New results from HAYSTAC’s phase II operation with a squeezed state receiver

Abstract

A search for dark matter axions with masses $>10\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}/{\mathrm{c}}^{2}$ has been performed using the HAYSTAC experiment's squeezed state receiver to achieve subquantum limited noise. This work includes details of the design and operation of the experiment previously used to search for axions in the mass ranges 16.96--17.12 and $17.14--17.28\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}/{\mathrm{c}}^{2}$ (4.100--4.140 GHz and 4.145--4.178 GHz) as well as upgrades to facilitate an extended search at higher masses. These upgrades include improvements to the data acquisition routine which have reduced the effective dead time by a factor of 5, allowing for the new region to be scanned $\ensuremath{\sim}1.6$ times faster with comparable sensitivity. No statistically significant evidence of an axion signal is found in the range $18.44--18.71\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{eV}/{\mathrm{c}}^{2}$ (4.459--4.523 GHz), leading to an aggregate upper limit exclusion at the 90% level on the axion-photon coupling of $2.06\ifmmode\times\else\texttimes\fi{}{g}_{\ensuremath{\gamma}}^{\mathrm{KSVZ}}$.