  We study the sensitivity of the ATLAS experiment to Wtb anomalous couplings
in top pair production with semileptonic decay, pp -> t tbar -> W+ b W- bbar,
with one of the W bosons decaying leptonically and the other hadronically.
Several observables are examined, including the W helicity fractions and new
quantities recently introduced, such as the ratios of helicity fractions and
some angular asymmetries defined in the W rest frame. The dependence on
anomalous couplings of all these observables has been previously obtained. In
this work we show that some of the new observables also have smaller systematic
uncertainties than the helicity fractions, with a dependence on anomalous
couplings similar or stronger than for helicity fractions. Consequently, their
measurement can significantly improve the limits on anomalous couplings.
Moreover, the most sensitive measurements can be combined. In this case, the
precision achieved in the determination of Wtb anomalous couplings can be of a
few percent in the semileptonic channel alone.
