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 $f_1^L$, $f_1^R$, $f_2^L$ and $f_2^R$. 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 $x_0$, $x_m$, $x_p$ and
$x_5$ to study the correlations among Wtb effective couplings. We show that (i)
improving the measurements of $\sigma_t$ and $\sigma_{tW}$ is important in
constraining the correlation of $(f_1^R,f_2^R)$ and $(f_2^L,f_2^R)$; (ii)
$f_1^L$ and $f_2^R$ are anti-correlated, which is sensitive to all the
experiments; (iii) $f_1^R$ and $f_2^L$ are also anti-correlated, which is
sensitive to the W-helicity measurements; (iv) the correlation between $f_2^L$
and $f_2^R$ is sensitive to the precision of $\sigma_t$, $\sigma_{tW}$ and
$F_0$ measurements. The effective Wtb couplings are studied in three kinds of
new physics models: $SU(2)_1 \times SU(2)_2 \times U(1)_X$ 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 ($M_{W'}$) is
less than several hundred GeV, but the constraint is loose if $M_{W'}>1$ 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