Speaker
Description
We present a Monte Carlo generator for off-shell top-quark pair production and decay in the semileptonic channel. The generator takes form of an extension to a generator which implements $t\bar{t}$ production in the dileptonic channel in terms of the exact matrix elements for $pp\to \ell^+\nu_{\ell}\, l^-\bar{\nu}_{l}b\bar{b}$ at order $\alpha^4 \alpha_s^2$ plus full NLO QCD corrections. The extension transforms existing dileptonic events into semileptonic ones by means of the matrix elements for $pp\to \ell^{\pm}{\nu} q \bar{q} b\bar{b}(+g)$ in a $W^+W^- b\bar{b}$ double-pole approximation. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements with massive $b$ quarks. Thus, the contributions from $t\bar t$ and $Wt$ single-top production, plus contributions without top resonances and all relevant quantum interferences between different channels are fully included. Matrix elements are matched to the Pythia8 parton shower using the improved resonance-aware POWHEG method within the POWHEG BOX RES framework. These theoretical improvements are especially important for the interpretation of precision measurements of the top-quark mass, for single-top analyses in the $Wt$ channel, and for $t\bar t$ and $Wt$ backgrounds in the presence of jet vetoes or cuts that enhance off-shell effects.
