Input and output formats¶
Les Houches Event Files¶
In order to use partonic events generated by an external matrix-element generator,
LesHouchesEventHandler object has to be created in the repository.
This object is supplied with at least one
LesHouchesReader objects represent event files according to the Les Houches accord or the reading of events in the same format ‘on the fly’ using pipes.
Here we give an example on how to use LHA event files. Reading of events is performed by the
LesHouchesReader. This class can handle arbitrary event files in the Les Houches format.
The corresponding libraries need to be loaded in the input file
Suppose the event file is named
myEvents.lhe. We assume it contains some process of interest at LHC. First, a LesHouchesReader object needs to be created and given the name of the sample file
cd /Herwig/EventHandlers create ThePEG::LesHouchesFileReader myReader set myReader:FileName myEvents.lhe
In principle, the information needed to generate full events, i.e. beam energies, incoming particles and parton distributions is extracted from the event file, but may also be set explicitly. For these switches, see the interface documentation of
In some cases ThePEG has to scan through the file in order to get an idea about the weights appearing and to precalculate the cross section. In this a cache file has to be specified with e.g.
set myReader:CacheFileName cache.tmp
If the file is very long one can limit the effort of scanning e.g. to 1000 events with
set myReader:maxScan 1000
Cuts can not be extracted from event files in general. If the interface value
set myReader:InitCuts 0
is assigned, the reader expects to be given a Cuts object. For example, typical cuts for hadron collisions may be chosen
set myReader:Cuts /Herwig/Cuts/Cuts
Completely open cuts can be set with
create ThePEG::Cuts NoCuts set myReader:Cuts NoCuts
Several cuts can be found in the repository directory
/Herwig/Cuts and cuts are discussed in more detail in the section on cuts.
Similar remarks apply to the use of parton distribution functions, which can be set explicitly using
set myReader:InitPDFs 0 set myReader:PDFA firstBeamPDF set myReader:PDFB secondBeamPDF
secondBeamPDF are PDFBase objects. Here, either the build in PDF sets or
LHAPDF may be used, see using LHAPDF. Note that the PDF sets have to match the ones being used for matrix element event generation.
Next an LesHouchesEventHandler object has to be created. Objects of this class inherit from EventHandler and provide the same interfaces. Thus the setup is similar to the setup of a usual event handler, which needs to be equipped with showering, hadronization and decay handlers
create ThePEG::LesHouchesEventHandler myLesHouchesHandler set myLesHouchesHandler:CascadeHandler /Herwig/Shower/ShowerHandler set myLesHouchesHandler:HadronizationHandler /Herwig/Hadronization/ClusterHadHandler set myLesHouchesHandler:DecayHandler /Herwig/Decays/DecayHandler set myLesHouchesHandler:PartonExtractor /Herwig/Partons/QCDExtractor insert myLesHouchesHandler:PreCascadeHandlers 0 /Herwig/NewPhysics/DecayHandler insert myLesHouchesHandler:PostSubProcessHandlers 0 /Herwig/QEDRadiation/QEDRadiationHandler
A Cuts object applying to all processes may be set as for every EventHandler. Finally, the readers from which the event handler should draw events from have to be specified:
insert myLesHouchesHandler:LesHouchesReaders 0 myReader insert myLesHouchesHandler:LesHouchesReaders 1 myOtherReader ...
An arbitrary number of readers may be used.
A default or custom EventGenerator object may now use the LesHouchesEventHandler object myLesHouchesHandler and a run file can be created from this event generator:
cd /Herwig/Generators cp EventGenerator myLesHouchesGenerator set myLesHouchesGenerator:EventHandler /Herwig/EventHandlers/myLesHouchesHandler saverun myLesHouches myLesHouchesGenerator
There is also support for multiple alternative weights according to the latest Les Houches accord (version 3). The supported files should include in their <init> block the following
<initrwgt> <weightgroup type='type information' combine='combination information'> <weight id='XXXX'> weight information </weight> ... </weightgroup> ... </initrwgt>
and each event should contain the relevant information in the form
<rwgt> <wgt id='XXXX'> value_of_weight </wgt> ... </rwgt>
An example analysis of weights is included in the Contrib
MultiWeight library found in Contrib.
Herwig supports the option of using the
QNumbers block to define new particles and their decays in Les Houches event files. This
is switched off by default and can be enabled using
set myReader:QNumbers Yes
In addition a
Decayer object is needed to perform the decays and can be specified using
set myReader:Decayer /Herwig/Decays/Mambo
Often the new particles have to be decayed as part of the showering process as their decays produce new coloured particles which must be showered. This must be done using
insert /Herwig/Shower/ShowerHandler:DecayInShower 0 PDG_NUMBER
PDG_NUMBER is the particle’s PDG code.
In addition in SUSY models we can read the SLHA file from the header of the Les Houches event file. You should select the relevant SUSY model, for example for the MSSM, using
and instruct it to read the SLHA information from the header using
setup /Herwig/NewPhysics/MSSM/Model myEvents.lhe
For some of the simplified model scenarios which are currently used the SLHA files are not consistent and it is advisable to switch to phase space decays and guard against missing information using
set /Herwig/NewPhysics/NewModel:WhichOffshell Selected clear /Herwig/NewPhysics/NewModel:DecayParticles set /Herwig/NewPhysics/MSSM/Model:CreateDiagonalMixingMatrices Yes
N.B. these commands should be executed before the setup command for the model and after the model is read. We would advise the commands are just after the selection of the collider type at the top of the input file.
Output and analysis formats¶
While there is an internal mechanism for performing the analysis of the events generated by Herwig
we would recommend that you only make use of it to either output the events to a file in the
or call the
Rivet package to analyse the events. This mechanism works by calling a number of
classes from the
HepMCFile file analysis can be used to output the events in the file in the HepMC format
read snippets/HepMC.in set HepMC:PrintEvent NNN
where NNN ist the number of events stored in the file
NameOfEventGenerator.hepmc. Caution: If you run fixed order, ie NO shower, you should use instead
read snippets/HepMCFixedOrder.in set HepMCFixedOrder:Print NNN
The Rivet package can be used to analyse events generated by Herwig by directly calling the Rivet code from within Herwig. In order to do this ThePEG should be configured with
--with-rivet=location of installed Rivet code --with-hepmc=location of installed HepMC code
which is done by default with the booststrap script.
This is will ensure that the
AnalysisHandler class is built as a dynamically loadable library.
This AnalysisHandler can be used by adding
read snippets/Rivet.in insert Rivet:Analyses 0 name_of_rivet_analysis
to your Herwig input files. Any number of Rivet analyses can then be enabled using the Analyses interface of the
RivetAnalysis class. Caution: If you run fixed order, ie NO shower, you should use instead
read snippets/RivetFixedOrder.in insert RivetFixedOrder:Analyses 0 name_of_rivet_analysis
Delphes is a C++ framework, performing a fast multipurpose detector response simulation. Herwig does not possess a custom interface to Delphes, however, a HepMC stream can be used for this purpose.
For example, one uses the LHC.in file to generate a .hepmc file called LHC.hepmc:
insert EventGenerator:AnalysisHandlers 0 /Herwig/Analysis/HepMCFile set /Herwig/Analysis/HepMCFile:PrintEvent 1000 set /Herwig/Analysis/HepMCFile:Format GenEvent set /Herwig/Analysis/HepMCFile:Units GeV_mm set /Herwig/Analysis/HepMCFile:Filename LHC.hepmc
run Herwig, e.g. for 1000 events:
Herwig read LHC.in; Herwig run LHC.run -N1000
This generates a .hepmc file: LHC.hepmc. Then run, for example, the following:
rm test.root; cat LHC.hepmc | [path_to_delphes]/DelphesHepMC [path_to_delphes]/cards/delphes_card_ATLAS.tcl test.root
The Delphes output file is test.root.