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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 October 2022, pythia8.3.0.7 is the most current version,
thus 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 https://pythia.org/download/pythia83/pythia8307.tgz
tar xzf pythia8307.tgz
3. Build/install Pythia8
cd pythia8307
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 pythia8307, 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/pythia8307/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)
Gabriele Cosmo