Probing anomalous Wtb couplings in top pair decays

TL;DR

This paper systematically investigates how non-standard Wtb couplings would manifest in top-quark pair decays at the LHC. It develops and analyzes a broad set of observables—W helicity fractions and their ratios, angular and energy asymmetries, and spin-correlation asymmetries—within an effective Wtb vertex framework, including bottom-mmass effects and CP-conserving phases. The authors provide analytic expressions for helicity fractions, quantify the sensitivity of each observable to anomalous couplings (notably g_R and V_R), and demonstrate that angular observables can yield competitive limits even without full distribution fits, while spin-correlation observables are comparatively robust to Wtb deviations. They also study complex phases and CP-violating observables, identifying when such effects could be detectable and how to disentangle production versus decay contributions, with all results integrated into the TopFit tool for combining multiple measurements.

Abstract

We investigate several quantities, defined in the decays of top quark pairs, which can be used to explore non-standard Wtb interactions. Two new angular asymmetries are introduced in the leptonic decay of top (anti)quarks. Both are very sensitive to anomalous Wtb couplings, and their measurement allows for a precise determination of the W helicity fractions. We also examine other angular and energy asymmetries, the W helicity fractions and their ratios, as well as spin correlation asymmetries, analysing their dependence on anomalous Wtb couplings and identifing the quantities which are most sensitive to them. It is explicitly shown that spin correlation asymmetries are less sensitive to new interactions in the decay of the top quark; therefore, when combined with the measurement of other observables, they can be used to determine the t tbar spin correlation even in the presence of anomalous Wtb couplings. We finally discuss some asymmetries which can be used to test CP violation in t tbar production and complex phases in the effective Wtb vertex.

Figures (9)

• Figure 1: Differential distribution in Eq. (\ref{['ec:dist']}) within the SM, calculated analytically and with a Monte Carlo simulation.
• Figure 2: Dependence of the helicity fractions $F_i = \Gamma_i/\Gamma$ on the anomalous couplings in Eq. (\ref{['ec:lagr']}), in the CP-conserving case.
• Figure 3: Dependence of the helicity ratios $\rho_{R,L} = \Gamma_{R,L}/\Gamma_0$ on the anomalous couplings in Eq. (\ref{['ec:lagr']}), in the CP-conserving case.
• Figure 4: Dependence of the asymmetries $A_+$, $A_-$ and $A_\mathrm{FB}$ on the couplings $g_L$, $g_L$ and $V_R$, for the CP-conserving case.
• Figure 5: Differential distribution in Eq. (\ref{['ec:dist2']}) within the SM, calculated analytically and with a Monte Carlo simulation.