# Probing ALPs and the Axiverse with Superconducting Radiofrequency Cavities

## TL;DR

This paper proposes a novel superconducting radiofrequency cavity experiment to detect axion-like particles (ALPs), leveraging nonlinear effects in Maxwell's equations, which could surpass current detection methods and also observe QED effects.

## Contribution

The paper introduces a new SRF cavity setup sensitive to light ALPs independent of dark matter, with improved sensitivity and potential to observe QED effects at low energies.

## Key findings

- Potential to detect ALPs beyond existing experiments
- First observation of Euler-Heisenberg QED effects below electron mass
- Designs for cavity geometries to distinguish ALP and QED contributions

## Abstract

Axion-like particles (ALPs) with couplings to electromagnetism have long been postulated as extensions to the Standard Model. String theory predicts an "axiverse" of many light axions, some of which may make up the dark matter in the universe and/or solve the strong CP problem. We propose a new experiment using superconducting radiofrequency (SRF) cavities which is sensitive to light ALPs independent of their contribution to the cosmic dark matter density. Off-shell ALPs will source cubic nonlinearities in Maxwell's equations, such that if a SRF cavity is pumped at frequencies $\omega_1$ and $\omega_2$, in the presence of ALPs there will be power in modes with frequencies $2\omega_1 \pm \omega_2$. Our setup is similar in spirit to light-shining-through-walls (LSW) experiments, but because the pump field itself effectively converts the ALP back to photons inside a single cavity, our sensitivity scales differently with the strength of the external fields, allowing for superior reach as compared to experiments like OSQAR while utilizing current technology. Furthermore, a well-defined program of increasing sensitivity has a guaranteed physics result: the first observation of the Euler-Heisenberg term of low-energy QED at energies below the electron mass. We discuss how the ALP contribution may be separated from the QED contribution by a suitable choice of pump modes and cavity geometry, and conclude by describing the ultimate sensitivity of our proposed program of experiments to ALPs.