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Abstract

Given the elusive nature of neutrinos, their self-interactio is particularly difficult to probe. Nevertheless, upper limits on the strength of such an interaction can be set by using data from terrestrial experiments. In this work, we focus on additional contributions to the invisible decay width of Z boson as well as the leptonic tau decay width in the presence of a neutrino coupling to a relatively light scalar. For invisible Z decays, we derive a complete set of constraints by considering both three-body bremsstrahlung as well as the loop correction to two-body decays. While the latter is usually regarded to give rather weak limits, we find that through the interference with the Standard Model diagram it actually yields a competitive constraint. As far as leptonic decays of tau are concerned, we derive a limit on neutrino self-interactions that is valid across the whole mass range of a light scalar mediator. Our bounds on the neutrino self-interaction are leading for m(phi) greater than or similar to 300 MeV and interactions that prefer nu(tau). Bounds on such nu-philic scalars are particularly relevant in light of the recently proposed alleviation of the Hubble tension in the presence of such couplings.