\section{Monte Carlo generators}
\label{mc:generators}

There are two physics event generators available for use within the
\hd\ Monte Carlo framework, known as \emph{genr8}~\cite{Genr8} and
\emph{cwrap}~\cite{Cwrap}.  Both programs are capable of describing
a complex decay chain of intermediate states, where decays into two
or three bodies are supported at each step.  The invariant masses of
each particle produced is sampled from a Breit Wigner distribution,
whose mass and width is taken from the PDG.  A general $t$-channel
process is assumed, with the distribution in $t$ drawn at random from
the standard form for a peripheral reaction
\[	\frac{d\sigma}{dt} \propto e^{-b|t|}   \]
where the $b$ parameter is specified by the user.  Both meson and
baryon decay chains are allowed.  In the case of \emph{genr8} the
user may specify the $t$-distribution in the form of a histogram
in place of specifying a value for $b$ in the above formula.
be specified by an input histogram.

The angular distributions at each decay vertex are generated according
to phase space.  This may appear to be a severe restriction in an
experiment whose goal is partial-wave analysis, but in fact that is
not the case.  To see how the physical model for particle spins and
decay asymmetries are applied to phase-space Monte Carlo data, see
section \ref{mc:weights}.

Both \emph{genr8} and \emph{cwrap} were imported from other experiments,
and so write their output events in different and somewhat esoteric
formats.  To incorporate them into the \hd\ framework it was sufficient
to provide translators from their private formats to a common hddm
format that can be viewed as xml.  The present draft specification
for the standard xml interface to generated events is found in
Ref.~\cite{hddm}.  At present a second standard interface is also
being supported known as \emph{stdhep}.  This somewhat archaic 
Fortran-based standard was in use by many HEP experiments over the
last decade, and there are a number of useful Monte Carlo tools that
rely on it, including \emph{MCFast} (see section \ref{mc:hdfast}).
Currently translators exist to supply generated events from either
generator through either the xml or the stdhep interface.

Both generators use cryptic private formats for the input data that
specify the reaction and desired number of events.  At present there
does not exist a single unified interface for specifying the reaction
to be generated.  The task of incorporating \emph{genr8} and \emph{cwrap}
into the \hd\ framework will not be complete until that interface has
been specified, and translators have been written to convert that
information from xml to a form understandable to the generators.
