A $\nu$ scalar in the early universe and $(g-2)_{\mu}$

TL;DR

This paper proposes a light scalar around 1 MeV that connects neutrino mass origins and muon g-2 anomaly, relaxing early universe bounds and offering a UV-complete model linking cosmology and particle physics.

Contribution

It introduces a novel light scalar model with feeble neutrino couplings that explains muon g-2 discrepancy and neutrino masses while satisfying cosmological constraints.

Findings

Scalar can relax bounds on N_eff from photon decays

Scalar explains muon g-2 anomaly with Higgs-like diphoton coupling

Ultraviolet completion links early universe physics and neutrino mass generation

Abstract

We investigate a concrete scenario of a light scalar with a mass around 1 MeV which can be connected to the origin of neutrino masses and simultaneously survive current bounds on relativistic degrees of freedom in the early universe. In particular we show that a feeble coupling to the Standard Model neutrinos can relax the stringent bounds on the decays to photons inferred from the measured value of $N_{\rm eff}$. Interestingly, we find that such a scalar whose diphoton coupling is generated by a tree-level coupling to the muon of similar strength as that of the Standard Model Higgs boson can simultaneously explain the long-standing discrepancy in the measured value of the muon magnetic moment. We present a possible ultraviolet completion of this scenario providing a link between new physics in the early universe and the generation of neutrino masses.