Gabriele Cosmo
191 lines
7.8 KiB
Plaintext
191 lines
7.8 KiB
Plaintext
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------------------------------------------------------------
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Example of the external decayer implementation with Pythia8
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------------------------------------------------------------
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This example demonstrates how to outfit Pythia8-based decay features
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to those resonances in Geant4 where decay tables are not implemented
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by default. In addition, it showns how to replace existing
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Geant4 decay tables to such resonances as tau+/- or B+/- with
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the Pythia8-based ones.
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This example is activated by setting up PYTHIA8 environment variable
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to point to the area where Pythia8 is installed.
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The complete Pythia8 information, including on download, and documentation
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is available from the following site:
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https://pythia.org
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The original version of this example has been implemented by Julia Yarba
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(FNAL, USA)
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For the complete list of the classes that compose this example please
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see later in this document.
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Location of example:
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examples/extended/eventgenerator/pythia/py8decayer
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Installation of Pythia8:
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NOTE: As of October 2022, pythia8.3.0.7 is the most current version,
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thus it is used in this example.
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In the future, please check updates at Pythi8 site: https://pythia.org
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1. cd path/to/your/pythia8/area
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2. Download desired version of Pythia8 and un-tar it, e.g.
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wget https://pythia.org/download/pythia83/pythia8307.tgz
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tar xzf pythia8307.tgz
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3. Build/install Pythia8
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cd pythia8307
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export CXX=\`which g++\`
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./configure --prefix=$PWD --cxx=$CXX
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make
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NOTE: By default, Pythia8 (as of 8.3.0.7) builds with C++11 standards.
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If one wants to turn to e.g. C++17 standard, one needs to override flags
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via --cxx-common argument to configure script.
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Alternatively, one can setup CXX_COMMON environment variable.
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As of pythia8307, the default flags are the following:
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-O2 -std=c++11 -pedantic -W -Wall -Wshadow -pthread
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Please note use of -pthread which was not among default flags in earlier
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relesaes of Pythia8.
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Example of specifying C++17 standards by overriding the default flags via
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use of --cxx_common argument to configure script:
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./configure --prefix=$PWD --cxx=$CXX \
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--cxx-common='-O2 -std=c++17 -pedantic -W -Wall -Wshadow -fPIC -pthread'
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4. Setup PYTHIA8 environment variable to point to the area where Pythia8
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is built/installed:
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export PYTHIA8=$PWD
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Building example:
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Upon setup of PYTHIA8 environment variable to point to the area where
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Pythia8 package is installed, the pythia/py8decayer example will be
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compiled together with several other features of the eventgenerator example.
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Description of classes:
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Py8Decayer class provides implementation of the G4VExternalDecayer interface
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with the use of PYTHIA8.
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It is reasonably annotated, and demonstrates what features of Pythia8 need
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to be activated and/or disactivated in order to make Pythia8 work only in
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the decay mode.
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It also illustrated how to control several other features of Pythia8, including
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some reduction of Pythia8 verbosity (by default, Pythia8 produces quite a large
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amount of printouts, thus reducing it could be useful in some cases).
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Last but not least, it also shown how to deactivate decays of pi0's by Pythia8
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as the idea is to handle pi0's back to Geant4 for decays.
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Py8DecayerPhysics class implements a G4VPhysicsConstructor type of component
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with the use of Py8Decayer; this component can later be used with a ddsired
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physics list (see main program).
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Specifically, in the Py8DecayerPhysics::ConstructProcess() the Py8Decayer is
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instantiated and is used to
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a) replace existing decay tables of such resonances as tau+/- and B+/-
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b) supplement decay features to those resonances in Geant4 where the decay
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tables are not implemnted by defaukt
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In principle, classes Py8Decayer and Py8DecayerPhysics can be directly reused with
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another user application.
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Alternatively, they can be used as an inspiration to implement similar, or perhaps
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even more extensive Pythia8-based functionalities of user's choice.
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Class DetConstruction demostrates how to implement minimalistic detector geometry.
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Class SingleParticleGun demonstrates how to implement generaton of the primary
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particle.
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Class ActionInitialization instantiates and registers to Geant4 kernel all user
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action classes; in this case it is SingleParticleGun.
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Main program:
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pythia8_decayer.cc
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This application explicitly uses SerialOnly RunManager.
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Transition to MT/Tasking operation is in the plans.
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Executable:
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pythia8_decayer
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Execution:
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At present, the pythia8_decayer executable does not take any input arguments.
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Everything, including the choine of primary particle, is hardcoded.
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Although in the future some configurability may be added.
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By default it'll run 5 single tau events using Pythia8 to decays them.
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It should print some Pythia8 event information, including on decays.
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Once again, please bear in mind that the decay of pi0's by Pythia8 is disabled
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(see Py8Decayer constructor) since the idea is to hand the pi0's back to Geant4
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and make Geant4 decay them.
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Additional notes on the contents of Geant4 and Pythia8 Particle Data Tables (PDT) :
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In their default form, PDT's in Geant4 and Pythia8 (v3.0.7) have a number
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of differences that need to be kept in mind.
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In the case of the Geant4 py8decayer example those differences are unlikely
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to cause any major issues.
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But Pythia8 can, in principle, be used within Geant4 in more ways that just
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as an external decayer.
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Thus, if one is potentially interested in more sophisticated use of Pythia8
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in Geant4, one may want to consider whatever differences exist between (default)
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Geant4 and Pythia8 PDT's.
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To be more precise, by default Pythia8 PDT contains 669 entries, of which
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525 particles have an antiparticle (it looks like antiparticles do not make
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separate entries in Pythia8 PDT, but the total number of available species
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should be considered as 1194).
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Geant4 PDT contains 508 entries.
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Of those, 239 particles/antiparticles match by Particle ID's (on the Geant4 side
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it is explicitly called "PDG encoding" while on Pythia8 side it is just "id").
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Many of Pythia8 PDT's entries are not available in Geant4 PDT, e.g. Z or W bosons
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are not in Geant4.
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Some of the species in the Geant4 PDT do not seem to be in the Pythia8 PDT
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(e.g. excited nucleons do not seem to be in the Pythia8 PDT).
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Also, there are entries in both PDT's that mean the same particles but are
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marked with different ID's.
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For example, excited Delta(s) are present in both PDT's but in Geant4 each one
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is marked with a 4-digit number as an ID (PDG encoding) while in Pythia8 an ID
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for such particle would be a 6-digit number starting with "20" and the last
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4 digits would be the same as the Geant4 ID for such particle.
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Speaking of the 239 particles that match by ID (PDG ID), there may be further
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differences, e.g. by mass, either central value or width, or both (there might be
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other aspects but they have not been checked for).
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As of October 2022, check has been made for differences larger that 1 keV in
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either mass central value or width.
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The largest differences have been observed for quarks/diquarks.
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It appears that Geant4 sets (at least) masses of quarks as listed in PDG.
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For details on default settings for the quark masses in Pythia8 please refer
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to the Pythia8 manual:
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https://pythia.org/manuals/pythia8307/Welcome.html
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See Particles and Decays section, Particle Data subsection.
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Beyond quarks/diquarks some differences in mass central values or width have also
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been observed, mainly for resonances.
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But even for such particles as proton or muon there may be differences on the order
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of a few keV (e.g. central value of the proton mass is 938.27 MeV in Pythia8 and
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938.272 MeV in Geant4)
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