A connection between flavour anomaly, neutrino mass, and axion

Seungwon Baek

doi:10.1007/jhep10(2020)111

Abstract

We propose a minimal model in which the flavour anomaly in the b → sμ+μ− 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′ couplings, which generates the b → sμ+μ− 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.

Highlights

JHEP10(2020)111 sub-eV scale when neutrino Yukawa coupling is of order one, yν ∼ O(1)
We propose a minimal model in which the flavour anomaly in the b → sμ+μ− 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

Summary

The model

The new particles in the model as well as the SM ones are shown in table 1 with their representations under the SM gauge group SU(3)C × SU(2)L × U(1)Y and their charges under the local U(1)X symmetry and the global U(1)PQ symmetry. To keep the hierarchy between the PQ scale (∼ 1012 GeV), the U(1)X scale (∼ 105 GeV), the EW scale (∼ 100 GeV), and the neutrino mass scale (∼ 0.1 eV) from the radiative corrections, we need to suppress the corresponding mixing parameters λφS, λ1(2)S, λ1(2)φ. The smallness of these parameters is technically natural due to the extended Poincaré symmetry [1, 43]. It is noted that the right-handed neutrinos do not have the Majorana mass terms because they are charged under U(1)PQ.

The axion and the Dirac neutrino

Conclusions