geant4

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  Example of the external decayer implementation with Pythia8
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  This example demonstrates how to outfit Pythia8-based decay features 
  to those resonances in Geant4 where decay tables are not implemented
  by default. In addition, it showns how to replace existing
  Geant4 decay tables to such resonances as tau+/- or B+/- with 
  the Pythia8-based ones.
  
  This example is activated by setting up PYTHIA8 environment variable
  to point to the area where Pythia8 is installed.
 
  The complete Pythia8 information, including on download, and documentation 
  is available from the following site:
  https://pythia.org
 
  The original version of this example has been implemented by Julia Yarba
  (FNAL, USA)

  For the complete list of the classes that compose this example please 
  see later in this document.

  Location of example:
  
  examples/extended/eventgenerator/pythia/py8decayer
  

  Installation of Pythia8: 

  NOTE: As of June 2021, pythia8.3.0.5 is the most current version,
  this it is used in this example.
  In the future, please check updates at Pythi8 site: https://pythia.org
  
  1. cd path/to/your/pythia8/area

  2. Download desired version of Pythia8 and un-tar it, e.g. 
     wget http://home.thep.lu.se/~torbjorn/pythia8/pythia8305.tgz
     tar xzf pythia8305.tgz

  3. Build/install Pythia8
     cd pythia8305
     export CXX=\`which g++\`
     ./configure --prefix=$PWD --cxx=$CXX
     make
     NOTE: By default, Pythia8 (as of 8.3.0.5) builds with C++11 standards. 
     If one wants to turn to e.g. C++17 standard, one needs to override flags 
     via --cxx-common argument to configure script. 

   4. Setup PYTHIA8 environment variable to point to the area where Pythia8
      is built/installed:
      export PYTHIA8=$PWD


  Building example:
  
  Upon setup of PYTHIA8 environment variable to point to the area where 
  Pythia8 package is installed, the pythia/py8decayer example will be
  compiled together with several other features of the eventgenerator example.


  Description of classes:
  
  Py8Decayer class provides implementation of the G4VExternalDecayer interface 
  with the use of PYTHIA8.
  It is reasonably annotated, and demonstrates what features of Pythia8 need
  to be activated and/or disactivated in order to make Pythia8 work only in
  the decay mode.
  It also illustrated how to control several other features of Pythia8, including
  some reduction of Pythia8 verbosity (by default, Pythia8 produces quite a large
  amount of printouts, thus reducing it could be useful in some cases).
  Last but not least, it also shown how to deactivate decays of pi0's by Pythia8
  as the idea is to handle pi0's back to Geant4 for decays.
  
  Py8DecayerPhysics class implements a G4VPhysicsConstructor type of component
  with the use of Py8Decayer; this component can later be used with a ddsired
  physics list (see main program).
  Specifically, in the Py8DecayerPhysics::ConstructProcess() the Py8Decayer is
  instantiated and is used to
  a) replace existing decay tables of such resonances as tau+/- and B+/-
  b) supplement decay features to those resonances in Geant4 where the decay
     tables are not implemnted by defaukt
  
  In principle, classes Py8Decayer and Py8DecayerPhysics can be directly reused with 
  another user application.
  Alternatively, they can be used as an inspiration to implement similar, or perhaps 
  even more extensive Pythia8-based functionalities of user's choice.

  Class DetConstruction demostrates how to implement minimalistic detector geometry.
  
  Class SingleParticleGun demonstrates how to implement generaton of the primary
  particle.
  

  Main program:
  
  pythia8_decayer.cc
  

  Executable:
  
  pythia8_decayer
  

  Execution:
  
  At present, the pythia8_decayer executable does not take any input arguments.
  Everything, including the choine of primary particle, is hardcoded in the main.
  Although in the future some configurability may be added.

  By default it'll run 5 single tau events using Pythia8 to decays them. 

  It should print some Pythia8 event information, including on decays. 
  Once again, please bear in mind that the decay of pi0's by Pythia8 is disabled 
  (see Py8Decayer constructor) since the idea is to hand the pi0's back to Geant4 
  and make Geant4 decay them.