# Matrix Element Method at NLO for (anti-)$\mathbf{k_t}$-jet algorithms

## TL;DR

This paper introduces a new method to compute NLO QCD event weights for jet events defined by the anti-$k_t$ algorithm, enabling more precise analysis and application of the Matrix Element Method at the LHC.

## Contribution

It extends existing Monte Carlo tools by allowing NLO weight calculation for individual events, facilitating more accurate event analysis and systematic studies of parton shower effects.

## Key findings

- Demonstrated NLO weight calculation for anti-$k_t$ jet events.
- Applied the method to single top-quark events with POWHEG and Pythia.
- Provided the first systematic study of parton shower effects within the MEM.

## Abstract

In this article, we present a method to calculate a posteriori event weights at next-to-leading-order (NLO) QCD accuracy for a given jet event defined by the (anti-)$k_t$ algorithm relying on the conventional $2\to 1$ recombination. This is an important extension compared to existing Monte-Carlo tools which generate jet events together with the corresponding weight but do not allow one to calculate the weight for a given event. The method can be used to generate unweighted events distributed according to the fixed-order NLO cross section. In addition, the method allows one to calculate NLO accurate weights for events recorded by experiments. The potential of this ability is illustrated by applying the Matrix Element Method (MEM) to single top-quark events generated with POWHEG in combination with Pythia. For the first time, a systematic study of parton shower effects within the MEM is provided. The method is completely general and can be applied to arbitrary LHC processes.