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 April 2023, pythia8.3.0.9 is the most current version, thus it is used in this example.
In the future, please check updates at Pythia8 site: https://pythia.org

1. cd path/to/your/pythia8/area

2. Download desired version of Pythia8 and un-tar it, e.g.

wget https://pythia.org/download/pythia83/pythia8309.tgz
tar xzf pythia8309.tgz

3. Build/install Pythia8
```
cd pythia8309
export CXX=\`which g++\`
./configure --prefix=$PWD --cxx=$CXX
make
```
NOTE: By default, Pythia8 (as of 8.3.0.7) 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. Alternatively, one can setup CXX_COMMON environment variable. As of pythia8309, the default flags are the following: -O2 -std=c++11 -pedantic -W -Wall -Wshadow -pthread Please note use of -pthread which was not among default flags in earlier relesaes of Pythia8. Example of specifying C++17 standards by overriding the default flags via use of –cxx_common argument to configure script:
```
./configure --prefix=$PWD --cxx=$CXX \
        --cxx-common='-O2 -std=c++17 -pedantic -W -Wall -Wshadow -fPIC -pthread'
```
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.

Class ActionInitialization instantiates and registers to Geant4 kernel all user action classes; in this case it is SingleParticleGun.

Main program:

pythia8_decayer.cc

This application explicitly uses SerialOnly RunManager. Transition to MT/Tasking operation is in the plans.

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. 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.

Additional notes on the contents of Geant4 and Pythia8 Particle Data Tables (PDT) :

In their default form, PDT's in Geant4 and Pythia8 (v3.0.7) have a number of differences that need to be kept in mind.

In the case of the Geant4 py8decayer example those differences are unlikely to cause any major issues.

But Pythia8 can, in principle, be used within Geant4 in more ways that just as an external decayer. Thus, if one is potentially interested in more sophisticated use of Pythia8 in Geant4, one may want to consider whatever differences exist between (default) Geant4 and Pythia8 PDT's.

To be more precise, by default Pythia8 PDT contains 669 entries, of which 525 particles have an antiparticle (it looks like antiparticles do not make separate entries in Pythia8 PDT, but the total number of available species should be considered as 1194).

Geant4 PDT contains 508 entries.

Of those, 239 particles/antiparticles match by Particle ID's (on the Geant4 side it is explicitly called "PDG encoding" while on Pythia8 side it is just "id").

Many of Pythia8 PDT's entries are not available in Geant4 PDT, e.g. Z or W bosons are not in Geant4.

Some of the species in the Geant4 PDT do not seem to be in the Pythia8 PDT (e.g. excited nucleons do not seem to be in the Pythia8 PDT).

Also, there are entries in both PDT's that mean the same particles but are marked with different ID's. For example, excited Delta(s) are present in both PDT's but in Geant4 each one is marked with a 4-digit number as an ID (PDG encoding) while in Pythia8 an ID for such particle would be a 6-digit number starting with "20" and the last 4 digits would be the same as the Geant4 ID for such particle.

Speaking of the 239 particles that match by ID (PDG ID), there may be further differences, e.g. by mass, either central value or width, or both (there might be other aspects but they have not been checked for).

As of October 2022, check has been made for differences larger that 1 keV in either mass central value or width.

The largest differences have been observed for quarks/diquarks. It appears that Geant4 sets (at least) masses of quarks as listed in PDG. For details on default settings for the quark masses in Pythia8 please refer to the Pythia8 manual: https://pythia.org/manuals/pythia8309/Welcome.html See Particles and Decays section, Particle Data subsection.

Beyond quarks/diquarks some differences in mass central values or width have also been observed, mainly for resonances. But even for such particles as proton or muon there may be differences on the order of a few keV (e.g. central value of the proton mass is 938.27 MeV in Pythia8 and 938.272 MeV in Geant4)