\chapter[Introduction]
   {Introduction}

\section{Scope of This Manual} 

The Physics Reference Manual provides detailed explanations of the physics
implemented in the Geant4 toolkit.  The manual's purpose is threefold:

\begin{itemize}

\item to present the theoretical formulation, model, or parameterization of 
      the physics interactions included in Geant4,

\item to describe the probability of the occurrence of an interaction and the
      sampling mechanisms required to simulate it, and

\item to serve as a reference for toolkit users and developers who wish to 
      consult the underlying physics of an interaction.

\end{itemize}

This manual does not discuss code implementation or how to use the 
implemented physics interactions in a simulation.  These topics are
discussed in the User's Guide for Application Developers.  Details 
of the object-oriented design and functionality of the Geant4 toolkit 
are given in the User's Guide for Toolkit Developers.  The 
Installation Guide for Setting up Geant4 in Your Computing Environment
describes how to get the Geant4 code, install it, and run it. 

\section{Definition of Terms}

Several terms used throughout the Physics Reference Manual have specific
meaning within Geant4, but are not well-defined in general usage.  The 
definitions of these terms are given here.

\begin{itemize}

\item {\bf process} - a C++ class which describes how and when a specific 
                      kind of physical interaction takes place along a 
                      particle track.  A given particle type typically 
                      has several processes assigned to it.  
                      Occaisionally ``process'' refers to the interaction
                      which the process class describes.

\item {\bf model} - a C++ class whose methods implement the details of an 
                    interaction, such as its kinematics.  One or more
                    models may be assigned to each process.  In sections
                    discussing the theory of an interaction, ``model'' may 
                    refer to the formulae or parameterization on which 
                    the model class is based. 

\item {\bf Geant3} - a physics simulation tool written in Fortran,
                     and the predecessor of Geant4.  Although many 
                     references are made to Geant3, no knowledge 
                     of it is required to understand this manual.

\end{itemize}

\section{Status of this document}
  4.12.01   created by D.H. Wright