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 \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  
\FeynArt\cite{FeynArt}, \GRACE\cite{GRACE}, \HELAS\cite{HELAS}, 
\MADGRAPH\cite{MADGRAPH}. See also the review \cite{Harlander}.


  \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.
The numerical part is written in \C~ too.
The numerical part has comfortable interface as symbolic part. 


\vskip 0.2cm  
\noindent
The symbolic part of \CompHEP~ lets the user:

\begin{itemize}    
\item  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;
   
\item   generate  Feynman diagrams,  display them, and create the
corresponding \LaTeX~ output;

\item   exclude some diagrams;

\item   generate and display squared Feynman diagrams;

\item   calculate analytical expressions corresponding to squared diagrams
   by using the fast built-in symbolic calculator;
          
\item   save symbolic results corresponding to the squared diagrams
   calculated in the \REDUCE~ and \MATHEMATICA~
% and \FORM~ 
   codes for  further symbolic manipulations;

\item   generate the optimized \Fortran~ and \C~  codes for the squared matrix
   elements for further numerical calculations;

\item   generate the \REDUCE~ and \MATHEMATICA~  codes for 
   the squared matrix elements for further symbolical calculations;

\item   launch the numerical calculations;  
\end{itemize}

\noindent
The numerical part of \CompHEP~ offers to:

\begin{itemize}
\item  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;

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

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

\item   introduce various kinematic cuts.

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


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

\item   perform  a   Monte Carlo phase space  integration by \VEGAS;

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

\item   generate events; 

\item   display distributions in various kinematic variables;

\item    create the graphical and \LaTeX~ outputs for histograms.
\end{itemize}

See the review \cite{Results}  about physical results  produced by means 
of \CompHEP.
