EvtGen README

Introduction

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Building the code

To simplify the procedure you can use the setupEvtGen.sh script, which automates the process of downloading and building EvtGen and all dependencies from source. There are variables in the script that can be edited to set the installation location and the versions of EvtGen and each dependency (for more details see below) to be used.

Alternatively you can perform a manual build, for example if you are building against an existing installation of the dependencies, e.g. LCG releases. This procedure is described in the rest of this section.

To build the EvtGen code, first make sure that there is a valid (C++) version of HepMC avilable:

HepMC http://hepmc.web.cern.ch/hepmc/

HepMC is used to store particle information. We now recommend using HepMC3 but support for HepMC2 continues to be available.

Optionally, it is possible to use other external generators, such as Pythia8 (for Pythia decays in the DECAY.DEC file, for example), Photos or Sherpa's Photons++ (for radiative corrections) and Tauola (for tau decays):

Pythia8 https://pythia.org/

Photos https://photospp.web.cern.ch/

Tauola https://tauolapp.web.cern.ch/

Sherpa https://sherpa-team.gitlab.io/

All of these packages have instructions for building them.

For HepMC3 support the following versions are required: Pythia8: 8.201 or newer Photos: 3.64 or newer Tauola: 1.1.8 or newer Sherpa: 2.0.0 or newer

Once these packages are available, build the EvtGen release by creating a build directory alongside the EvtGen source directory (assumed here to be called evtgen.git) and running:

cmake ../evtgen.git <options>

within the EvtGen build directory, using the following options:

-DCMAKE_INSTALL_PREFIX=<location> : Location in which to install EvtGen (highly recommended)

-DEVTGEN_HEPMC3=ON : Enable HepMC3 support (default) To use HepMC2, set this to OFF instead.

-DHEPMC3_ROOT_DIR=<location> : Location of HepMC3 install directory While linking with either HepMC2 or HepMC3 is mandatory, depending on your environment the installation may be detected automatically. Failing this, you can specify the location via this option.

-DHEPMC2_ROOT_DIR=<location> : Location of HepMC2 install directory While linking with either HepMC2 or HepMC3 is mandatory, depending on your environment the installation may be detected automatically. Failing this, you can specify the location via this option.

-DEVTGEN_PYTHIA=ON : Enable linking with Pythia 8 (OFF by default)

-DPYTHIA8_ROOT_DIR=<location> : Location of Pythia8 install directory As with HepMC this may be automatically detected depending on your build environment, otherwise the location can be specified via this option.

-DEVTGEN_PHOTOS=ON : Enable linking with Photos++ (OFF by default)

-DPhotos++_ROOT_DIR=<location> or -DPHOTOSPP_ROOT_DIR=<location> : Location of Photos++ install directory As with HepMC this may be automatically detected depending on your build environment, otherwise the location can be specified via this option.

-DEVTGEN_TAUOLA=ON : Enable linking with Tauola++ (OFF by default)

-DTauola++_ROOT_DIR=<location> or -DTAUOLAPP_ROOT_DIR=<location> : Location of Tauola++ install directory As with HepMC this may be automatically detected depending on your build environment, otherwise the location can be specified via this option.

-DEVTGEN_SHERPA=ON : Enable linking with Sherpa (OFF by default)

-DSHERPA_ROOT_DIR=<location> : Location of Sherpa install directory As with HepMC this may be automatically detected depending on your build environment, otherwise the location can be specified via this option.

-DEVTGEN_BUILD_DOC=ON : Enable building documentation in 'doc' directory (OFF by default)

-DEVTGEN_BUILD_TESTS=ON : Enable building executables in 'test' directory (OFF by default)

-DEVTGEN_BUILD_VALIDATIONS=ON : Enable building executables in 'validation' directory (OFF by default)

Then compile and (optionally, although highly recommended) install the EvtGen code using

make
make install

This should create the libraries libEvtGen.so and libEvtGenExternal.so, as well as the archives libEvtGen.a and libEvtGenExternal.a (the "EvtGenExternal" library/archive will not be created if linking is not enabled for any of the external generators).

A series of validation and test executables are also created (discussed below). The CMakeLists.txt file in these two directories can be used as a template for building any user executables.

The build/install will also create CMake config files that allow the library targets to be easily imported into other projects.

To use Pythia 8, the environment variable PYTHIA8DATA needs to be set to the location of the corresponding xml documentation directory, which also contains the default values for particle decays and models: <location of Pythia 8 base directory>/xmldoc

Code validation

The validation sub-directory contains code validation test cases for Pythia, Tauola and B mixing models.

Note that these executables also depend on ROOT, which is only used to create ntuples and plots for the validation examples.

The script genAllDecayExamples.sh runs other scripts that generate a range of decay modes using the genExampleRootFiles.cc program. The script compareAllDecays.sh runs a range of scripts that creates comparison plots using the compareRootFiles.cc program. For now, the comparisons use the same plots, but each of the "compare.sh" files can be edited to compare any two versions of ROOT data created by the genExampleRootFiles.cc program.

The testCPVDecays.cc program runs a test for the B mixing decay model.

Examples

Some examples are provided in the test sub-directory.

Note that these executables also depend on ROOT, which is only used to create ntuples and plots for the validation examples.

Running the script ./do_tests will run a series of EvtGen examples. Example decay files are in the test/exampleFiles sub-directory.

Release notes

Please see History.md for a detailed list of changes to this package.

The major points comparing this version with the 2009 release are the following:

1. This version requires HepMC (version 2.04 and above) for storing event structures of particle decays, and can also use Pythia 8 (version 8.180 and above is required) and the C++ interfaced packages Photos (version 3.5.2 and above) and Tauola (version 1.0.7 and above). These external generators are included via engine classes in the new sub-directory EvtGenExternal.

   Two libraries can be created for EvtGen: i) libEvtGen.so contains the EvtGenBase and EvtGenModel core code/decay models, ii) libEvtGenExternal.so contains only the code within EvtGenExternal. This means that the external generator interface can be ignored by not loading/creating the 2nd library libEvtGenExternal.so.

   In the installation instructions below, it is possible to select which external generators you want to use. Note, however, that the generic "DECAY.DEC" file contains Pythia decays, and if Pythia is not included in the build, these decays are not generated (in fact, they would need a new decay model specified). Likewise, if Photos is not included, there will be no radiative corrections done for particle decays.

   To use the external generators, use the following code:

   #include "EvtGenBase/EvtAbsRadCorr.hh"
   #include "EvtGenBase/EvtDecayBase.hh"
    
   #include "EvtGenExternal/EvtExternalGenList.hh"
    
   // Set up the default external generator list: Photos, Pythia and/or Tauola
   EvtExternalGenList genList;
   EvtAbsRadCorr* radCorrEngine = genList.getPhotosModel();
   std::list<EvtDecayBase*> extraModels = genList.getListOfModels();
    
   // Create the EvtGen generator object
   EvtGen myGenerator( "decayFile.dec", "evt.pdl", randomEnginePointer,
                       radCorrEngine, &extraModels );
    
   //If you don't want to use external generators, use the following:
   //EvtGen myGenerator("decayFile.dec", "evt.pdl", randomEnginePointer);

   To use Sherpa's PHOTONS++ generator instead of Photos, replace above getPhotosModel() with getSherpaPhotonsModel().

   The files Pythia8_README.md and Tauola_README.md have more details about using the new Pythia 8 and Tauola generators (called via the PYTHIA and TAUOLA "decay.dec" model names). The new Photos generator is still called via the PHOTOS "decay.dec" model name.

   It is now possible to use alias particle decays for the Pythia 8 model. Two Pythia 8 instances are used in EvtPythiaEngine for normal and aliased decays. Since the underlying code for Photos and Tauola is still Fortran, it is only possible to have one unique instance of each of these external generators. This can only be fixed if these packages are converted to pure C++ code.

2. This version of EvtGen is effectively a merger of the latest LHCb and BaBar EvtGenBase and EvtGenModels code. Various decay models have been added, and there have been a range of bug fixes.

3. There is also a new Dalitz decay class model "GENERIC_DALITZ" (EvtGenModels/EvtGenericDalitz.cpp), that should be used instead of EvtDDalitz. The generic Dalitz model uses xml files to configure the resonance amplitude parameters (instead of being hardcoded in EvtDDalitz):

   Decay D+
   1.0  K-  pi+  pi+  GENERIC_DALITZ MyDalitzParameters.xml;
   Enddecay

   Examples of xml Dalitz parameter files are given in the sub-directory validation/DalitzFiles, e.g. see DalitzDecays.xml.

4. It is possible to use decay files in xml format. Use the python script scripts/convertDecayFile.py for converting decay files to the new format. The src/EvtGen.cpp constructor has an additional boolean argument useXml that needs to be set to true if xml decay files are to be used (default is useXml=false). For example, DECAY_2010.XML is the xml version of DECAY_2010.DEC.
5. Bug fixes for Bs mixing decay/CP violation amplitudes. Added the capability to use either coherent or incoherent mixing in EvtCPUtil. One or the other can be chosen as the mixing method for the B system by choosing 0 (coherent) or 1 (incoherent) for the last integer argument in the EvtGen() constructor.