New Physics in the Third Generation: A Comprehensive SMEFT Analysis and Future Prospects

TL;DR

This paper conducts a comprehensive analysis of dimension-six SMEFT operators in the $U(2)^5$-symmetric limit, assessing current and future collider bounds on new physics primarily coupled to the third generation.

Contribution

It provides the first complete bounds on 124 $U(2)^5$-invariant operators, including renormalization group effects, and explores future collider sensitivities.

Findings

Current bounds allow new physics scales as low as 1.5 TeV.

Future FCC-ee collider could improve bounds by an order of magnitude.

Most models with third-generation coupling remain viable under present constraints.

Abstract

We present a comprehensive analysis of electroweak, flavor, and collider bounds on the complete set of dimension-six SMEFT operators in the $U(2)^5$-symmetric limit. This operator basis provides a consistent framework to describe a wide class of new physics models and, in particular, the motivated class of models where the new degrees of freedom couple mostly to the third generation. By analyzing observables from all three sectors, and consistently including renormalization group evolution, we provide bounds on the effective scale of all 124 $U(2)^5$-invariant operators. The relation between flavor-conserving and flavor-violating observables is analyzed taking into account the leading $U(2)^5$ breaking in the Yukawa sector, which is responsible for heavy-light quark mixing. We show that under simple, motivated, and non-tuned hypotheses for the parametric size of the Wilson coefficients at the high scale, all present bounds are consistent with an effective scale as low as 1.5 TeV. We also show that a future circular $e^+ e^-$ collider program such as FCC-ee would push most of these bounds by an order of magnitude. This would rule out or provide clear evidence for a wide class of compelling new physics models that are fully compatible with present data.