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Phenomenology of non-Abelian gauge and Goldstone bosons in a U(2) flavor model

School of Physics, Nankai University, Tianjin 300071, China

^*Contact author: yijiangyi@nankai.edu.cn

Phys. Rev. D 113, 075041 – Published 29 April, 2026

DOI: https://doi.org/10.1103/1mwt-bqsm

Abstract

We investigate the phenomenological implications of the bosons associated with the $S U (2)_{F}$ subgroup in a simple and realistic $U (2)_{F}$ flavor model. While the Nambu-Goldstone boson of the $U (1)_{F}$ factor behaves as a standard QCD axion (an axiflavon) with suppressed flavor-violating couplings, the three degrees of freedom from $S U (2)_{F}$ have not been studied before. This work focuses on these states, considering both the case where $S U (2)_{F}$ is a global symmetry, yielding pseudo-Nambu-Goldstone bosons (PNGBs), and the case where it is a gauge symmetry with a potentially small coupling, yielding a triplet of (possibly) light gauge bosons. In both scenarios, these new bosons naturally feature unsuppressed flavor-violating couplings to Standard Model fermions in the mass basis. We derive the resulting predictions for flavor-changing neutral currents and lepton flavor violation, including exotic decays of mesons and leptons. Our analysis shows that processes like $K \to π X$ and $μ \to e X$ place the most stringent constraints, probing the flavor symmetry breaking scale $v_{ϕ}$ up to $10^{11} - 10^{12} GeV$ for light bosons, while heavier states are tested in $B$ and $τ$ decays, as well as by $K − \bar{K}$ mixing and $μ \to e γ$ . We demonstrate that low-energy flavor experiments provide a powerful probe of this framework, capable of testing ultrahigh symmetry breaking scales that surpass the limits set by astrophysical observations.

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Phys. Rev. D 113, 075041– Published 29 April, 2026

Vol. 113, Iss. 7 — 1 April 2026

Received 19 November 2025
Accepted 13 April 2026

DOI: https://doi.org/10.1103/1mwt-bqsm

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

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Phenomenology of non-Abelian gauge and Goldstone bosons in a U(2) flavor model

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