Precise determination of the Wtb couplings at LHC

TL;DR

The paper analyzes how to precisely constrain Wtb couplings at the LHC by exploiting the lb forward-backward asymmetry in ttbar decays, which is highly sensitive to nonstandard $V_{tb}^R$, $g^L$, and $g^R$ and can reveal one-loop QCD corrections. It compares the FB asymmetry with ttbar spin-correlation observables and finds A_FB to be the most powerful and least dependent on production details. A detailed numerical study with detector effects shows that A_FB can detect loop-induced effects with ~6 sigma significance and constrain $g^R$ more strongly than single-top channels, while spin observables provide complementary information. The results support using A_FB in ttbar events as a robust, model-independent probe of the Wtb vertex at the LHC and benchmark the Tevatron and future linear-collider sensitivities.

Abstract

Top pair production at LHC is the ideal place to search for nonstandard Wtb couplings in t -> W b -> l nu b decays. The lb forward-backward asymmetry in the W rest frame is very sensitive to sigma_{mu nu} couplings, and can spot one-loop QCD corrections to the decay vertex with more than 5 sigma statistical significance. We discuss the potential of this asymmetry to signal nonstandard gamma_mu and sigma_{mu nu} couplings and compare with top-antitop spin correlation asymmetries, which have a lower sensitivity. We also briefly summarise the results for Tevatron.

Figures (2)

• Figure 1: Dependence of $A_\mathrm{FB}$ on $\delta g^R$ (solid line), $\delta g^L$ (dashed line) and $\delta V_{tb}^R$ (dotted line) using the analytical expression.
• Figure 2: Dependence of $A_\mathrm{FB}$ on the top mass for $m_t = 175 \pm 1$ GeV. The thickness of the lines shows the variation for $M_W = 80.33 \pm 0.015$ GeV.