Results from phase 1 of the HAYSTAC microwave cavity axion experiment (1803.03690v1)

Abstract: We report on the results from a search for dark matter axions with the HAYSTAC experiment using a microwave cavity detector at frequencies between 5.6-5.8$\, \rm Ghz$. We exclude axion models with two photon coupling $g_{a\gamma\gamma}\,\gtrsim\,2\times10^{-14}\,\rm GeV^{-1}$, a factor of 2.7 above the benchmark KSVZ model over the mass range 23.15$\,<\,$$m_a \,$<$\,$24.0$\,\mu\rm eV$. This doubles the range reported in our previous paper. We achieve a near-quantum-limited sensitivity by operating at a temperature $T<h\nu/2k_B$ and incorporating a Josephson parametric amplifier (JPA), with improvements in the cooling of the cavity further reducing the experiment's system noise temperature to only twice the Standard Quantum Limit at its operational frequency, an order of magnitude better than any other dark matter microwave cavity experiment to date. This result concludes the first phase of the HAYSTAC program utilizing a conventional copper cavity and a single JPA.

• The paper demonstrates a near-quantum-limited axion search in the 5.6-5.8 GHz band using a microwave cavity and a Josephson parametric amplifier.
• It achieves a system noise temperature around twice the Standard Quantum Limit and excludes axion models with gₐγγ > 2×10⁻¹⁴ GeV⁻¹.
• Engineering improvements like piezoelectric motor tuning and enhanced thermal links lay the groundwork for more sensitive future dark matter detection.

An Overview of the HAYSTAC Microwave Cavity Axion Experiment Results

The research paper presents the findings from the initial phase of the HAYSTAC (Haloscope At Yale Sensitive To Axion Cold dark matter) experiment, a critical venture in the search for dark matter axions using a microwave cavity detector. This phase focused on detecting axions in the frequency range of 5.6 to 5.8 GHz, corresponding to axion masses between 23.15 and 24.0 $\mu$eV. Despite the longstanding theoretical underpinning of axion physics, identifying these particles empirically has presented substantial challenges due to their minimal interaction with conventional matter and low mass. The HAYSTAC experiment makes strides in this search by utilizing a high-quality microwave cavity and a Josephson parametric amplifier (JPA) to achieve near-quantum-limited sensitivity, a significant milestone in dark matter detection efforts.

The experiment addresses the imperatives of resonant axion-photon conversion, critical for leveraging observable signals from hypothetical axionic interactions. The constructed apparatus incorporates a strong magnetic field to enhance this conversion within a highly controlled copper cavity environment. Operating at an exceptionally low temperature ($T<h\nu/2k_B$) further ensures the reduction of thermal noise, thereby optimizing detection sensitivity.

A significant technical achievement in this phase was the reduction of the system noise temperature to about twice the Standard Quantum Limit (SQL), an enhancement made possible through the cooling advancements and utilization of the JPA. The enhancements made the detection system an order of magnitude more sensitive than any prior dark matter microwave cavity experiment. Notably, the experiment excluded axion models with two-photon coupling $g_{a\gamma\gamma}$ greater than approximately $2 \times 10^{-14}$ GeV${^{-1}}$, which is 2.7 times higher than the benchmark KSVZ model, thereby extending the detectable mass range.

Several notable engineering improvements were made between data collection runs, addressing challenges observed in the initial configurations. These enhancements included the implementation of a piezoelectric motor tuning system, allowing for precise frequency tuning and therefore more robust analysis of potential axion signals. Moreover, an increase in the thermal link efficiency reduced the noise associated with the cavity to improve overall experimental sensitivity reliably.

The results confirm that this method of axion detection continues to hold tangible promise, particularly within higher-mass parameter spaces even as experiments reach increasingly sophisticated levels of operational precision. Despite its limited volume, the ingenuity of the HAYSTAC setup is indicative of the substantial progress in detecting extremely weak signals close to the quantum noise limit.

The implications for future work in the field include an ambition to incorporate a squeezed-vacuum state receiver, potentially offering enhanced sensitivity and expediting the search effort. The technical innovations and empirical outcomes of the first phase serve as a foundation for the subsequent phases, likely facilitating expanded searches for dark matter axions across diverse theoretical models. These findings underscore the vital role of continued experimental adaptation in the pursuit of understanding and discovering dark matter constituents.