A connection between flavour anomaly, neutrino mass, and axion

TL;DR

This paper introduces a minimal model linking flavour anomalies in B-meson decays to neutrino mass generation, axion dark matter, and solving the strong CP problem through a unified symmetry framework.

Contribution

It presents a novel minimal model connecting flavour anomalies, neutrino masses, and axion dark matter via a shared Peccei-Quinn symmetry.

Findings

Explains $b o s \, ext{mu}^+ \, ext{mu}^-$ anomaly with new vector-like fermions.

Provides a solution to the strong CP problem with an axion from PQ symmetry breaking.

Links neutrino Dirac masses to the same symmetry, integrating flavour, neutrino, and dark matter physics.

Abstract

We propose a minimal model in which the flavour anomaly in the $b \to s \mu^+ \mu^-$ transition is connected to the breaking of Peccei-Quinn (PQ) symmetry. The flavour anomaly is explained from new physics contribution by introducing one generation of heavy quark and heavy lepton which are vector-like under the standard model (SM) gauge group but charged under a local $U(1)_X$ group. They mix with the SM quarks and leptons, inducing flavour-changing $Z^\prime$ couplings, which generates the $b \to s \mu^+\mu^-$ anomaly at tree level. On the other hand the new fermions are chiral under the global Peccei-Quinn(PQ) symmetry. The pseudo-Goldstone boson coming from the spontaneous breaking of the PQ symmetry becomes an axion, solving the strong CP problem and providing a cold dark matter candidate. The same symmetry prevents the right-handed neutrino from having a Majorana mass term. But the introduction of a neutrino-specific Higgs doublet allows neutrino to have Dirac mass term without fine-tuning problem. The model shows an interplay between axion, neutrino, dark matter, and flavour physics.