Global $U(1)_{L}$ Breaking in Neutrinophilic 2HDM: From LHC Signatures to X-Ray Line

TL;DR

This paper explores a neutrinophilic two-Higgs-doublet model with spontaneous lepton number violation, highlighting its unique collider signatures and potential to explain astrophysical X-ray line excesses through a keV-scale Majoron as decaying dark matter.

Contribution

It introduces a novel neutrinophilic 2HDM framework with spontaneous lepton number breaking, identifying distinctive collider signatures and dark matter implications not previously detailed.

Findings

Current experiments constrain the model's parameters due to large invisible Higgs decay.

The model predicts a unique same-sign tri-lepton signature distinguishing it from other seesaw models.

A keV-scale Majoron can serve as decaying dark matter explaining 3.5 keV and 511 keV X-ray line excesses.

Abstract

Lepton number violation plays an essential role in many scenarios of neutrino mass generation and also provides new clues to search new physics beyond the standard model. We consider the neutrinophilic two-Higgs-doublet model ($\nu$-2HDM) where additional right-handed neutral fermions $N_{Ri}$ and a complex singlet scalar $\sigma$ are also involved. In scalar sector, the global $U(1)_{L}$ symmetry is spontaneous broken, leading to Nambu-Goldstone boson, the Majoron $J$, accompanied by the Majorana neutrino mass generation. We find that the massless Majoron will induce large invisible Higgs decay, and current experiments have already set constraints on relevant parameters. For the first time, we point out that the $\nu$-2HDM with $N_{Ri}$ can be distinguished from other seesaw by the same sign tri-lepton signature $3\ell^\pm4j+\cancel{E}_T$. More interesting, for $\mathcal{O}(\keV)$ scale Majoron, it is a good candidate of decaying dark matter to interpret the $3.5\keV$ and $511\keV$ line excesses.