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Main facilities.

CompHEP  is a package for automatic calculations of elementary particle decay and collision properties in the lowest order of perturbation theory (the tree approximation). The main idea prescribed into the CompHEP  is to make available passing on from the Lagrangian to the final distributions effectively with a high level of automation. Other packages created to solve a similar problem are FeynArts/FeynCalc[1], GRACE[2], HELAS[3], MADGRAPH[4]. See also the review [5].

CompHEP  is a menu-driven system with the context help. The notations used in CompHEP  are very similar to those used in particle physics.

The present version has 4 built-in physical models. Two of them are the versions of the Standard Model (SU(3)xSU(2)xU(1)) in the unitary and t'Hooft - Feynman gauges. The user can change particle interaction and model parameters. It is also possible to create new models of particle interaction.

In the present version polarizations are not taken into account. Averaging over initial and summing over final polarizations are performed automatically.

The CompHEP  package consists of two parts: symbolic and numerical. The symbolic part is written in the C  programming language. It produces Fortran  and C  codes for a squared matrix element, and they are used in the numerical calculation later on. There are two versions of the numerical part: one is written in Fortran  and another one is done in C. The facilities of both versions are almost equal. The C  version has more comfortable interface but it does not possess an option to generate events and does not perform calculations with a quadruple precision.

The symbolic part of CompHEP  lets the user:

• select a process by specifying incoming and outgoing particles for the decays of $1 \rightarrow 2, \ldots ,1 \rightarrow 5$ types and the collisions of $2 \rightarrow 2, \ldots , 2 \rightarrow 4$ types;

• generate Feynman diagrams, display them, and create the corresponding L^ATEX  output;

• exclude some diagrams;

• generate and display squared Feynman diagrams;

• calculate analytical expressions corresponding to squared diagrams by using the fast built-in symbolic calculator;

• save symbolic results corresponding to the squared diagrams calculated in the Reduce  and Mathematica  codes for further symbolic manipulations;

• generate the optimized Fortran  and C  codes for the squared matrix elements for further numerical calculations;

• launch the built-in interpreter for numerical calculations;

The numerical part of CompHEP  offers to:

• convolute the squared matrix element with structure functions and beam spectra. CTEQ  and MRS  parton distribution functions, the ISR and Beamstrahlung spectra of electrons, the laser photon spectrum, and the Weizsaecker-Williams photon structure functions are available;

• modify physical parameters (total energy, charges, masses etc.) involved in the process;

• select the scale parameter for evaluation of the QCD coupling constant and parton structure functions;

• introduce various kinematic cuts.

• define the kinematic scheme (phase space parameterization) for effective Monte Carlo integration;

• introduce a phase space mapping in order to smooth sharp peaks of a squared matrix element and structure functions;

• perform a Monte Carlo phase space integration by Vegas;

• store values of the calculated matrix element in a file for subsequent event generation;

• generate events;

• display distributions in various kinematic variables;

• create the graphical and L^ATEX  outputs for histograms.

See the review [6] about physical results produced by means of CompHEP.