Extended Haloscope Search and Candidate Validation near 1.036G Hz

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.

Figures (5)

• Figure 1: Schematic of the axion haloscope setup, including the microwave cavity in a superconducting magnet and the associated cryogenic and room-temperature receiver chain.
• Figure 2: Power spectra centered on the candidate frequency for different frequency-bin widths: (a) 50 Hz and (b) 200 Hz. The expected signal lineshape from the standard axion halo model is overlaid as red dashed lines.
• Figure 3: Signal strength as a function of frequency detuning $\nu-\nu_c$, normalized to the cavity bandwidth ($\Delta\nu_c=28.25$ kHz). The error bars indicate statistical uncertainties. The data are fitted with a Lorentzian profile whose linewidth is consistent with the measured cavity resonance.
• Figure 4: Measured system noise temperature $T_{\rm sys}$ for this scan (blue), compared with the original scans employing JPA-based (green) and HEMT-based (yellow) receiver chains CAPP-MAX. The red dashed line indicates the standard quantum limit.
• Figure 5: Experimental limits on the axion–photon coupling $g_{a\gamma\gamma}$ as a function of axion mass $m_a$ (frequency $\nu_{a}$). The upper panel shows the 90% confidence-level limits set by this work, assuming a local axion density of $\rho_a=0.45\,{\rm GeV/cm^3}$; the black dashed lines indicate the KSVZ and DFSZ benchmark models. The vertical red dashed line denotes the candidate frequency. The lower panel compares the results of this work with previously reported haloscope limits ADMXADMX2018ADMX2019_1_2ADMX2021ADMX2024ADMX2025ADMX_SidecarADMX_Sidecar_JTWPACAPP-1CAPP-2CAPP-3CAPP-4CAPP-5CAPP-6CAPP-7CAPP-8CAPP-9CAPP-MAXCAPP-8TB-PR2DMAG-12TCASTCAPPGrAHalHAYSTAC2HAYSTAC3HAYSTAC4ORGANQUAXQUAX2QUAX3QUAX4QUAX5RADESRADES2RBFUFTASEH, with the exclusion data taken from the AxionLimits GitHub repository AxionLimits.