ALPGEN, a generator for hard multiparton processes in hadronic collisions

TL;DR

ALPGEN delivers a comprehensive LO event generator for multiparton final states in hadronic collisions, enabling exact matrix-element calculations for a wide array of processes and interfacing with shower programs via the Les Houches format. The code emphasizes a modular structure, efficient phase-space sampling, and careful treatment of flavor, color, and spin correlations, with two running modes: weighted parton-level calculations and unweighted events for subsequent showering with HERWIG or PYTHIA. It covers extensive processes, including W/Z plus heavy quarks, multi-jet production, and Higgs-associated channels, offering configurable PDFs, αs, EW inputs, and scale choices, along with detailed phase-space grids and unweighting procedures. The work provides substantial practical tools for SM background studies at the Tevatron, LHC, and future colliders, along with benchmarks and a pathway for future process additions and Higgs decay implementations.

Abstract

This paper presents a new event generator, ALPGEN, dedicated to the study of multiparton hard processes in hadronic collisions. The code performs, at the leading order in QCD and EW interactions, the calculation of the exact matrix elements for a large set of parton-level processes of interest in the study of the Tevatron and LHC data. The current version of the code describes the following final states: (W -> ffbar') QQbar+ N jets (Q being a heavy quark, and f=l,q), with N<=4; (Z/gamma* -> f fbar)+QQbar+Njets (f=l,nu), with N<=4; (W -> ffbar') + charm + N jets (f=l,q), N<= 5; (W -> f fbar') + N jets (f=l,q) and (Z/gamma* -> f fbar)+ N jets (f=l,nu), with N<=6; nW+mZ+lH+N jets, with n+m+l+N<=8 and N<=3 including all 2-fermion decay modes of W and Z bosons, with spin correlations; Q Qbar+N jets (N<=6) with t -> b f fbar' decays and relative spin correlations included if Q=t; Q Qbar Q' Qbar'+N jets, with Q and Q' heavy quarks (possibly equal) and N<=4; H Q Qbar+N jets (N<=4) with t -> b f fbar' decays and relative spin correlations included if Q=t; N jets, with N<=6. Parton-level events are generated, providing full information on their colour and flavour structure, enabling the evolution of the partons into fully hadronised final states.