Leif Lönnblad

High-Energy-Physics Event Generation with PYTHIA 6.1

Pythia version 6 represents a merger of the Pythia 5, Jetset 7 and SPythia programs, with many improvements. It can be used to generate high-energy-physics ‘events’, i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions. The underlying physics is not understood well enough to give an exact description; the programs therefore contain a combination of analytical results and various models. The emphasis in this article is on new aspects, but a few words of general introduction are included. Further documentation is available on the web.

Ariadne version 4 — A program for simulation of QDC cascades implementing the colour dipole model

Abstract The fourth version of the Ariadne program for generating QCD cascades in the colour dipole approximation is presented. The underlying physics issues are discussed and a manual for using the program is given together with a few sample programs. The major changes from previous versions are the introduction of photon radiation from quarks and inclusion of interfaces to the LEPTO and PYTHIA programs.

Comparative study of various algorithms for the merging of parton showers and matrix elements in hadronic collisions

We compare different procedures for combining fixed-order tree-level matrix-element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW and MLM schemes using different matrix-element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.

Rivet user manual

This is the manual and user guide for the Rivet system for the validation and tuning of Monte Carlo event generators. As well as the core Rivet library, this manual describes the usage of the rivet program and the AGILe generator interface library. The depth and level of description is chosen for users of the system, starting with the basics of using validation code written by others, and then covering sufficient details to write new Rivet analyses and calculational components. Program summary Program title: Rivet Catalogue identifier: AEPS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEPS_v1_0.html Program obtainable from: CPC Program Library, Queens University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 571126 No. of bytes in distributed program, including test data, etc.: 4717522 Distribution format: tar.gz Programming language: C++, Python. Computer: PC running Linux, Mac. Operating system: Linux, Mac OS. RAM: 20 MB Classification: 11.9, 11.2. External routines: HepMC (https://savannah.cern.ch/projects/hepmc/), GSL (http://www.gnu.org/software/gsl/manual/gsl-ref.html), FastJet (http://fastjet.fr/), Python (http://www.python.org/), Swig (http://www.swig.org/), Boost (http://www.boostsoftware.com/), YAML (http://www.yaml.org/spec/1.2/spec.html) Nature of problem: Experimental measurements from high-energy particle colliders should be defined and stored in a general framework such that it is simple to compare theory predictions to them. Rivet is such a framework, and contains at the same time a large collection of existing measurements. Solution method: Rivet is based on HepMC events, a standardised output format provided by many theory simulation tools. Events are processed by Rivet to generate histograms for the requested list of analyses, incorporating all experimental phase space cuts and histogram definitions. Restrictions: Cannot calculate statistical errors for correlated events as they appear in NLO calculations. Unusual features: It is possible for the user to implement and use their own custom analysis as a module without having to modify the main Rivet code/installation. Running time: Depends on the number and complexity of analyses being applied, but typically a few hundred events per second

General-purpose event generators for LHC physics

We review the physics basis, main features and use of general-purpose Monte Carlo event generators for the simulation of proton-proton collisions at the Large Hadron Collider. Topics included are: the generation of hardscattering matrix elements for processes of interest, at both leading and nextto-leading QCD perturbative order; their matching to approximate treatments of higher orders based on the showering approximation; the parton and dipole shower formulations; parton distribution functions for event generators; non-perturbative aspects such as soft QCD collisions, the underlying event and diractive processes; the string and cluster models for hadron formation; the treatment of hadron and tau decays; the inclusion of QED radiation and beyond-Standard-Model processes. We describe the principal features of the Ariadne, Herwig++, Pythia 8 and Sherpa generators, together with the Rivet and Professor validation and tuning tools, and discuss the physics philosophy behind the proper use of these generators and tools. This review is aimed at phenomenologists wishing to understand better how parton-level predictions are translated into hadron-level events as well as experimentalists wanting a deeper insight into the tools available for signal and background simulation at the LHC.

Correcting the colour-dipole cascade model with fixed order matrix elements

An algorithm is presented in which the Colour-Dipole Cascade Model as implemented in the ARIADNE program is corrected to match the fixed order tree-level matrix elements for e+e− → n jets. The result is a full parton level generator for e+e− annihilation where the generated states are correct on tree-level to fixed order in αs and to all orders with modified leading logarithmic (MLLA) accuracy. In addition, virtual corrections are taken into account to all orders with MLLA accuracy. In this paper, matrix elements are used up to second order in αs, but the scheme is applicable also for higher orders. An improvement to also include exact virtual corrections to fixed order is suggested and the possibility to extend the scheme to hadronic collisions is discussed.An algorithm is presented in which the Colour-Dipole Cascade Model as implemented in the ARIADNE program is corrected to match the fixed order tree-level matrix elements for e?e+?n jets. The result is a full parton level generator for e?e+ annihilation where the generated states are correct on tree-level to fixed order in ?s and to all orders with modified leading logarithmic (MLLA) accuracy. In this paper, matrix elements are used up to second order in ?s, but the scheme is applicable also for higher orders. A strategy for also including exact virtual corrections to fixed order is suggested and the possibility to extend the scheme to hadronic collisions is discussed.An algorithm is presented in which the Colour-Dipole Cascade Model as implemented in the Ariadne program is corrected to match the fixed order tree-level matrix elements for e e(+) --> n jets. The result is a full parton level generator for e e(+) annihilation where the generated states are correct on tree-level to fixed order in alpha(s) and to all orders with modified leading logarithmic (MLLA) accuracy. In this paper, matrix elements are used up to second order in alpha(s), but the scheme is applicable also for higher orders. A strategy for also including exact virtual corrections to fixed order is suggested and the possibility to extend the scheme to hadronic collisions is discussed.

Extending CKKW-merging to one-loop matrix elements

We extend earlier schemes for merging tree-level matrix elements with parton showers to include also merging with one-loop matrix elements. In this paper we make a first study on how to include one-loop corrections, not only for events with a given jet multiplicity, but simultaneously for several different jet multiplicities. Results are presented for the simplest non-trivial case of hadronic events at LEP as a proof-of-concept.

JETNET 3.0—A versatile artificial neural network package

An F77 package for feed-forward artificial neural network data processing, JETNET 3.0, is presented. It represents a substantial extension and generalization of an earlier release, JETNET 2.0. The package, which consists of a set of subroutines, is focused on multilayer perceptron architectures. As compared to earlier versions it contains a variety of minimization options, measures for monitoring the learning process, limited precision emulation, etc. Also, the reader is provided with a set of guidelines for when to use the different options.

Matching parton showers and matrix elements

We compare different procedures for combining fixed-order tree-level matrix element generators with parton showers. We use the case of W-production at the Tevatron and the LHC to compare different implementations of the so-called CKKW scheme and one based on the so-called MLM scheme using different matrix element generators and different parton cascades. We find that although similar results are obtained in all cases, there are important differences.

Merging multi-leg NLO matrix elements with parton showers

A bstractWe discuss extensions of multi-jet matrix element and parton shower merging approaches, to also include next-to-leading order accuracy. Specifically, we generalise the so-called CKKW-L prescription and the recently developed unitarised matrix element + parton shower (UMEPS) scheme. Endowing tree-level merging methods with NLO corrections greatly enhances the perturbative accuracy of parton shower Monte Carlo programs.To generalise the CKKW-L approach, we augment the Nils-Lavesson-Leif-Lönnblad (NL3) scheme, which was previously developed for e+e−-annihilation, with a careful treatment of parton densities. This makes the application of the NL3 method to hadronic collisions possible. NL3 is further updated to use for more readily accessible next-to-leading order input calculations.We also extend the UMEPS scheme to NLO accuracy. The resulting approach, dubbed unitarised next-to-leading order + parton shower (UNLOPS) merging, does not inherit problematic unitarity-breaking features of CKKW-L, and thus allows for a theoretically more appealing definition of NLO order merging.Both schemes have been implemented in PYTHIA8. We present results for the merging of W- and Higgs-production events, where the zero- and one-jet contribution are corrected to next-to-leading order simultaneously, and higher jet multiplicities are described by tree-level matrix elements. We find that NL3 and UNLOPS yield a very similar description for W production. For Higgs production however, UNLOPS produces more stable results.The implementation of the NLO merging procedures is completely general and can be used for higher jet multiplicities and other processes, subject to the availability of programs able to correctly generate the corresponding partonic states to leading and next-to-leading order accuracy.