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Manuscript Title: The Monte Carlo event generator AcerMC version 1.0 with interfaces to PYTHIA 6.2 and HERWIG 6.3.
Authors: B.P. Kersevan, E. Richter-Was
Program title: AcerMC version 1.0
Catalogue identifier: ADQQ_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 149(2003)142
Programming language: Fortran.
Computer: PIII 800 MHz PC.
Operating system: Linux.
Keywords: Standard Model backgrounds at LHC, Massive matrix elements, Monte Carlo generator, Heavy flavor production, Multi-channel phase space generation, Elementary particle physics.
Classification: 11.2.

Nature 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 expected physics potential 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 Event Generator itself provides library of the matrix-element-based generators for a few example 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 supervising generator. The interfaces to PYTHIA 6.2 and HERWIG 6.3, as such generators, are provided. At present, the following matrix-element-based processes have been implemented: gg, qqbar -> ttbarbbbar, qqbar -> W(-> lnu)bbbar; qqbar -> W(->lnu)ttbar, gg, qqbar -> Z/gamma*(-> ll)bbbar; gg, qqbar -> Z/gamma*(-> ll, nunu, bbbar)ttbar and complete EW gg -> (Z/W/gamma* ->)ttbarbbbar. Both interfaces allow the use of the PDFLIB library of parton density functions.
The program uses CERNLIB library.

Restrictions:
The package is dedicated for the 14 TeV pp collision simulated in the LHC environment. In particular, using it for the 2 TeV ppbar collision although technically possible might not be efficient and would require dedicated optimisation of the phase space generation. The consistency between results of the complete generation using PYTHIA 6.2 or HERWIG 6.3 interfaces is technically limited by the different approaches taken in both these generators for evaluating alphaQCD and alphaQED couplings and by the different models for fragmentation/hadronisation. For the consistency check, in the AcerMC library contains coded definitions of the alphaQCD and alphaQED. Using these native definitions leads to the same total cross-sections both with PYTHIA 6.2 or HERWIG 6.3 interfaces.

Running time:
On a PIII 800 MHz PC it amounts to ~0.05 -> 1.1 events/sec, depending on the choice of process.

References:
[1] T. Sjostrand et al., High energy physics generation with PYTHIA 6.2, reprint hep-ph/0108264, LU-TP 01-21, August 2001.
[2] G. Marchesini et al., Comp. Phys. Commun. 67 (1992) 465, G. Corcella et al., JHEP 0101 (2001) 010.
[3] E. Boos et al., Generic user process interface for event generators, hep-ph/0109068.