New constraints on tau-coupled Heavy Neutral Leptons with masses $m_N=280-970$ MeV

Abstract

A search for Heavy Neutral Leptons has been performed with the ArgoNeuT detector exposed to the NuMI neutrino beam at Fermilab. We search for the decay signature $N\to \hat{I}½\hat{I}{¼}^{+}\hat{I}{¼}^{-}$, considering decays occurring both inside ArgoNeuT and in the upstream cavern. In the data, corresponding to an exposure to $1.25\times 10^{20}$ POT, zero passing events are observed consistent with the expected background. This measurement leads to a new constraint at 90\% confidence level on the mixing angle ${\left|U_{\ddot{I}„N}\right|}^2$ of tau-coupled Dirac Heavy Neutral Leptons with masses $m_N=$ 280 - 970 MeV, assuming ${\left|U_{eN}\right|}^2={\left|U_{\hat{I}¼N}\right|}^2=0$.

Accepted by PRL