Anomalous tau neutrino appearance from light mediators in short-baseline neutrino experiments

We point out a new mechanism giving rise to anomalous tau neutrino appearance at the near detectors of beam -focused neutrino experiments, without extending the neutrino sector. The charged mesons (re +/-, K +/-) produced and focused in the target -horn system can decay to a (neutrino-philic) light mediator via the helicityunsuppressed three -body decays. If such a mediator carries non -vanishing hadronic couplings, it can also be produced via the bremsstrahlung of the incident proton beam. The subsequent decay of the mediator to a tau neutrino pair results in tau neutrino detection at the near detectors, which is unexpected under the standard three -flavor neutrino oscillation paradigm. We argue that the signal flux from the charged meson decays can be significant enough to discover the light mediator signal at the on -axis liquid -argon near detector of the DUNE experiment, due to the focusing of charged mesons. In addition, we show that ICARUS-NuMI, an off -axis near detector of the NuMI beam, as well as DUNE, can observe a handful of tau neutrino events induced by beamproton bremsstrahlung.