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 $\tau$ neutrino with a mixing strength of $|U_{\tau 4}{|}^2$, corresponding to the absolute square of the extended Pontecorvo-Maki-Nakagawa-Sakata matrix element. Data from the BABAR experiment, with a total integrated luminosity of $424{\mathrm{fb}}^{-1}$, 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 $|U_{\tau 4}{|}^2$ 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 $100<m_4<1300\mathrm{MeV}/c^2$; the more stringent limits being placed at higher masses.