Search for Heavy Neutral Leptons Using Tau Lepton Decays at BABAR

Abstract

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 (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 $|U_{\tau 4}{|}^2$ 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 $100<m_4<1300\text{MeV}/c^2$; the more stringent limits being placed at higher masses.

20 pages, 10 figures

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