Abstract
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 $10^9\, G_F$.
Highlights
The discrepancy between cosmic microwave background (CMB) and local measurements of the Hubble constant, known as the Hubble tension, has grown to about 4σ [1,2,3,4,5]
If a physical fact, it would imply that nonstandard particle physics or cosmology is required
The required strength of νSI needs to be much larger than the Fermi effective interactions predicted in the Standard Model (SM) [6,7,8,9,10]
Summary
Neutrino self-interactions (νSI) beyond the Standard Model are an attractive possibility to soften cosmological constraints on neutrino properties and 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. 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
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