A Comprehensive Study on Top Quark FCNC Interactions in SMEFT Framework

TL;DR

This paper conducts a comprehensive, model-independent analysis of top quark FCNC interactions within SMEFT, deriving constraints from various experimental data and providing predictions for rare decay observables and CP asymmetries.

Contribution

It offers the first global, model-independent constraints on top FCNC couplings in SMEFT, incorporating complex dipole operators and EDM bounds, with detailed predictions for future collider tests.

Findings

Stringent limits on FCNC couplings from EDM constraints.

Strong bounds on SMEFT Wilson coefficients involving top FCNCs.

Predictions for rare top decay branching ratios and CP asymmetries.

Abstract

We present a comprehensive, model-independent analysis of rare flavour-changing neutral current (FCNC) interactions of the top quark within an effective field theory framework. Beginning with a general parametrisation of top-FCNC couplings, we match these interactions onto the Standard Model Effective Field Theory (SMEFT) operator basis at the top-quark scale. We then perform a global study that incorporates constraints from low-energy flavour observables, electroweak precision data, Higgs and gauge-boson measurements, and electric dipole moment (EDM) bounds. By treating the dipole operators as complex, we derive stringent limits on both the real and imaginary components of left- and right-handed FCNC couplings. In particular, we show that constraints from the neutron EDM impose especially strong bounds on products of FCNC couplings involving the transition $t \to u$. Translating these results into the SMEFT framework, we obtain robust constraints on several products of the corresponding SMEFT Wilson coefficients. Finally, we provide predictions for branching ratios and CP asymmetries in rare top-quark FCNC decays. We identify well-motivated benchmark scenarios for future collider searches and emphasise the crucial role of CP-violating effects in probing the flavour structure of the top quark.