Title

Measuring axion gradients with photon interferometry (MAGPI)

Abstract

I will present a novel small-scale experimental concept to search for axions that have a CP-violating monopole coupling to bulk SM charges, such as baryon or lepton number. Gradients in the static axion field configurations sourced by nearby ordinary matter can induce achromatic circular photon birefringence via the axion-photon coupling. Circularly polarized light fed into an optical or (open) radio-frequency (RF) Fabry-Perot (FP) cavity then develops a phase shift that is enhanced by the cavity finesse. The fixed axion spatial gradient prevents a cancellation known to occur for an axion dark-matter (DM) search, allowing an important experimental simplification. The relative phase shift between two FP cavities fed with opposite circular polarizations can be detected interferometrically. This time-independent signal can be modulated up to non-zero frequency by altering the cavity orientations with respect to the field gradient. Multi-wavelength co-metrology techniques can be used to address chromatic measurement systematics and noise sources. I will show projections indicating that, with Earth acting as the axion source, there is a possibility to reach beyond current constraints on the product of the CP-violating monopole coupling and the axion-photon coupling for axion masses below ~10 micro-eV. If shot-noise-limited sensitivity can be achieved, an experiment using high-finesse RF FP cavities could reach a factor of up to ~10^5 into new parameter space for the product of these couplings for axion masses below ~0.05 nano-eV. I'll also relate this work to some older work I have done on DM-axion-induced birefringence in the context of the CMB.