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A General Analysis of Wtb anomalous Couplings

Abstract

We investigate new physics effects on the Wtb effective couplings in a model-independent manner. The new physics effects are summarized as four independent couplings f1Lf_1^L, f1Rf_1^R, f2Lf_2^L and f2Rf_2^R. Using single-top-quark productions and W-helicity fraction measurements at the LHC and Tevatron, we perform a global fit to impose constraints on top quark effective couplings. We introduce a set of parameters x0x_0, xmx_m, xpx_p and x5x_5 to study the correlations among Wtb effective couplings. We show that (i) improving the measurements of Οƒt\sigma_t and ΟƒtW\sigma_{tW} is important in constraining the correlation of (f1R,f2R)(f_1^R,f_2^R) and (f2L,f2R)(f_2^L,f_2^R); (ii) f1Lf_1^L and f2Rf_2^R are anti-correlated, which is sensitive to all the experiments; (iii) f1Rf_1^R and f2Lf_2^L are also anti-correlated, which is sensitive to the W-helicity measurements; (iv) the correlation between f2Lf_2^L and f2Rf_2^R is sensitive to the precision of Οƒt\sigma_t, ΟƒtW\sigma_{tW} and F0F_0 measurements. The effective Wtb couplings are studied in three kinds of new physics models: SU(2)1Γ—SU(2)2Γ—U(1)XSU(2)_1 \times SU(2)_2 \times U(1)_X models, vector-like quark models and Littlest Higgs model with and without T-parity. 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 (MWβ€²M_{W'}) is less than several hundred GeV, but the constraint is loose if MWβ€²>1M_{W'}>1 TeV. 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. We also include the constraints of the oblique T-parameter and Zbb couplings which impose much tighter constraints on the mixing angles. We show that the Wtb coupling constraints become relevant if the precision of single top production cross section measurements could be reduced to 1\% relative to the SM predictions in future.Comment: Chin. Phys. C in pres

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