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 -> nu mu(+)mu(-) , considering decays occurring both inside ArgoNeuT and in the upstream cavern. In the data, corresponding to an exposure to 1.25 x 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 vertical bar U-tau N vertical bar(2) of tau-coupled Dirac heavy neutral leptons with masses m(N) = 280-970 MeV, assuming vertical bar U-eN vertical bar(2) = vertical bar U-mu N vertical bar(2) = 0.