A Comprehensive Study on Top Quark FCNC Interactions in SMEFT Framework

TL;DR

This work develops a comprehensive, model‑independent analysis of rare top quark FCNC interactions within the SMEFT framework, matching top‑scale EFT operators to SMEFT WCs and running them through multiple energy regimes. A global fit to low‑energy flavour observables, EWPOs, Higgs and gauge measurements, and EDM bounds yields robust, CP‑sensitive constraints on both the real and imaginary parts of top FCNC couplings and their SMEFT counterparts. The study provides predictions for branching fractions and CP asymmetries in top FCNC decays, identifies well‑motivated collider benchmarks, and demonstrates that EDMs, especially neutron EDM, profoundly constrain CP‑violating combinations, notably for $t\to u$ transitions. The results highlight the complementarity of low‑ and high‑energy data, quantify RG running effects, and show that future HL‑LHC or lepton colliders can probe substantial portions of the constrained parameter space, particularly in CP‑violating channels.

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.

Figures (14)

• Figure 1: Feynman diagrams for various FCNC processes receiving contributions from top FCNC interactions. Here, $u_i$ and $u_j$ represent up-type quarks, where one is the top quark ($t$) and the other can be either an up ($u$) or charm ($c$) quark.
• Figure 2: Feynman diagrams contributing to the FCCC processes $(b \to c(u)\, \ell \nu)$. Similarly, top FCNC couplings also affect the quark-level transitions $c \to s(d)\, \ell \nu$ and $s \to u\, \ell \nu$, although those diagrams are not explicitly shown here.
• Figure 3: Effects of top FCNC operators on the $t \to W d_i$ form factor.
• Figure 4: Gauge boson $(V_i=\gamma,Z)$ self-energy correction affected due to the presence of top-FCNC processes.
• Figure 5: Feynman diagrams of $Z\to d_i \bar{d}_i$ decay, modified by the top FCNC operators.