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New Constraints on Tau-Coupled Heavy Neutral Leptons with Masses <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>m</mml:mi></mml:mrow><mml:mrow><mml:mi>N</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>280</mml:mn><mml:mi>โ</mml:mi><mml:mn>970</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>MeV</mml:mi></mml:mrow></mml:math>
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 $1.25\ifmmode\times\else\texttimes\fi{}{10}^{20}\text{ }\text{ }\mathrm{POT}$, โฆ