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 $\mathrm{\mid }{U}_{\tau 4}{\mathrm{\mid }}^2$, 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 $\mathrm{\mid }{U}_{\tau 4}{\mathrm{\mid }}^2$ at the 95$\mathrm{\%}$ 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 $100<m_4<1300\text{ MeV}\mathrm{/}{c}^2$; the more stringent limits being placed at higher masses.