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Searching for axions and light-by-light scattering with superconducting RF cavities

Axion-like particles which couple to photons can induce small nonlinearities in electromagnetism, leading to light-by-light scattering. We will describe a proposed experiment at the SQMS National Quantum Center at Fermilab to detect this process, using a high-Q superconducting radiofrequency (SRF) cavity. When the cavity is pumped with two resonant modes ω1 and ω2, the axion-induced nonlinearity will source a small signal at ωs = 2ω1 − ω2, which may be detected if ωs is also a resonant frequency of the cavity. An experiment sensitive enough to set world-leading limits on axions may also be sensitive to the nonlinearity already present in quantum electrodynamics (QED) as encapsulated by the Euler-Heisenberg Lagrangian, which has never been probed at energies below the electron mass.