Neutrino self-interactions ($\nu \mathrm{SI}$) 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 $\nu \mathrm{SI}$ to explain the $4\sigma$ Hubble tension is in terms of a pointlike 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 $\nu \mathrm{SI}$ 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 are used to constrain $\nu \mathrm{SI}$, which rules out the regime around ${10}^9{G}_F$.

• Received 4 May 2020
• Accepted 19 August 2020

DOI:https://doi.org/10.1103/PhysRevD.102.051701

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society