Searching for axions with kaon decay at rest

Abstract

We describe a novel search strategy for axions (or hadronically coupled axion-like particles) in the mass range of $m_a\lesssim 350\mathrm{MeV}$. The search relies on kaon decay at rest, which produces a mono-energetic signal in a large volume detector (e.g.\ a tank of liquid scintillator) from axion decays $a\to \gamma \gamma$ or $a\to e^{+}{e}^{-}$. The decay modes $K^{+}\to {\pi }^{+}a$ 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 $e^{+}{e}^{-}$ pairs, and study prospects at JSNS${}^2$ and other near-term facilities. We find that JSNS${}^2$ will have world-leading sensitivity to hadronically coupled axions in the mass range of $40\mathrm{MeV}\lesssim m_a\lesssim 350\mathrm{MeV}$.

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