Search for post-inflationary QCD axions with a quantum-limited tunable microwave receiver

A search for cosmological axions has been performed by scanning a frequency region of $38\,$MHz centered at about $10.2\,$GHz, corresponding to an axion mass $m_a \simeq 42\,\mu$eV. The QUAX experimental apparatus, a haloscope comprised of a 1-liter volume tunable cavity immersed in an $8\,$T magnetic field and a quantum-limited detection chain, set limits on the axion-photon coupling at the $10^{-14}\,$GeV$^{-1}$ level. As no signal candidate has been observed, viable hadronic axion models are ruled out in a currently preferred post-inflationary region $m_a > 40\,\mu$eV.