Minimal Flavor Protection for TeV-scale New Physics

TL;DR

The paper confronts the challenge of TeV-scale new physics coexisting with stringent FCNC constraints by introducing Minimal Flavor Protection (MFP), which identifies the minimal approximate flavor symmetry $SU(2)_q \times U(1)_X$ and a small set of spurions of size $O(10^{-2})$. This structure reproduces SM Yukawa hierarchies and CKM mixing while maintaining alignment between mass and interaction bases, allowing $O(1)$ flavor universality violation in the SMEFT and enriching collider-accessible flavor dynamics beyond MFV and $U(2)^5$. A concrete benchmark demonstrates that a substantial portion of SMEFT operator space remains viable with $\Lambda_{\rm eff}$ in the TeV range, while enabling flavor-charged currents not present in MFV. The framework thus broadens the landscape of testable TeV-scale flavor phenomena and motivates both experimental searches across flavor sectors and UV-completion scenarios involving vector-like fermions and flavons.

Abstract

We determine how much TeV-scale new physics can deviate from flavor universality, $U(3)^5$, while respecting stringent bounds on flavor-changing neutral currents. The minimal continuous subgroup that must be approximately preserved is identified as $SU(2)_{q} \times U(1)_{X}$. With only a few symmetry-breaking spurions of $\mathcal{O}(10^{-2})$, all observed fermion hierarchies may be reproduced, offering a new perspective on the SM flavor puzzle. Remarkably, this framework provides structural flavor protection for generic TeV-scale new physics within the SMEFT, enlarging the space of collider-accessible scenarios beyond MFV and $U(2)^5$ and allowing for richer patterns of flavor violation.

Figures (2)

• Figure 1: Bounds on selected SMEFT operators: Anarchy (yellow), $SU(2)_{q} \times U(1)_{X}$ (blue), MFV (green). See Sec. \ref{['sec:SMEFT']} for details.
• Figure 2: The chains of vector-like fermions yielding $Y_{u,d,e}$ in the TeV-scale flavor model introduced in \ref{['sec:model']}. Flavor-spurion mixing is shown in green, and the vector-like fermions in red.