Single top quark production at LHC with anomalous Wtb couplings

TL;DR

This paper develops a gauge-invariant EFT framework to study single top quark production at the LHC in the presence of anomalous Wtb couplings. It shows that, due to gauge invariance, only the on-shell-like gamma^mu and sigma^{mu u} q_nu structures are relevant for production and decay, with off-shell contributions canceling against quartic gWtb terms, reducing the parameter set. The three single-top channels are treated with process-specific κ factors that encode the dependence on anomalous couplings, and careful matching/subtraction schemes are used to define consistent cross sections. By combining cross-section measurements with top-decay observables (A±, ρRx, and r_bl), the work demonstrates substantial improvements in limits on V_tb and anomalous couplings, highlighting the important role of decay observables in breaking parameter degeneracies. The results provide a concrete, detector-aware strategy for constraining Wtb couplings at the LHC and show that a precise determination of V_tb is achievable alongside stringent bounds on new physics in the Wtb vertex.

Abstract

We investigate single top production in the presence of anomalous Wtb couplings. We explicitly show that, if these couplings arise from gauge invariant effective operators, the only relevant couplings for single top production and decay are the usual gamma^mu and sigma^mu nu q_nu terms, where q is the W boson momentum. This happens even in the single top production processes where the Wtb interaction involves off-shell top and/or bottom quarks. With this parameterisation for the Wtb vertex, we obtain expressions for the dependence on anomalous couplings of the single top cross sections, for (i) the t-channel process, performing a matching between tj and t bbar j production, where j is a light jet; (ii) s-channel t bbar production; (iii) associated tW- production, including the correction from tW- bbar. We use these expressions to estimate, with a fast detector simulation, the simultaneous limits which the measurement of single top cross sections at LHC will set on V_tb and possible anomalous couplings. Finally, a combination with top decay asymmetries and angular distributions is performed, showing how the limits can be improved when the latter are included in a global fit to Wtb couplings.

Figures (13)

• Figure 1: Cancellation between contributions to the $gb \to tW$ amplitude. The gray circles in the diagrams stand for: (i) a $Wtb$ interaction through $\mathcal{O}_1$ in the first and second diagrams, and a $gWtb$ interaction through $\mathcal{O}_1^{(4)}$ in the third one; or (ii) the same involving $\mathcal{O}_2$ and $\mathcal{O}_2^{(4)}$.
• Figure 2: Sample Feynman diagrams for single top production in the $t$-channel process. Additional diagrams are obtained by crossing the light quark fermion line, and/or replacing $(u,d)$ by $(c,s)$. The diagrams for antitop production are the charge conjugate ones.
• Figure 3: Normalised transverse momentum distribution for the $\bar{b}$ quark in the $t$-channel process, when it is generated from the hard process or from ISR in the parton-shower Monte Carlo.
• Figure 4: Feynman diagram for single top production in the $t \bar{b}$ process. An additional diagram is obtained replacing $(u,d)$ by $(c,s)$. For antitop production the diagrams are the charge conjugate ones.
• Figure 5: Feynman diagrams for single top production in the $gb \to tW^-$ process.