Flavour and cosmological probes of Diracon models

Abstract

We present and analyze two minimal extensions of the Standard Model featuring a spontaneously broken global, chiral, and anomaly-free $U(1)_D$ symmetry. This breaking generates naturally small Dirac neutrino masses via a seesaw mechanism and yields a physical massless Goldstone boson, the Diracon. Although both models share the same particle content and scalar potential, their distinct symmetry breaking pattern leads to remarkably different phenomenological and cosmological signatures. In the first model, the Diracon couples weakly to charged leptons but right-handed neutrinos can be efficiently produced in the early Universe, resulting in stringent constraints from the effective number of relativistic species, $\Delta N_{\text{eff}}$. Conversely, in the second one, right-handed neutrino production is suppressed, and flavour-violating processes such as $\mu \to e \mathcal{D}$ provide the most promising probes. These simple but elegant models showcase the complementarity between cosmological observations and low-energy flavour experiments in the search for physics beyond the Standard Model.