Neutrino self-interactions and double beta decay

Neutrino self-interactions (νSI) beyond the Standard Model are an attractive possibility to soften cosmological constraints on neutrino properties and also to explain the tension in late and early time measurements of the Hubble expansion rate. The required strength of νSI to explain the 4σ Hubble tension is in terms of a pointlike effective four-fermion coupling that can be as high as 10⁹GF, where GF is the Fermi constant. In this work, we show that such strong νSI can cause significant effects in two-neutrino double beta decay, leading to an observable enhancement of decay rates and to spectrum distortions. We analyze self-interactions via an effective operator as well as when mediated by a light scalar. Data from observed two-neutrino double beta decay are used to constrain νSI, which rules out the regime around 10⁹GF.