A CMB Search for the Neutrino Mass Mechanism and its Relation to the $H_0$ Tension

TL;DR

This paper explores how massive majorons, linked to neutrino mass models, could leave detectable imprints on the CMB and potentially address the H0 tension, with current data constraining their interactions.

Contribution

It investigates the impact of massive majorons on the CMB and identifies parameter space where they can help resolve the H0 tension, providing new constraints on neutrino-majoron couplings.

Findings

Planck2018 data sensitive to neutrino-majoron couplings as small as 10^{-13}

Majoron-neutrino interactions can contribute to resolving the H0 tension

Constraints imply a lepton number breaking scale around 100 GeV

Abstract

The majoron, a pseudo-Goldstone boson arising from the spontaneous breaking of global lepton number, is a generic feature of many models intended to explain the origin of the small neutrino masses. In this work, we investigate potential imprints in the Cosmic Microwave Background (CMB) arising from massive majorons, should they thermalize with neutrinos after Big Bang Nucleosynthesis via inverse neutrino decays. We show that Planck2018 measurements of the CMB are currently sensitive to neutrino-majoron couplings as small as $\lambda \sim 10^{-13}$, which if interpreted in the context of the type-I seesaw mechanism correspond to a lepton number symmetry breaking scale $v_L \sim \mathcal{O}(100) \, {\rm GeV}$. Additionally, we identify parameter space for which the majoron-neutrino interactions, collectively with an extra contribution to the effective number of relativistic species $N_{\rm eff}$, can ameliorate the outstanding $H_0$ tension.