This is an example of the external decayer implementation with PYTHIA6.

The complete PYTHIA6 documentation can be found at: http://home.thep.lu.se/~torbjorn/pythiaaux/recent.html

The PYTHIA6 external decayer was originally developed within the AliRoot framework, by Andreas Morsch (CERN).
The dependence on the ALICE software was taken off by Christian Holm Christensen.
The dependence on the Root framework and the integration in the Geant4 framework was done by Ivana Hrivnacova (IPN Orsay).

The use of the external decayer is demonstrated with using the classes from common examples repository, see below their complete list.

The G4Pythia6Decayer class provides the implementation of the G4VExternalDecayer interface with using PYTHIA6. In order to be able to use PYTHIA6, which is written in FORTRAN, a C++ interface class Pythia6 is provided. This class interfaces only the PYTHIA6 functions relevant to decay.

The G4Pythia6Decayer is instantiated in the P6DExtDecayerPhysics::ConstructProcess() function where the external decayer is set to G4Decay process for all particles. To demonstrate the decay with external decayer, the B- meson is defined in ExG4PrimaryGeneratorAction01, as it has no own decay table defined within Geant4.

With PYTHIA6, it is possible to force a selected decay type. This selection can be chosen interactively via the implemented Geant4 UI command:

/pythia6Decayer/forceDecayType decayType

where the available decay types are listed in the EDecayType enumaration.

The classes Pythia6, G4Pythia6Decayer, G4Pythia6DecayerMessenger are independent from the example classes and can be reused in another user application.

Installation:

1. Download the PYTHIA6 source file from the PYTHIA6 download site:
http://www.hepforge.org/downloads/pythia6
2A. With CMake: Build pythia6 library

For a convenience a CMake file for building Pythia6 library from the source is provided in examples/extended/eventgenerator/CMakeLists.txt.pythia6. Build the pythia6 library following the instructions in this file and then define the environment variables:
```
PYTHIA6          the path where pythia6 library is installed
PYTHIA6_VERSION  the pythia version 
```
2B. With GNUmake: Define the environment variables:
```
PYTHIA6           the path to pythia-versionX.f source code
PYTHIA6_VERSION   the pythia version 
```
e.g. If you download pythia-6.4.26.f.gz and unzip it in $HOME, then you have to set: export PYTHIA6=$HOME export PYTHIA6_VERSION="6.4.26"

pythia6 will be then compiled together with example code.
3. Compilation:
Then the example is compiled in a standard way, see How to build and run an example.
Note that with GNUmake build, an additional step 'gmake setup' is needed before 'gmake'.
4. Execution:
```
% pythia6_decayer pythia6_decayer.in
```

This example uses the following user action classes from the extended examples common repository available in common subdirectory:

DetectorConstruction
GunPrimaryGeneratorAction