Sensitivity to Kaon Decays to ALPs at Fixed Target Experiments

TL;DR

This paper investigates how fixed target experiments can detect hadronically-coupled ALPs produced in kaon decays, highlighting the potential of current and future neutrino experiments to explore new axion parameter space.

Contribution

It demonstrates that kaon decay is the dominant ALP production mechanism below the decay threshold and reinterprets existing experimental results to constrain axion models, providing projections for upcoming experiments.

Findings

Kaon decay dominates ALP production below the decay threshold.

Existing experiments like CHARM and MicroBooNE constrain new axion parameter space.

Future experiments like DUNE will significantly extend sensitivity to ALPs.

Abstract

We study the sensitivity of fixed target experiments to hadronically-coupled axion like particles (ALPs) produced in kaon decays, with a particular emphasis on current and upcoming short-baseline neutrino experiments. We demonstrate that below the kaon decay mass threshold ($m_a < m_K - m_\pi$) kaon decay is the dominant production mechanism for ALPs at neutrino experiments, larger by many orders of magnitude than production in psuedo-scalar mixing. Such axions can be probed principally by the di-photon and di-muon final states. In the latter case, even if the axion does not couple to muons at tree level, such a coupling is induced by the renormalization group flow from the UV scale. We reinterpret prior results by CHARM and MicroBooNE through these channels and show that they constrain new areas of heavy axion parameter space. We also show projections of the sensitivity of the SBN and DUNE experiments to axions through these channels, which reach up to multiple decades higher in the axion decay constant beyond existing constraints. DUNE projects to have a sensitivity competitive with other world-leading upcoming experiments.