---
author:
- |
    Paolo Torrielli\
    ITPP, EPFL, CH-1015 Lausanne, Switzerland\
    E-mail:
- |
    Stefano Frixione [^1]\
    PH Department, TH Unit, CERN, CH-1211 Geneva 23, Switzerland\
    ITPP, EPFL, CH-1015 Lausanne, Switzerland\
    E-mail:
title: 'Matching NLO QCD computations with PYTHIA using MC@NLO'
---

Introduction\[sec:intro\]
=========================

Perturbation theory offers a systematic way to improve theoretical predictions for any given infrared-safe observable. Depending on whether the expansion parameter in the series is ${\alpha_{{\scriptscriptstyle}S}}$, or is ${\alpha_{{\scriptscriptstyle}S}}$ times a logarithm or a logarithm squared of some function of the kinematics of the hard process, one obtains a fixed-order result or a resummed result, respectively. Cross sections expanded up to a certain order in ${\alpha_{{\scriptscriptstyle}S}}$ and those resummed are relevant to complementary kinematic regions of the phase-space. It is therefore convenient to combine the features of these two expansions, by defining a matched cross section which is equal to the former or to the latter in the appropriate phase-space region.

Fixed-order results are now generally available at the next-to-leading order (NLO), which corresponds to including in the cross section the coefficients of terms of order ${\alpha_{{\scriptscriptstyle}S}}^b$ (the leading order or Born level) and of order ${\alpha_{{\scriptscriptstyle}S}}^{b+1}$. Thanks to the recent and rapid progress in the 