Heavy neutral leptons below the kaon mass at hodoscopic neutrino detectors

Abstract

Heavy neutral leptons ($N$) below the kaon mass are severely constrained by cosmology and lab-based searches for their decays in flight. If $N$ interacts via an additional force, $N\ensuremath{\rightarrow}\ensuremath{\nu}{e}^{+}{e}^{\ensuremath{-}}$ decays are enhanced and cosmological limits can be avoided. We show that the T2K and MicroBooNE neutrino experiments provide the best limits on the mixing of $N$ with muon neutrinos, outperforming past-generation experiments, previously thought to dominate. We constrain models with electromagnetically decaying and long-lived $N$, such as in a transition-magnetic-moment portal and in a leptophilic axionlike particle portal, invoked to explain the MiniBooNE excess. By considering these models as representative examples, our results show that explanations of the MiniBooNE excess that involve ${e}^{+}{e}^{\ensuremath{-}}$ pairs from long-lived particles are in tension with T2K, PS191, and MicroBooNE data. Similarly, these searches also constrain MiniBooNE explanations based on single photons due to the associated ${e}^{+}{e}^{\ensuremath{-}}$ decay mode via a virtual photon.

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