A Comprehensive Study on Top Quark FCNC Interactions in SMEFT Framework

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.

💡 Research Summary

The paper presents a comprehensive, model‑independent study of rare flavour‑changing neutral current (FCNC) interactions of the top quark within the Standard Model Effective Field Theory (SMEFT) framework. Starting from a general parametrisation of top‑FCNC couplings that includes gluon, photon, Z‑boson and Higgs final states for both up‑type transitions $t\to u_j X$ with $u_j=u,c$, the authors match these couplings onto a selected set of dimension‑six SMEFT operators in the Warsaw basis. The matching is performed at the top‑mass scale and retains the full complex structure of tensor and scalar operators, thereby allowing CP‑violating phases to be probed.

A key technical component is the renormalisation‑group evolution (RGE) of the SMEFT Wilson coefficients from a high new‑physics scale $\Lambda_{\rm NP}$ down to the electroweak scale $\mu_{\rm EW}$, using the known anomalous‑dimension matrix for the chosen operators. Below $\mu_{\rm EW}$ the theory is matched onto the Low‑Energy Effective Theory (LEFT), and QCD/QED running is applied down to the scales relevant for low‑energy observables. This two‑step matching enables the authors to compute contributions of the top‑FCNC operators to a wide array of precision observables: low‑energy flavour processes (e.g. $D^0$–$\bar D^0$ mixing, $K\to\pi\nu\bar\nu$, $B\to X_s\gamma$), electroweak precision observables (S, T, U parameters, Z‑pole asymmetries), Higgs and triple‑gauge‑boson measurements, and electric dipole moments (EDMs) of the neutron, electron and proton.

The authors assemble a global likelihood that incorporates all these data sets. They treat the Wilson coefficients (or equivalently the effective top‑FCNC couplings) as complex parameters, performing a Markov‑Chain Monte‑Carlo (MCMC) scan to obtain 95 % confidence intervals for both real and imaginary parts. The analysis reveals that EDM constraints, especially the neutron EDM bound $|d_n|<1.8\times10^{-26},e\cdot{\rm cm}$, impose the most stringent limits on products of couplings involving the $t\to u$ transition. In particular, the imaginary part of the product $\xi_L^{tu}\xi_R^{tu*}$ must be below $\mathcal{O}(10^{-5})$, translating into branching‑ratio limits $BR(t\to u g)\lesssim10^{-6}$ and $BR(t\to u\gamma)\lesssim10^{-8}$. For the $t\to c$ channels, the combined low‑energy and electroweak data constrain the effective couplings to the $10^{-5}$–$10^{-6}$ level, comparable to current LHC direct search limits ($\sim10^{-4}$). Vector‑type couplings derived from $O_{\varphi q}^{(1,3)}$ and $O_{\varphi u}$ are bounded at $|X_{L,R}^{tq}|\lesssim0.02$ under the assumption of real coefficients.

Correlation matrices are provided, showing that dipole operators mix into scalar operators under RGE, and that EDM limits dominate the constraints on CP‑violating phases. The paper also proposes several benchmark scenarios for future collider studies. For example, if a large CP‑violating phase is present (e.g. $\arg\xi_L^{tu}\approx\pi/2$), upcoming EDM experiments with an order‑of‑magnitude improvement could either discover a signal or further tighten the top‑FCNC parameter space. Conversely, the High‑Luminosity LHC (HL‑LHC) and future hadron colliders (FCC‑hh) are expected to improve direct limits on $t\to c\gamma$ and $t\to cZ$, which, when combined with the indirect constraints, will significantly shrink the allowed SMEFT Wilson‑coefficient region.

In summary, the work demonstrates the power of a global EFT approach that bridges high‑energy collider searches with low‑energy precision measurements. By retaining complex Wilson coefficients, the analysis captures both magnitude and CP‑violating aspects of possible new physics in the top sector. The results provide robust, model‑independent bounds on the relevant SMEFT operators and outline clear pathways—through improved EDM experiments and next‑generation colliders—to further probe or potentially discover top‑quark FCNC phenomena.