Search for Heavy Neutral Leptons Using Tau Lepton Decays at BABAR

This article presents a model independent search for an additional, mostly sterile, Heavy Neutral Lepton (HNL), that is capable of mixing with the Standard Model $τ$ neutrino with a mixing strength of $|U_{τ4}|^{2}$, corresponding to the absolute square of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix element. Data from the \babar~ experiment, with a total integrated luminosity of $424~ \text{fb}^{-1}$, are analyzed using a kinematic approach that makes no assumptions on the model behind the origins of the HNL, its lifetime or decay modes. No significant signal is found. Upper limits on $|U_{τ4}|^{2}$ at the 95$\%$ confidence level, depend on the HNL mass hypothesis and vary from $2.31 \times 10^{-2}$ to $5.04 \times 10^{-6}$ (with all uncertainties considered), across the mass range $100 < m_{4} < 1300 ~\text{MeV}/c^{2}$; the more stringent limits being placed at higher masses.

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