Cavity-mode couplings in axion dark matter searches

TL;DR

This paper analyzes how cavity-mode couplings in two-port microwave cavities affect axion dark matter searches, highlighting the importance of measuring weakly coupled ports to maintain sensitivity.

Contribution

It reveals the dependence of measured coupling strengths on each other in two-port cavities and recommends measuring weak port couplings to reduce systematic uncertainties.

Findings

Measured coupling strength of one port depends on the other in two-port cavities.

Systematic uncertainty mainly arises from the weakly coupled port.

Neglecting weak port measurement can cause up to 10% sensitivity loss.

Abstract

Axion dark matter searches use a microwave cavity for the resonant conversion of axions into photons to enhance experimental sensitivity, with the cavity generally configured as a two-port system for both signal pickup and cavity characterization measurements. In this study, we investigated cavity-mode couplings in such a two-port system and examined their impact on axion dark matter search experiments, which typically use one strongly coupled port and one weakly coupled port. We found that, in such a two-port cavity system, the measured coupling strength of one port depends on that of the other; hence, the coupling coefficients appearing in the relation for the unloaded quality factor of the cavity mode can vary substantially with the measured coupling strengths. Meanwhile, the scanning rate, the figure-of-merit for axion dark matter searches, cancels the systematic contribution from the strongly coupled port; hence, the remaining systematic uncertainty arises only from the weakly coupled port and may be negligible, depending on its coupling strength. Nevertheless, we recommend measuring the coupling strength of the weakly coupled port to eliminate this systematic uncertainty and thereby recover any experimental sensitivity that may have been lost, for example by approximately 10\% when the coupling strength of the weakly coupled port is 0.05.