Eur. Phys. J. C (2012) 72: 2078
https://doi.org/10.1140/epjc/s10052-012-2078-y

Special Article - Tools for Experiment and Theory

Importance of a consistent choice of α _S in the matching of AlpGen and Pythia

¹ Department of Physics & Astronomy, University College London, London, WC1E 6BT, UK
² Deutsches Electron Synchroton, 22603, Hamburg, Germany
³ European Organization for Nuclear Research, CERN, 1211, Geneva 23, Switzerland
⁴ CEA-Saclay, IRFU/SPP, bât. 141, 91191, Gif sur Yvette Cedex, France

^* e-mail: michelangelo.mangano@cern.ch

Received: 7 November 2011
Revised: 21 June 2012
Published online: 31 July 2012

Abstract

We consider the impact of varying α _s choices (and scales) on each side of the so-called “matching scale” in MLM-matched matrix-element + parton-shower predictions of collider observables. We explain how inconsistent prescriptions can lead to counter-intuitive results and present a few explicit examples, focusing mostly on W/Z+jets processes. We give a specific prescription for how to improve the consistency of the matching and also address how to perform consistent tune variations (e.g., of the renormalization scale) around a central choice. Comparisons to several collider processes are included to illustrate the properties of the resulting improved matching, relying on AlpGen + Pythia 6, with the latter using the so-called Perugia 2011 tunes, developed as part of this effort. Our observations, nevertheless, apply to the large class of tools where matrix-element generators are merged with independent codes for the parton-shower evolution.