ν scalar in the early Universe and (g−2)μ

Jia Liu,Carlos E M Wagner,Navin Mcginnis,Xiao-Ping Wang

doi:10.1103/physrevd.105.l051702

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.

Highlights

The theoretical and experimental establishment of the Standard Model (SM) of particle physics [1] and general relativity ushered in the so-called ΛCDM model, which provides an excellent description of nucleosynthesis and cosmic microwave background data [2]
We have presented a model of a real scalar, with a mass in the MeV range, that leads to the active neutrino mass generation and to an explanation of the observed muon anomalous magnetic moment
Neutrino masses are generated through the addition of sterile neutrinos, with masses in the hundreds of MeV range

Summary

INTRODUCTION

The theoretical and experimental establishment of the Standard Model (SM) of particle physics [1] and general relativity ushered in the so-called ΛCDM model, which provides an excellent description of nucleosynthesis and cosmic microwave background data [2]. The current observation of a deviation of the anomalous magnetic moment [12] from its theoretically predicted value in the SM [13–33] (see a recent review [34]) demands an explanation and could be connected to the presence of new scalars coupled to the muon [35–42]. Motivated by these observations, in this paper we present a low-energy model of a real scalar particle with mass Oð1Þ MeV which, at tree level, couples to the muon and SM neutrinos, and whose UV completion provides the source of neutrino masses. We find that a scalar with mass in this range can simultaneously generate the necessary correction to explain the recently measured value of the muon magnetic moment [12], Δaμ 1⁄4 ð2.51 Æ 0.59Þ × 10−9; ð1Þ and, for an appropriate coupling to neutrinos, satisfy all cosmological, astrophysical and laboratory constraints

EFFECTIVE MODEL

Neff CALCULATION

UV MODEL

CONCLUSION