New Constraints on Tau-Coupled Heavy Neutral Leptons with Masses m_{N}=280-970 MeV.

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 $N\ensuremath{\rightarrow}\ensuremath{\nu}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, considering decays occurring both inside ArgoNeuT and in the upstream cavern. In the data, corresponding to an exposure to $1.25\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{POT}$, 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 ${|{U}_{\ensuremath{\tau}N}|}^{2}$ of tau-coupled Dirac heavy neutral leptons with masses ${m}_{N}=280--970\text{ }\text{ }\mathrm{MeV}$, assuming ${|{U}_{eN}|}^{2}={|{U}_{\ensuremath{\mu}N}|}^{2}=0$.