  Large numbers of $\tau$ leptons are produced at Belle II. These could
potentially decay into sterile neutrinos that, for the mass range under
consideration, are typically long-lived, leading to displaced-vertex
signatures. Here, we study a displaced-vertex search sterile-neutrino-extended
Standard Model Effective Field Theory. The production and decay of the sterile
neutrinos can be realized via either the standard active-sterile neutrino
mixing or higher-dimensional operators in the effective Lagrangian. We perform
Monte-Carlo simulations to estimate the Belle II sensitivities to such
interactions. We find that Belle II can probe non-renormalizable dimension-six
operators involving a single sterile neutrino up to a few TeV in the
new-physics scale.
