Interleaved resonance decays and electroweak radiation in the Vincia parton shower

Abstract

We propose a framework for high-energy interactions in which resonance decays and electroweak branching processes are interleaved with the QCD evolution in a single common sequence of decreasing resolution scales. The interleaved treatment of resonance decays allows for a new treatment of finite-width effects in parton showers. At scales above their offshellness (i.e., typically Q > \GammaQ>Γ), resonances participate explicitly as incoming and outgoing states in branching processes, while they are effectively ``integrated out’’ of the description at lower scales. We implement this formalism, together with a full set of antenna functions for branching processes involving electroweak (W/Z/HW/Z/H) bosons in the Vincia shower module in Pythia 8.3, and study some of the consequences.