Neutrino Self-Interactions and Double Beta Decay
  Neutrino Self-Interactions ($\nu$SI) 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 $\nu$SI to explain the
$4\sigma$ Hubble tension is in terms of a point-like effective four-fermion
coupling that can be as high as $10^9\, G_F$, where $G_F$ is the Fermi
constant. In this work, we show that such strong $\nu$SI 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 is used to constrain $\nu$SI, which
rules out the regime around $10^9\, G_F$.
