The increasingly high accuracy of top-quark studies at the LHC calls for a
theoretical description of ttˉ production and decay in terms of exact
matrix elements for the full 2→6 process that includes the off-shell
production and the chain decays of ttˉ and tW intermediate states,
together with their quantum interference. Corresponding NLO QCD calculations
matched to parton showers are available for the case of dileptonic channels and
are implemented in the bb4l Monte Carlo generator, which is based on the
resonance-aware POWHEG method. In this paper, we present the first NLOPS
predictions of this kind for the case of semileptonic channels. In this
context, the interplay of off-shell ttˉ+tW production with various other
QCD and electroweak subprocesses that yield the same semileptonic final state
is discussed in detail. On the technical side, we improve the resonance-aware
POWHEG procedure by means of new resonance histories based on matrix elements,
which enable a realistic separation of ttˉ and tW contributions.
Moreover, we introduce a general approach which makes it possible to avoid
certain spurious terms that arise from the perturbative expansion of decay
widths in any off-shell higher-order calculation, and which are large enough to
jeopardise physical finite-width effects. These methods are implemented in a
new version of the bb4l Monte Carlo generator, which is applicable to all
dileptonic and semileptonic channels, and can be extended to fully hadronic
channels. The presented results include a NLOPS comparison of off-shell against
on-shell ttˉ+tW production and decay, where we highlight various
non-trivial aspects related to NLO and parton-shower radiation in leptonic and
hadronic top decays.Comment: 78 pages, 20 figures, 6 table