Beyond the Narrow-Width Limit for Off-Shell and Boosted Differential Top Quark Decays

The standard approaches for describing top quark production and its decay dynamics are currently mostly either based on the narrow-width (NW) limit or on off-shell fixed-order calculations. In this article we present a factorised approach for boosted top quarks that combines the properties of the NW limit and off-shell computations accounting for the dominant off-shell effects in an expansion in $m_t\mathrm{/}Q$ with the hard scattering scale $Q$. We discuss the key ideas of our approach and show some preliminary results at tree-level.