Phenomenology of Non-Abelian Gauge and Goldstone Bosons in a U(2) Flavor Model (2511.10468v1)

Abstract: We investigate the phenomenological implications of the bosons associated with the $SU(2)F$ subgroup in a simple and realistic $U(2)_F$ flavor model. While the PNGB 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 $SU(2)_F$ have not been studied before. This work focuses on these states, considering both the case where $SU(2)_F$ 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 $K \to πX$ and $μ\to e X$ place the most stringent constraints, probing the flavor symmetry breaking scale $vφ$ 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 ultra-high symmetry breaking scales that surpass the limits set by astrophysical observations.