Abstract
Axionlike 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 radio-frequency (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 ω_{1} and ω_{2}, in the presence of ALPs there will be power in modes with frequencies 2ω_{1}±ω_{2}. Our setup is similar in spirit to light-shining-through-walls 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.
Highlights
Axionlike particles (ALPs) with couplings to electromagnetism have long been postulated as extensions to the standard model
We propose a new experiment using superconducting radio-frequency (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 ω1 and ω2, in the presence of ALPs there will be power in modes with frequencies 2ω1 Æ ω2
Summary
An off-shell ALP will induce small nonlinearities in electromagnetism. An experiment that is designed to look for nonlinearities induced by ALPs would, if sensitive enough, have the guaranteed physics result of discovering lightby-light scattering at low energies for the first time ever. We will consider an SRF cavity pumped simultaneously at two frequencies ω1 and ω2, such that ALP- or EH-induced nonlinearities will give signal photons at ωs 1⁄4 2ω1 − ω2 (see Fig. 1). Eqs. (5) and (6) can be interpreted as an effective ALP charge and current: ρa 1⁄4 gaγγBp · ∇a; Ja 1⁄4 gaγγ
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