90% confidence level. The achieved sensitivity approaches the KSVZ benchmark prediction, setting the most stringent limits to date in this range.