The Monte Carlo event generator AcerMC versions 2.0 to 3.8 with interfaces to PYTHIA 6.4, HERWIG 6.5 and ARIADNE 4.1

Abstract

The AcerMC Monte Carlo generator is dedicated to the generation of Standard Model background processes which were recognised as critical for the searches at LHC, and generation of which was either unavailable or not straightforward so far. The program itself provides a library of the massive matrix elements (coded by MADGRAPH) and native phase space modules for generation of a set of selected processes. The hard process event can be completed by the initial and the final state radiation, hadronisation and decays through the existing interface with either PYTHIA, HERWIG or ARIADNE event generators and (optionally) TAUOLA and PHOTOS. Interfaces to all these packages are provided in the distribution version. The phase-space generation is based on the multi-channel self-optimising approach using the modified Kajantie–Byckling formalism for phase space construction and further smoothing of the phase space was obtained by using a modified ac-VEGAS algorithm. An additional improvement in the recent versions is the inclusion of the consistent prescription for matching the matrix element calculations with parton showering for a select list of processes. Program summaryProgram title: AcerMC version 3.8Catalogue identifier: ADQQ_v2_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADQQ_v2_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 3853309No. of bytes in distributed program, including test data, etc.: 68045728Distribution format: tar.gzProgramming language: FORTRAN 77 with popular extensions (g77, gfortran).Computer: All running Linux.Operating system: Linux.Classification: 11.2, 11.6.External routines: CERNLIB (http://cernlib.web.cern.ch/cernlib/), LHAPDF (http://lhapdf.hepforge.org/)Catalogue identifier of previous version: ADQQ_v1_0Journal reference of previous version: Comput. Phys. Comm. 149(2003)142Does the new version supersede the previous version?: YesNature of problem: Despite a large repertoire of processes implemented for generation in event generators like PYTHIA [1] or HERWIG [2] a number of background processes, crucial for studying the expected physics of the LHC experiments, is missing. For some of these processes the matrix element expressions are rather lengthy and/or to achieve a reasonable generation efficiency it is necessary to tailor the phase space selection procedure to the dynamics of the process. That is why it is not practical to imagine that any of the above general purpose generators will contain every, or even only observable, processes which will occur at LHC collisions. A more practical solution can be found in a library of dedicated matrix-element-based generators, with the standardised interfaces like that proposed in [3], to the more universal one which is used to complete the event generation.Solution method: The AcerMC EventGenerator provides a library of the matrix-element-based generators for several processes. The initial- and final-state showers, beam remnants and underlying events, fragmentation and remaining decays are supposed to be performed by the other universal generator to which this one is interfaced. We will call it a supervising generator. The interfaces to PYTHIA 6.4, ARIADNE 4.1 and HERWIG 6.5, as such generators, are provided. Provided is also an interface to TAUOLA [4] and PHOTOS [5] packages for τ-lepton decays (including spin correlations treatment) and QED radiations in decays of particles. At present, the following matrix-element-based processes have been implemented: gg,qq̄→tt̄bb̄, qq̄→W(→ℓν)bb̄; qq̄→W(→ℓν)tt̄; gg,qq̄→Z/γ∗(→ℓℓ)bb̄; gg,qq̄→Z/γ∗(→ℓℓ,νν,bb̄)tt̄; complete EW gg,qq̄→(Z/W/γ∗→)tt̄bb̄; gg,qq̄→tt̄tt̄; gg,qq̄→(tt̄→)ff̄bff̄b̄; gg,qq̄→(WWbb→)ff̄ff̄bb̄. Both interfaces allow the use of the LHAPDF/LHAGLUE library of parton density functions. Provided is also a set of control processes: qq̄→W→ℓν; qq̄→Z/γ∗→ℓℓ; gg,qq̄→tt̄ and gg→(tt̄→)WbWb̄;Reasons for new version: Implementation of several new processes and methods.Summary of revisions: Each version added new processes or functionalities, a detailed list is given in the section “Changes since AcerMC 1.0”.Restrictions: The package is optimised for the 14 TeV pp collision simulated in the LHC environment and also works at the achieved LHC energies of 7 TeV and 8 TeV. The consistency between results of the complete generation using PYTHIA 6.4 or HERWIG 6.5 interfaces is technically limited by the different approaches taken in both these generators for evaluating αQCD and αQED couplings and by the different models for fragmentation/hadronisation. For the consistency check, in the AcerMC library contains native coded definitions of the QCD and αQED. Using these native definitions leads to the same total cross-sections both with PYTHIA 6.4 or HERWIG 6.5 interfaces.Additional comments: !!!!! The distribution file for this program is over 67 Mbytes and therefore is not delivered directly when download or Email is requested. Instead an html file giving details of how the program can be obtained is sent. !!!!!Running time: On an PIII 800 MHz PC it amounts to ∼0.05→1.1 events/sec, depending on the choice of process.