Constraining anomalous $W tb$ and related SMEFT couplings using low-energy and electroweak precision observables

TL;DR

This work links high-scale SMEFT operators that modify the $Wtb$ vertex to low-energy flavour and electroweak observables by performing tree-level matching, one-loop SMEFT-to-LEFT transitions, and RG evolution across scales. A global fit to FCNC, FCCC, and EWPO data yields tight bounds on the SMEFT coefficients $\mathcal{C}^{\phi q(3)}_{33}$, $\mathcal{C}^{\phi u d}_{33}$, $\mathcal{C}^{* dW}_{33}$, and $\mathcal{C}^{uW}_{33}$, with scale dependence up to $\Lambda=10\,\mathrm{TeV}$ quantified. The analysis translates these bounds into much stronger constraints on the anomalous $Wtb$ couplings than current ATLAS/CMS limits and makes concrete predictions for various top FCNC decays, many of which could be probed at future colliders. The study highlights how distinct observables—neutral meson mixing, radiative and semileptonic decays, invisible modes, and EWPOs—complement each other to constrain heavy new physics affecting the $Wtb$ sector. Overall, the results demonstrate the power of a SMEFT-based, scale-spanning global fit to tightly bound NP scenarios that modify electroweak and flavor interactions of the top quark.

Abstract

We investigate constraints on couplings of Standard Model effective field theory (SMEFT) operators contributing to $Wtb$ effective vertex at tree level. We study the one-loop level impact of these couplings on the low-energy flavour changing charged and neutral current processes and on the electroweak precision observables. We use the available data on these relevant processes to constrain the associated SMEFT/$Wtb$ couplings. Solving the renormalisation group equations, we connect the SMEFT couplings at different scales and use the bounds at low energy to obtain the relevant bounds at the large scale $Λ$. Our findings indicate significantly improved constraints on the couplings compared to existing constraints on $Wtb$ couplings by ATLAS and CMS. Additionally, we predict branching ratios for various top-FCNC processes, which exceed SM expectations by several orders of magnitude but remain within the reach of future colliders. These SMEFT couplings, or anomalous couplings of the effective $Wtb$ vertex, can further constrain different UV-complete and simplified models that generate such interactions at the tree or loop level.

Figures (16)

• Figure 1: Energy scale hierarchy: effective $Wtb$ vertices are matched with the higher dimension SMEFT Lagrangian at scale $\mu_t$; eventually SMEFT Lagrangian is matched with the effective Hamiltonian governing B physics at the EW symmetry scale ($\mu_{EW}$). Wilson coefficients in different scales are connected via appropriate RGEs.
• Figure 2: Feynman diagrams contributing to the process of neutral meson-mixing. The blue square dots refer to the $Wtb$ effective vertex. The quark $d_{j}$ denotes down-type quarks $(s,d)$, which are corresponding to $B_{s}^{0}$ and $B^{0}$ mesons, respectievely.
• Figure 3: Feynman diagrams contributing to radiative process $b \to d_{j} \gamma(g)$, with blue squares being effective $Wtb$ vertex.
• Figure 4: Feynman diagrams contributing to FCNC process $b \to d_{j} (=s,d) \ell \ell$. The blue squares refer to the $Wtb$ effective vertex.
• Figure 5: Feynman diagrams contributing to invisible decay process $b \to s (d) \nu \bar{\nu}$. The blue square dots refer to the $Wtb$ effective vertex.