---
author:
- 'Tomáš Ježo,'
- Paolo Nason
title: 'On the Treatment of Resonances in Next-to-Leading Order Calculations Matched to a Parton Shower.'
---

The problem
===========

At present, several methods exist for the computation of next-to-leading order (NLO) corrections in the Standard Model. When strong and/or electromagnetic interactions are present, these calculations must deal properly with collinear and soft divergences, that must cancel when infrared insensitive (IR-safe) observables are computed. The so-called subtraction methods are generally used in order to deal with this problem. In essence, they work as follows. A generic NLO cross section can be written symbolically as $${\mathrm{d}}\sigma = {\mathrm{d}}\Phi_B (B (\Phi_B) + V (\Phi_B)) + {\mathrm{d}}\Phi_R R
  (\Phi_R), \label{eq:basicsigma}$$ where $\Phi_B$ stands for the Born phase space and $\Phi_R$ is the real emission phase space. $B (\Phi_B)$, $V(\Phi_B)$ and $R (\Phi_R)$ represent the Born, Virtual and Real cross section respectively. The real emission process corresponds to the Born process in association with an extra parton.[^1]  For the purpose of this example we assume that we do not have hadrons in the initial state, i.e. we consider processes like $Z$ decays into hadrons. The expression in eq. (\[eq:basicsigma\]), in order to make sense, must be evaluated 