Searching for axions with kaon decay at rest

Abstract

We describe a novel search strategy for axions (or hadronically coupled axionlike particles) in the mass range of ${\mathrm{mₐ}}^{}\lesssim 350\mathrm{MeV}$. The search relies on kaon decay at rest, which produces a monoenergetic signal in a large volume detector (e.g., a tank of liquid scintillator) from axion decays $a\to \gamma \gamma$ or $a\to {\mathrm{e⁺}}^{}{\mathrm{e⁻}}^{}$. The decay modes ${\mathrm{K⁺}}^{}\to {\pi }^{⁺}a$ and $a\to \gamma \gamma$ are induced by the axion’s coupling to gluons, which is generic to any model which addresses the strong $CP$ problem. We recast a recent search from MicroBooNE for ${\mathrm{e⁺}}^{}{\mathrm{e⁻}}^{}$ pairs and study prospects at ${\mathrm{JSNS²}}^{}$ and other near-term facilities. We find that ${\mathrm{JSNS}}^{\mathrm{²}}$ will have world-leading sensitivity to hadronically coupled axions in the mass range of $40\mathrm{MeV}\lesssim \; mₐ{}_{}\lesssim 350\mathrm{MeV}$.

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