Extending the Matrix Element Method beyond the Born approximation: Calculating event weights at next-to-leading order accuracy

TL;DR

This paper develops a method to compute event weights at NLO accuracy for jet events, enabling the application of the Matrix Element Method beyond the Born approximation, with a focus on top quark mass measurement.

Contribution

It introduces a modified jet recombination procedure allowing NLO event weight calculation, extending the Matrix Element Method beyond leading order.

Findings

Reproduces NLO differential distributions for sample processes.

Demonstrates a significant shift in top quark mass extraction when using NLO versus LO.

Provides a proof of concept for NLO application of the Matrix Element Method.

Abstract

In this article we illustrate how event weights for jet events can be calculated efficiently at next-to-leading order (NLO) accuracy in QCD. This is a crucial prerequisite for the application of the Matrix Element Method in NLO. We modify the recombination procedure used in jet algorithms, to allow a factorisation of the phase space for the real corrections into resolved and unresolved regions. Using an appropriate infrared regulator the latter can be integrated numerically. As illustration, we reproduce differential distributions at NLO for two sample processes. As further application and proof of concept, we apply the Matrix Element Method in NLO accuracy to the mass determination of top quarks produced in e+e- annihilation. This analysis is relevant for a future Linear Collider. We observe a significant shift in the extracted mass depending on whether the Matrix Element Method is used in leading or next-to-leading order.