===== Main facilities of CompHEP =====

CompHEP is a package for automatic calculations of elementary particle decays and collision processes in the lowest order of perturbation theory (the tree level approximation). The main idea behind the program is to make calculations and data manipulations from Lagrangians to final distributions with a high level of automation. There are several packages created to solve the problem in a similar way: FeynArts/FeynCalc, GRACE, HELAS, MadGraph.
This page describes CompHEP 4.6.
CompHEP is a GUI computer system with context help. A user makes all manupulations by means of graphical menus.

The notations used in CompHEP are very similar to those used in particle physics.

CompHEP is based on an idea of the physical model. ComHEP Models are very simular to physical models in high energy physics, like Standards Model or MSSM. The present version of the program has several built-in physical models (see later). Two of them are versions of the Standard Model (SU(3)xSU(2)xU(1)) in the unitary and t'Hooft - Feynman gauges. Users can change particle content of a model, interaction vertices, and other model parameters. It is also possible to create a new model of particle interaction.

The present version does not take into account separate polarization states of particles. All results are presented with averaged initial and summed final polarization states.

The CompHEP package consists of two parts, symbolic and numerical programs. The symbolic part is written in the C programming language. It produces C codes for squared matrix elements, and they are used in the numerical calculation later on.

The symbolic part of CompHEP has the following possibilities:

    * 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 $2 \rightarrow 2, \ldots , 2 \rightarrow 6$ types;
    * generate Feynman diagrams, display them, and create the corresponding LATEX 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, Mathematica and FORM codes for further symbolic manipulations;
    * generate the optimized C codes for the squared matrix elements for further numerical calculations;
    * launch the make program in order to prepare the numerical part; 

The numerical part of CompHEP offers to:

    * convolute the squared matrix element with structure functions and beam spectra: built-in CTEQ and LHAPDF 6 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, particle masses, charges, etc.) involved in the process;
    * select an evaluation scale for the QCD coupling constant and parton structure functions;
    * introduce various kinematic cuts. Some cuts are built in and there is a special code to define more involved cuts;
    * define a kinematic scheme (a phase space parameterization) for effective Monte Carlo integration;
    * introduce a phase space mapping in order to smooth sharp peaks of squared matrix elements and structure functions;
    * perform Monte-Carlo phase space integration by Vegas;
    * calculate all subprocesses of a process automatically, also in parallel;
    * scan cross sections and widths over model parameters;
    * use complex form factors, particle widths as functions of model parameters, and diagrams multiplied by user-defined functions;
    * generate and mix events for all subprocesses in the Les Houches Event File format (LHEF);
    * display distributions of various kinematic variables;
    * create graphical and LATEX outputs for the histograms;
    * create one-, two- and three-dimensional plots in ROOT format, and build confidence regions in the model parameter space with a χ² analysis.

All models use PDG 2024 physical constants. Results are cross-validated against MadGraph5 and WHIZARD.
See the paper on [[https://doi.org/10.3103/S0027134925702698|CompHEP 4.6]] and the [[https://gitverse.ru/comphep/comphep/content/master/README.md|README]] for details.

===== CompHEP Models. =====

A model is identified by its name, which is the first line of its files in the [[https://gitverse.ru/comphep/comphep/content/master/models|models directory]] and the name shown in the CompHEP menu. The first ten models have kept their numbers since version 4.2, but numbers above 10 can differ between versions and installations, so please refer to a model by its name.

The current release includes these models:

    * ''QED''
    * ''Effective 4-fermion'' (Fermi Model)
    * ''SM, unitary gauge''
    * ''SM, Feynman gauge''
    * ''MSSM, unitary gauge''
    * ''MSSM, Feynman gauge''
    * ''SUGRA, Feynman gauge''
    * ''GMSB, Feynman gauge''
    * ''_SM_ud'': Standard Model, with 2 quark generations unified (see more details in the [[http://arxiv.org/abs/hep-ph/0004194|paper]])
    * ''_SM_qQ'': QCD with 4 quarks unified (see more details in the [[http://arxiv.org/abs/hep-ph/0004194|paper]])
    * ''_H_R_DM'': Higgs-radion mixing with a dark matter fermion ''hi'' and effective couplings of H and R to gluons and photons ([[https://arxiv.org/abs/1505.05892|paper]])
    * ''_H_R_simple1'': simplified variant of ''_H_R_DM'' with free couplings ''CH'', ''CR'' and widths ''wH'', ''wR''
    * ''_H_R_simple2'': the same as ''_H_R_simple1'', without the loop form factors
    * ''_H_loop'': Higgs loop model, with effective Hgg and Hγγ form factors

Models outside the distribution are collected in the [[:comphep:extra:model:main|Community model library]].

See [[https://theory.sinp.msu.ru/~semenov/mssm.html|here]] more details about MSSM models.

===== LanHEP overview =====

The LanHEP program for Feynman rules generation in momentum representation is presented. It reads the Lagrangian written in the compact form close to one used in publications. It means that Lagrangian terms can be written with summation over indices of broken symmetries and using special symbols for complicated expressions, such as covariant derivative and strength tensor for gauge fields. The output is Feynman rules in terms of physical fields and independent parameters. This output can be written in LaTeX format and in the form of CompHEP model files, which allows one to start calculations of processes in the new physical model. Although this job is rather straightforward and can be done manually, it requires careful calculations and in the modern theories with many particles and vertices can lead to errors and misprints. The program allows one to introduce into CompHEP new gauge theories as well as various anomalous terms.

More details see [[http://theory.sinp.msu.ru/~semenov/lanhep.html|LanHEP]] home page

===== Passing events to other generators =====

CompHEP writes events in the Les Houches Event File format (LHEF), which shower generators such as PYTHIA and HERWIG read directly. The former interfaces to PYTHIA and HERWIG (cpyth, comphep-interfaces) are obsolete and are kept on the [[:comphep:archive|archive page]].

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