GenEvA (I): A new framework for event generation

TL;DR

GenEvA introduces a novel framework for event generation that seamlessly merges partonic calculations with parton showers using a new phase space concept and a matching scale, improving accuracy and avoiding double-counting.

Contribution

The paper presents a new phase space definition and matching scale in GenEvA, enabling consistent merging of various partonic calculations with parton showers, including NLO and LO results.

Findings

Developed a positive-weight NLO calculation for e+e- -> n jets

Merged NLO, LO, and LL calculations for improved accuracy

Implemented a new phase space generator for event simulation

Abstract

We show how many contemporary issues in event generation can be recast in terms of partonic calculations with a matching scale. This framework is called GenEvA, and a key ingredient is a new notion of phase space which avoids the problem of phase space double-counting by construction and includes a built-in definition of a matching scale. This matching scale can be used to smoothly merge any partonic calculation with a parton shower. The best partonic calculation for a given region of phase space can be determined through physics considerations alone, independent of the algorithmic details of the merging. As an explicit example, we construct a positive-weight partonic calculation for e+e- -> n jets at next-to-leading order (NLO) with leading-logarithmic (LL) resummation. We improve on the NLO/LL result by adding additional higher-multiplicity tree-level (LO) calculations to obtain a merged NLO/LO/LL result. These results are implemented using a new phase space generator introduced in a companion paper [arXiv:0801.4028].