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{\Large \bf CompHEP-interfaces package: a short manual}

\vspace*{1cm}
\textsc{E.~Boos$^a$, V.~Bunichev$^a$, M.~Dubinin$^a$, L.~Dudko$^a$, V.~Ilyin$^a$, 
A.~Kryukov$^a$, V.~Edneral$^a$, V.~Savrin$^a$, A.~Semenov$^a$ and A. Sherstnev$^b$\\
(the CompHEP collaboration)} \\

\vspace*{0.5cm} $^a$Skobeltsyn Institute of Nuclear Physics, Moscow
State University, 119992 Moscow, Russia\\
$^b$ Cavendish Laboratory, University of Cambridge, 
\\ JJ Thomson Avenue, Cambridge, CB3 0HE, UK\\ and Moscow State University, Moscow, Russia (on leave)\\
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\vspace*{0.5cm}

\begin{abstract}
This short manual describes a toolkit for passing event generated by CompHEP, as a 
partonic event generator, to PYTHIA or HERWIG, as a generator of final states of 
detectable objects. Thus, the tool provides simulation of (almost) arbitrary 
collision processes at the level of detectable particles. Exact (multiparticle) 
matrix elements, convolution with structure functions, decays, partons hadronization 
and (optionally) parton shower evolution are basic stages of calculations. 
\end{abstract}

\vspace{1cm}

The text is in preparation. 

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%%---------------------------------------------------------------- 
%
%In the widely used generators PYTHIA \cite{pythia}, ISAJET \cite{isajet}
%and HERWIG \cite{herwig} data bases of matrix elements of hard
%subprocesses are built in. It means that matrix elements are stored as
%formulas. Furthermore, the matrix element squared $|M|^2$ is represented
%by means of some function modelling the behaviour of the integrand to get
%effective Monte-Carlo integration and events generation. Thus, as one can
%see, mainly $2\to 2$ subprocesses are included in these data bases.
%
%However, the generation of events with 3, 4 and more bodies in the final
%states of hard subprocesses is needed for the Tevatron, LHC and future
%linear collider physics. One can note, in particular, that for such
%states there is no possibility to construct simple analytical formulae to
%match singular behaviour of $|M|^2$. Multidimensional phase space (4
%dimensions in the 3-body case plus 2 dimensions in case of hadron
%collisons for convilution with parton distributions, 7+2 dimensions in
%the 4-body case etc) with untrivial regions corresponding to the
%singularities of the martrix element leads to complicated symbolic
%structures. Thus, one needs a new approach to the generation of events at
%the partonic level.
%
%Partonic level final states with top quarks, Higgs bosons and
%intermediate vector bosons, like $Wtj$, $ttH$, $Wbb$, $ttbb$ and $tttt$,
%give practical examples of multidimensional phase space integration.
%Then, multiple production of light quarks, gluons, leptons and photons
%also assumes the evaluation of multiparticle final states if one is
%interested in effects at high $p_T$, large invariant masses etc. This
%problem stands as an important one especially if one should evaluate
%precisely background processes.
%
%The singular behaviour of the phase space integrand is connected with
%singular behaviour of propagators in Feynman diagrams: some masses are
%extremely small (light quarks and leptons) or even zero (gluons and
%photons), while other masses are of order 100 GeV ($M_W$, $M_Z$,
%$M_{top}$ $\ldots$) and collison energies are of hundreds or even
%thousands of GeV. As a result, huge energy scale difference for the
%parameters involved produces serious computational problems. One has to
%regularize the integration measure to smooth the singularities (see,
%e.g., the discussion in \cite{sings}).
%
%To get more partons in final state one can exploit, for example, the QCD
%{\it parton-shower} generation. However, this method is good only for
%soft regimes (small $p_T$, small angles etc.) and fails in case of hard
%production of these extra partons.
%
%\vspace{0.5cm}
%
%We propose to use programs created for automatic computation of matrix
%elements, like CompHEP \cite{comphep}, GRACE \cite{grace} and MADGRAPH
%\cite{madgraph}\footnote{see also programs presented on this Workshop by
%T.~Ohl and C.~Papadopoulos \cite{Ohl,HELAC}.} as a tool for generation of
%data base of hard subprocesses for generators like PYTHIA, ISAJET and
%HERWIG.  In particular, at the step of the evaluation of hard subprocess
%the phase space grid is adapted to match a singular behaviour of $|M|^2$.
%So, we develop the {\it two stage} approach:
%
%\begin{enumerate}
%\item 
%CompHEP produces cross sections and proper phase space grid. This
%information is stored in special data base, {\bf PEVLIB}, on the hard 
%disk;
%
%\item 
%events generated by CompHEP are used as an input to generators PYTHIA,
%ISAJET and HERWIG, for further decaying and hadronization of final
%partonic states.
%
%\end{enumerate}
%
%In this paper we present the interface to provide an automatic input of
%CompHEP events to PYTHIA. This interface is under the development still
%and new options are assumed to be realized, in particular, automatic
%addition of new events in the data base if the number of events already
%stored in the existing sample is not enough, and the regeneration of
%events in case of change of physical parameter set.
%
%Some comments why we propose to separate the {\it matrix element} and
%{\it decay/showers/hadroni\-zation} steps of the computations:
%
%\begin{itemize}
%\item 
%it is easy to automate the interface;
%
%\item 
%more flexibility of the computation model is reached:
%   \begin{itemize}
%   
%   \item 
%   it allows to develop/implement new options in these two steps
%   independently (what corresponds to the standard theoretical approach: 
%   at the "matrix element" 
%   step quantum effects are evaluated, interference etc, while the second
%   step corresponds to the probability processes);
%   
%   \item 
%   it gives a possibility to input partonic events in different
%   programs for the second step (PYTHIA, ISAJET, HERWIG etc), as well as to
%   create partonic
%   events by different programs, e.g. CompHEP, GRACE, MADGRAPH
%   etc. Of course the standardization of the partonic event files
%   is necessary.
%   \end{itemize}
%\end{itemize}
%
%\vspace{1cm}
%
%In PYTHIA we use the subroutine {\sf PYUPEV} to input CompHEP events as
%an external process.
%
%CompHEP generates events (unweighted in v.41) and writes them to the file
%
%\begin{center}
%\sf events\_N.txt
%\end{center}
%where {\sf N} is the number of working session. This is the text file with
%a header, where information about the subprocess is given, cross section 
%value is written, and some information about the beams is presented (in
%particular what PDF set was used and what is the QCD scale). Then, events
%are written (one event - one line).
%
%Then, a command
%
%\begin{center}
%\sf mixPEV 
%\end{center}
%is used to mix several subprocesses in one event flow according to their
%relative weight, $\sigma_i/\Sigma_j\sigma_j$, where $\sigma_i$ is the
%cross section of the $i$-th subprocess. This command, {\sf mixPEV},
%randomizes also the position of events from different subprocesses in the
%final event flow.
%
%As a result of the command {\sf mixPEV} the file {\sf Mixed.PEV} with the
%events from mixed subprocesses is created. In this file headers of all
%subprocesses contributed are listed in the beginning and then events are
%written. In the end of each line (event) the information about color
%flows is given, allowing the user to switch on, for example, Lund
%Fragmentation Model or other models using color flows in the $N_c\to
%\infty$ limit. This new option is realized in CompHEP v.41. The event
%line includes also: the number of subprocess, to which the event belongs,
%and components of the particles momenta (for in-coming particles only
%their z-components).
%
%When the command {\sf mixPEV} is completed the protocol file, {\sf
%Prt.PEV}, is created.
%
%\vspace{1cm}
%
%The CompHEP code is available from the following addresses:
%
%\begin{center}
%\sf http://theory.npi.msu.su/comphep/ 
%\end{center}
%\begin{center}
%/afs/cern.ch/cms/physics/COMPHEP/V\_41.10.tar.gz
%\end{center}
%
%The CompHEP-PYTHIA interface code and command {\sf mixPEV} are available
%from:
%
%\begin{center} /afs/cern.ch/cms/physics/PEVLIB/cpyth \end{center}
%where one can find the Fortran code in directory {\sf interf45} (for
%interface with PYTHIA 5.7/JETSET 7.4) and {\sf interf46} for interface
%with PYTHIA v.6.x. The code for the command {\sf mixPEV} is placed in the
%directory {\sf c\_source} (the {\sf mix.c} file).
%
%One should compile Fortran routine from the directory {\sf interf46} (or
%correspondingly from {\sf interf45}) and link with the PYTHIA object
%code. For some platforms a user should comment the dummy routine {\sf
%PYUPEV} in the original PYTHIA code.
%
%For more details on the PYTHIA switches and other comments we refer the
%reader to the file {\sf main.f}, where many comment lines are given with
%corresponding explanations.
%
%\vspace{1cm}
%
%Using the CompHEP-PYTHIA interface we create the PEVLIB library of
%partonic events for LHC processes. Each process is stored in the
%subdirectory
%\begin{center} /afs/cern.ch/cms/physics/PEVLIB/ \end{center}
%
%In each process subdirectory there is the {\sf README} file where some
%details are given concerning the partonic events computed.
%
%Among processes already stored in PEVLIB are the following:
%\begin{itemize}
%
%\item 
%{\sf SingleTop} (see details about the number of events generated and
%subprocesses in the corresponding {\sf README} file);
%
%\item 
%$(H\to \tau^+\tau^-)+2tagjets$. Here electroweak background events can be 
%found (about $10^5$ with $Z$ off-shell and $4\cdot 10^5$ with $Z$ on-shell), 
%and QCD background (about 65000 events);
%
%\item 
%$(H\to b\bar b)+t\bar t$. Here signal events are generated for $M_H=100$, 
%$115$, $120$ and $130$ GeV, as well as the events for background processes 
%$t\bar t b\bar b$ (about 300000 events) and $t\bar t +2jets$ (about 2 
%millions events);
%\end{itemize}
%
%\vspace{1cm}
%This work was supported by the INTAS-CERN 99-0377 grant and by the
%Programme "University of Russia" (grant 990588), and done in the
%framework of the CPP Collaboration \cite{CPP}.
%
%
%%----------------------------------------------------
%\nocite{*}
%\bibliographystyle{aipproc}
%\begin{thebibliography}{99}
%
%\bibitem{pythia} 
%T.~Sjostrand, Comput. Phys. Comm. {\bf 82} (1994) 74.
%
%\bibitem{isajet}  
%F.E.Paige et al., {\it ISAJET 7.40: a Monte Carlo event generator for $pp$,
%$\bar p p$, and $e^+e^-$ reactions}, BNL-HET-98-39, Oct 1998,
%hep-ph/9810440.
%
%\bibitem{herwig}  
%G.Marchesini  et al., {\it HERWIG VERSION 5.9.}, hep-ph/9607393.
%
%\bibitem{sings} 
%V.A.~Ilyin, D.N.~Kovalenko and A.E.~Pukhov, Int. J. Mod. Phys. 
%{\bf C7} (1996) 761.
%
%D.~Kovalenko and A.~Pukhov,  Nucl. Instr. and Meth. {\bf A389} (1997) 299.
%
%\bibitem{comphep}
%A.~Pukhov et al, {\it CompHEP - a package for evaluation of Feynman
%diagrams  and integration over multi-particle phase space. User's manual
%for version 33}, Preprint INP MSU 98-41/542, hep-ph/9908288.
%
%\bibitem{grace}  
%T.~Ishikawa et al., {\it GRACE manual}, KEK Report 92-19, February 1993.
%
%\bibitem{madgraph} 
%T.~Stelzer and W.F.~Long, Comput. Phys. Commun. {\bf 81} (1994).
%
%\bibitem{Ohl} 
%T.~Ohl, {\it O'Mega: An Optimizing Matrix Element Generator}, hep-ph/0011243.
%
%T.~Ohl, {\it O'Mega\&WHIZARD: Monte Carlo Event Generator Generation For
%Future Colliders}, hep-ph/0011287.
%
%\bibitem{HELAC}
%A.~ Kanaki and C.G.~Papadopoulos, {\it HELAC-PHEGAS: automatic computation of 
%helicity amplitudes and cross sections}, hep-ph/0012004; Comput. Phys. Commun. 
%{\bf 132} (2000) 306.
%
%\bibitem{CPP}
%{\sf http://wwwlapp.in2p3.fr/cpp/cpp.html}
%\end{thebibliography}

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