QCD matrix elements and truncated showers

TL;DR

This paper presents an improved method for merging matrix elements with parton showers in QCD, ensuring accurate jet simulations by incorporating flavor-dependent phase space separation and truncated shower algorithms.

Contribution

It introduces a new merging prescription extending the CKKW approach, including a flavor-dependent phase space criterion and a truncated shower algorithm, with applications to QCD processes.

Findings

Preserves logarithmic accuracy of the shower.

Incorporates explicit colour information from matrix elements.

Assesses systematic uncertainties of the method.

Abstract

We derive an improved prescription for the merging of matrix elements with parton showers, extending the CKKW approach. A flavour-dependent phase space separation criterion is proposed. We show that this new method preserves the logarithmic accuracy of the shower, and that the original proposal can be derived from it. One of the main requirements for the method is a truncated shower algorithm. We outline the corresponding Monte Carlo procedures and apply the new prescription to QCD jet production in e+e- collisions and Drell-Yan lepton pair production. Explicit colour information from matrix elements obtained through colour sampling is incorporated in the merging and the influence of different prescriptions to assign colours in the large N_C limit is studied. We assess the systematic uncertainties of the new method.