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>–</mml:mi><mml:mn>970</mml:mn><mml:mtext> </mml:mtext><mml:mtext> </mml:mtext><mml:mi>MeV</mml:mi></mml:mrow></mml:math>
R. Acciarri, C. Adams, J. Asaadi, B. Baller, V. Basque, F. Cavanna, André de Gouvêa, R. S. Fitzpatrick, B. Fleming, P. Green, C. James, Kevin J. Kelly, I. Lepetic, X. Luo, O. Palamara, G. Scanavini, M. Söderberg, J. Spitz, A. M. Szelc, W. Wu, T. Yang
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 $N\ensuremath{\rightarrow}\ensuremath{\nu}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$, 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 ${|{U}_{eN}|}^{2}={|{U}_{\ensuremath{\mu}N}|}^{2}=0$.