[Clas12_verystrange] Generator documentation

[Michael Dugger]

Hi,

At meeting on Monday, I was asked to provide some documentation for the 
vsGen event generator. I have appended a short description (a few 
paragraphs and a table) of the vsGen software to the bottom of this email.

Please feel free to modify the text, and let me know if I should produce a 
more detailed version.

Take care,
Michael


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

The vsGen software generates various final states for $\gamma p$
initial state reactions, using both TGenPhaseSpace \cite{genbod}
(ROOT implementation of GenBod), and Pythia \cite{pythia} for
much of the actual event generation. The vsGen software can be
used for real, as well as virtual initial state photons, and can
be directed to create predefined reactions, or run in "background''
mode.

When vsGen is run in background mode, the Pythia event generator
completely determines all intermediate and final states using an
implementation of parton model code within the Pythia generator
framework. For predefined reactions, vsGen uses Pythia to generate
the mass of the reaction particles, then TGenPhaseSpace is used
to phase-space generate the reaction of interest, after which,
Pythia takes over and determines all decays of the predefined
reaction.

As a concrete example, vsGen can be directed to generate the
reaction $\gamma p \rightarrow K^+ K^+ K^0 \; \Omega^-$. Once the
reaction is defined, vsGen obtains the masses of the Kaons and
$\Omega^-$ from Pythia, uses TGenPhaseSpace for phase-space
generation, and passes the Kaon's and $\Omega^-$ particles back
to Pythia which handles the $K^0$ and $\Omega^-$ decays.

Virtual photons are generated from an 11 GeV incident electron,
that after scattering, has a final state energy between 1 and 4
GeV, with polar angle between 1 and 5 degrees.

The vsGen event generator has several types of input variables
that can be controlled by the user. The default values for
parameters relevant to virtual photons can be found in Table
\ref{tbl:vsGen}.

\begin{table}[h]
\caption{Default parameters for the vsGen event generator.}
\vspace{2mm}
\begin{tabular}{|c|c|}
\hline
Parameter & Default value \\
\hline
\hline
Reaction type & background \\
\hline
Target length & 7.5 cm \\
\hline
Target center & 0.0 cm \\
\hline
Vertex resolution & 0.1 cm \\
\hline
Beam profile standard deviation & 0.01 cm \\
\hline
Incident photon type & virtual \\
\hline
\end{tabular}
\label{tbl:vsGen}
\end{table}

The output of vsGen is a ROOT tree that contains initial and
final state information for each event. The information includes
the identification and four-momentum of each initial and final
state particle, as well as the cross section weight of the event
(in units of micro-barns). When vsGen is run in background mode,
the weight is simply the total cross section derived from world
data. For the ground state $\Xi$ reactions, the weights come from
differential cross section predictions of Kanzo Nakayama \cite{kn}.

\begin{thebibliography}{0}
\bibitem{genbod} \url{http://root.cern.ch/root/html/TGenPhaseSpace.html}
\bibitem{pythia} \url{http://home.thep.lu.se/~torbjorn/Pythia.html}
\bibitem{kn} K. Nakayama, private communication.
\end{thebibliography}