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Manuscript Title: carlomat: A program for automatic computation of lowest order cross sections
Authors: Karol Kolodziej
Program title: carlomat
Catalogue identifier: AEDQ_v1_0
Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 180(2009)1671
Programming language: Fortran 90/95.
Computer: all.
Operating system: Linux.
Keywords: Lowest order multiparticle reactions, Standard Model, automatic calculation of cross sections, Monte Carlo event generation.
PACS: 12.15.-y, 12.15.Ji, 12.38.-t, 12.38.Bx.
Classification: 4.4, 11.2.

Nature of problem:
Description of two particle scattering reactions with possibly up to 10 particles in the final state with a complete set of the Feynman diagrams in the lowest order of the Standard Model.

Solution method:
The matrix element for a user specified process and phase space parametrizations, which are necessary for the multi-channel Monte Carlo integration of the lowest order cross sections and event generation, are generated automatically. Both the electroweak and quantum chromodynamics lowest order contributions are taken into account. Particle masses are not neglected in the program. Matrix elements are calculated numerically with the helicity amplitude method. Constant widths of unstable particles are implemented by modifying mass parameters in corresponding propagators.

Restrictions:
The number of external particles is limited to 12. Only the Standard Model is implemented at the moment in the program. No higher order effects are taken into account, except for assuming the fine structure constant and the strong coupling at appropriate scale and partial summation of the one particle irreducible loop corrections by introducing fixed widths of unstable particles.

Running time:
Generation of the Fortran code with carlomat on a PC with the Pentium 4 3.0 GHz processor for reactions with 8 particles in the final state relevant for the associated top quark pair and Higgs boson production and decay takes about 10 minutes CPU time. This relatively long time of the code generation is determined by a lot of write to and read from a disk commands which have to be introduced in order to circumvent limitations of the Fortran compilers concerning possible array sizes. The compilation time of generated routines depends strongly on a compiler used and an optimization option chosen. Typically, for the reactions mentioned, it takes about one hour to compile all the routines generated. Most of the time is used for the compilation of the kinematical routines. The execution time of the Monte Carlo (MC) integration with about 2 million calls to the integrand amounts typically to a few hours and, if the MC summing over polarizations is employed, it is dominated by computation of the phase space normalization.