QCD axion, colour-mediated neutrino masses, and $B^+\to K^+ + E_{\text{miss}}$ anomaly

TL;DR

This paper proposes a QCD axion model with a scalar leptoquark and vector-like quarks to explain the Belle II $B^+ o K^+ + E_{miss}$ anomaly, while addressing the strong CP problem and neutrino masses.

Contribution

It introduces a high-scale PQ symmetry-based model with a scalar leptoquark and vector-like quarks that explains the B decay anomaly and generates neutrino masses.

Findings

The model can account for the $2.7\sigma$ excess in $B^+ \to K^+ + E_{miss}$ events.

It simultaneously provides a solution to the strong CP problem and dark matter.

Neutrino masses are generated via two-loop radiative processes.

Abstract

Motivated by the recent Belle II result indicating a $2.7\,\sigma$ excess of $B^+\to K^+ + E_{\text{miss}}$ events compared to the Standard Model (SM) prediction for $B^+ \to K^+ \nu\bar{\nu}$, we explore an explanation to this anomaly based on a KSVZ-type QCD axion model featuring a Peccei-Quinn (PQ) symmetry breaking at high-scale, that can provide a solution to the strong CP problem with dark matter relic abundance. The model contains a PQ-charged scalar} leptoquark which can interact with the SM quarks only via mass-mixing of the latter with vector-like quarks. The mixing between SM and vector-like quarks is determined by the PQ mass scales and can explain the excess $B^+\to K^+ + E_{\text{miss}}$ events while respecting other flavour constraints. The same PQ-charged scalar leptoquarks and vector-like quarks also mediate the two-loop radiative neutrino masses.