Interleaved Resonance Decays and Electroweak Radiation in Vincia

TL;DR

This paper introduces a unified framework for simulating high-energy particle interactions that interleaves resonance decays with electroweak and QCD processes, improving the treatment of finite-width effects.

Contribution

It presents a novel formalism integrating resonance decays with electroweak radiation within the parton shower, implemented in Vincia for Pythia 8.3.

Findings

Enhanced modeling of resonance finite-width effects.

Explicit inclusion of electroweak bosons in parton showers.

Implementation in Pythia 8.3 with new antenna functions.

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, 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/H) bosons in the Vincia shower module in Pythia 8.3, and study some of the consequences.