Axion-Neutrino Interplay in a Gauged Two-Higgs-Doublet Model

TL;DR

This paper introduces a gauged two-Higgs-doublet model that links neutrino masses, dark matter, and the strong CP problem, predicting observable signals in future experiments and providing a framework for axion dark matter detection.

Contribution

It presents a novel 2HDM with an anomalous Peccei-Quinn symmetry and an extra gauge symmetry that suppresses flavor-changing neutral currents and relates neutrino masses to symmetry breaking scales.

Findings

Neutrino masses generated via Dirac seesaw mechanism.

Predicted axion as dark matter candidate within experimental reach.

Suppressed kaon decay into pion and axion, weakly constraining axion decay constant.

Abstract

We propose a gauged two-Higgs-doublet model (2HDM) featuring an anomalous Peccei-Quinn symmetry, $U(1)_{PQ}$. Dangerous tree-level flavour-changing neutral currents, common in 2HDMs, are forbidden by the extra gauge symmetry, $U(1)_X$. In our construction, the solutions to the important issues of neutrino masses, dark matter and the strong CP problem are interrelated. Neutrino masses are generated via a Dirac seesaw mechanism and are suppressed by the ratio of the $U(1)_X$ and the $U(1)_{PQ}$ breaking scales. Naturally small neutrino masses suggest that the breaking of $U(1)_X$ occurs at a relatively low scale, which may lead to observable signals in near-future experiments. Interestingly, spontaneous symmetry breaking does not lead to mixing between the $U(1)_X$ gauge boson, $Z^\prime$, and the standard $Z$. For the expected large values of the $U(1)_{PQ}$ scale, the associated axion becomes "invisible", with DFSZ-like couplings, and may account for the observed abundance of cold dark matter. Moreover, a viable parameter space region, which falls within the expected sensitivities of forthcoming axion searches, is identified. We also observe that the flavour-violating process of kaon decaying into pion plus axion, $K^+ \to \pi^+ a$, is further suppressed by the $U(1)_X$ scale, providing a rather weak lower bound for the axion decay constant $f_a$.