Extended Haloscope Search and Candidate Validation near 1.036G Hz

TL;DR

This paper reports a follow-up axion haloscope search near 1.036 GHz, including candidate validation and setting improved upper limits on axion-photon coupling, emphasizing the importance of validation strategies in sensitive searches.

Contribution

It extends previous axion search efforts with improved validation and sensitivity, and provides new upper limits on axion-photon coupling in the 1.026-1.045 GHz range.

Findings

Candidate excess was not confirmed as axion signal.

Extended search with quantum-noise-limited amplifier achieved high sensitivity.

Set new upper limits on axion-photon coupling in the targeted frequency range.

Abstract

We report a follow-up axion haloscope search near 1.036 GHz that completes and extends our previous work [Phys. Rev. X 14, 031023 (2024)], in which a portion of the HEMT-based data could not be analyzed due to unrecorded experimental information. While recovering this dataset, we identified an excess near 1.036 GHz that satisfied our candidate-selection criteria, motivating dedicated validation studies, including independent cross-checks and re-examination with the original apparatus. The excess did not persist under these investigations and was not confirmed as an axion dark-matter signal. We subsequently extended the search over a 20-MHz band surrounding the candidate using a quantum-noise-limited amplifier, achieving sensitivity close to the Dine-Fischler-Srednicki-Zhitnitsky benchmark. In the absence of a confirmed signal, we set improved 90% confidence-level upper limits on the axion-photon coupling over the frequency range 1.026-1.045 GHz. This work highlights the importance of robust candidate-validation strategies as haloscope searches approach discovery-level sensitivity.