Search for BSM new physics at neutrino facilities

We point out the novelty of dark-sector-origin mechanisms giving rise to an anomalous tau neutrino appearance at the near detectors of beam-focused neutrino experiments, without extending the neutrino sector. The charged mesons produced and focused in the target-horn system can decay to a (neutrinophilic) mediator together with a charged lepton and its neutrino counterpart. If such a mediator also couples to hadrons, it can be additionally produced via the bremsstrahlung of the incident proton beam. The mediator subsequently decays to a tau neutrino pair resulting in tau neutrino detection at the near detectors, which is unexpected under the standard three-flavor neutrino model. We show that ICARUS-NuMI, an off-axis near detector of the NuMI beam, as well as DUNE can observe tau neutrino events induced by beam-proton bremsstrahlung.