A general analysis of Wtb anomalous couplings

Abstract: We investigate new physics effects on the Wtb effective couplings in a model-independent framework. The new physics effects can be parametrized by four independent couplings, f₁^L, f₁^R, f₂^L and f₂^R. We further introduce a set of parameters x₀, x_m, x_p and x₅ which exhibit a linear relation to the single top production cross sections. Using recent data for the t-channel single top production cross section σ_t, tW associated production cross section σ_tW, s-channel single top production cross section σ_s, and W-helicity fractions F₀, F_L and F_R collected at the 8 TeV LHC and Tevatron, we perform a global fit to impose constraints on the top quark effective couplings. Our global fitting results show that the top quark effective couplings are strongly correlated. We show that (i) improving the measurements of σ_t and σ_tW is important in constraining the correlation of (f₁^R,f₂^R) and (f₂^L,f₂^R); (ii) f₁^L and f₂^R are anti-correlated, and are sensitive to all the four experiments; (iii) f₁^R and f₂^L are also anti-correlated, and are sensitive to the F₀ and F_L measurements; (iv) the correlation between f₂^L and f₂^R is sensitive to the precision of the σ_t, σ_tW and F₀ measurements. The effective Wtb couplings are studied in three kinds of new physics models: the G(221)=SU(2)₁⊗SU(2)₂⊗U(1)_X models, the vector-like quark models and the Littlest Higgs model with and without T-parity. We show that the Wtb couplings in the left-right model and the un-unified model are sensitive to the ratio of gauge couplings when the new heavy gauge boson's mass (M_W') is less than several hundred GeV, but the constraint is loose if M_W' >1 TeV. Furthermore, the Wtb couplings in vector-like quark models and the Littlest Higgs models are sensitive to the mixing angles of new heavy particles and SM particles.