Dark Matter Axion Search with HAYSTAC Phase II

This Letter reports new results from the HAYSTAC experiment's search for dark matter axions in our galactic halo. It represents the widest search to date that utilizes squeezing to realize subquantum limited noise. The new results cover 1.71  μeV of newly scanned parameter space in the mass ranges 17.28-18.44  μeV and 18.71-19.46  μeV. No statistically significant evidence of an axion signal was observed, excluding couplings |g_{γ}|≥2.75×|g_{γ}^{KSVZ}| and |g_{γ}|≥2.96×|g_{γ}^{KSVZ}| at the 90% confidence level over the respective region. By combining this data with previously published results using HAYSTAC's squeezed state receiver, a total of 2.27  μeV of parameter space has now been scanned between 16.96-19.46  μeVμ  eV, excluding |g_{γ}|≥2.86×|g_{γ}^{KSVZ}| at the 90% confidence level. These results demonstrate the squeezed state receiver's ability to probe axion models over a significant mass range while achieving a scan rate enhancement relative to a quantum-limited experiment.