ATLAS sensitivity to Wtb anomalous couplings in top quark decays

TL;DR

This study evaluates ATLAS sensitivity to anomalous Wtb couplings in $pp\to t\bar t$ with semileptonic decays by analyzing $W$ helicity fractions, helicity ratios, and angular asymmetries in the $W$ rest frame. Using a detailed simulation, detector effects, and a probabilistic event‑discrimination approach, the authors quantify statistical and systematic uncertainties for observables and translate them into limits on $V_R$, $g_L$, and $g_R$. They show that certain observables, notably $\rho_R$ and $A_+$, have smaller systematics and stronger coupling dependence, and that combining $\rho_{R,L}$ with $A_\pm$ yields substantial improvements—achieving percent‑level precision in the semileptonic channel and tightening constraints beyond previous ATLAS studies. However, the observables studied are not sufficient for model‑independent constraints when more than one coupling is nonzero, underscoring the need for additional observables and channels (e.g., dilepton spin asymmetries and single‑top processes). Overall, the work demonstrates a viable path to probing the $Wtb$ vertex with high precision at the LHC and outlines avenues for further enhancement.

Abstract

We study the sensitivity of the ATLAS experiment to Wtb anomalous couplings in top pair production with semileptonic decay, pp -> t tbar -> W+ b W- bbar, with one of the W bosons decaying leptonically and the other hadronically. Several observables are examined, including the W helicity fractions and new quantities recently introduced, such as the ratios of helicity fractions and some angular asymmetries defined in the W rest frame. The dependence on anomalous couplings of all these observables has been previously obtained. In this work we show that some of the new observables also have smaller systematic uncertainties than the helicity fractions, with a dependence on anomalous couplings similar or stronger than for helicity fractions. Consequently, their measurement can significantly improve the limits on anomalous couplings. Moreover, the most sensitive measurements can be combined. In this case, the precision achieved in the determination of Wtb anomalous couplings can be of a few percent in the semileptonic channel alone.

Figures (4)

• Figure 1: Kinematical distributions at the pre-selection level for the transverse momentum of the charged lepton (a), the neutrino (b), the $p_T$ of the two non $b$ jets used in the hadronic $W$ reconstruction (c),(d), the $b$ jet from the hadronic (e) and leptonic (f) top quarks. Invariant mass distributions of the hadronic $W$ boson (g), the hadronic top (h) and the leptonic top (i). The $t\bar{t}$ signal (full line) and the SM backgrounds (shaded region) are normalised to $L=10$ fb$^{-1}$.
• Figure 2: Discriminant variable for the SM background (shaded region) and the $t\bar{t}$ signal (full line), normalised to $L=10$ fb$^{-1}$.
• Figure 3: Simulated $\cos \theta_{\ell}^*$ distribution (a) and its correction function (b). In the first plot the $t\bar{t}$ signal (full line) and the SM backgrounds (shaded region) are normalised to $L=10$ fb$^{-1}$.
• Figure 4: 68.3% CL confidence regions on anomalous couplings: $g_L$ and $g_R$, for $V_R=0$ (a); $V_R$ and $g_R$, for $g_L = 0$ (b). The $1\sigma$ combined limits in Table \ref{['tab:lim2']} are also displayed.