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 ∣Uτ4∣2, corresponding to the absolute square of the extended Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix element. Data from the experiment, with a total integrated luminosity of 424fb−1
Nearby in the stack
, 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
∣Uτ4∣2
at the 95
%
confidence level, depend on the HNL mass hypothesis and vary from
2.31×10−2
to
5.04×10−6
(with all uncertainties considered), across the mass range
100<m4<1300MeV/c2
; the more stringent limits being placed at higher masses.