Run Delphes with external FastJet

Introduction

This section explains how you can run your own FastJet code on Delphes particles. We provide two working codes in the "examples" directory, examples/ExternalFastJetBasic.cpp and examples/ExternalFastJetHepMC.cpp.

Both examples convert generated events into Delphes candidates, perform event reconstruction, and pass reconstructed particles to anti-kT clustering. ExternalFastJetHepMC takes as input an HepMC event file, while ExternalFastJetBasic has internally defined input particles.

The cards/delphes_card_CMS_NoFastJet.tcl​ configuration detector card is a truncated version of the cards/delphes_card_CMS.tcl​, as the reconstruction is stopped after producing collections of calorimeter towers and particle-flow objects, before jet and high level object isolation and identification. Similar cards for the ATLAS and the FCC detectors can be easily produced by truncating cards/delphes_card_ATLAS.tcl​ and cards/delphes_card_FCC_basic.tcl likewise.

The input particle collection to jet clustering are specified inside examples/ExternalFastJetBasic.cpp and examples/ExternalFastJetHepMC.cpp via:

inputArray = modularDelphes->ImportArray("ModuleInstanceName/arrayName");

By default the particle-flow collection EFlowMerger/eflow is taken. This can be easily changed to simple calorimeter towers, tracks, or stable generated particles (or any other collection defined in the configuration card) by defining instead:

inputArray = modularDelphes->ImportArray("EFlowMerger/eflow");  // particle-flow objects

or

inputArray = modularDelphes->ImportArray("Calorimeter/towers"); // calorimeter objects

or

inputArray = modularDelphes->ImportArray("TrackMerger/tracks"); // charged tracks

or

inputArray = modularDelphes->ImportArray("Delphes/stableParticles"); // all gen-level stable particles

Run

In order to run Delphes with your own FastJet installation you first need to install Delphes and FastJet (to install Delphes, see here, for FastJet see ​here).

Then set the paths to your Delphes, FastJet and ROOT installations (DELPHES_DIR, FASTJET_DIR and ROOT_DIR):

DELPHES_DIR=<path to Delphes installation>  # main Delphes directory
FASTJET_DIR=<path to FastJet installation>  # by default should be xyz/fastjet-install
ROOT_DIR=<path to ROOT installation>        

Then run the following commands to build the executable:

DELPHES_LIB="-Wl,-rpath $DELPHES_DIR -L$DELPHES_DIR -lDelphesNoFastJet"

FASTJET_INC=`$FASTJET_DIR/bin/fastjet-config --cxxflags`
FASTJET_LIB=`$FASTJET_DIR/bin/fastjet-config --libs`

ROOT_INC=`$ROOT_DIR/bin/root-config --incdir`
ROOT_LIB=`$ROOT_DIR/bin/root-config --libs`

CXXFLAGS="$FASTJET_INC -I$ROOT_INC -I$DELPHES_DIR -I$DELPHES_DIR/external"
LDFLAGS="$FASTJET_LIB $ROOT_LIB $DELPHES_LIB"

The libDelphesNoFastJet.so library is automatically created when compiling Delphes the first time. This avoids conflict between your FastJet installation and the one in Delphes.

If you have a ​FastJet Contrib installation, assuming you installed it in the CONTRIB_DIR directory (by default FastJet contrib will be installed in the FastJet directory, in that case you can replace in what follows CONTRIB_DIR with FASTJET_DIR), you have to define:

CONTRIB_DIR=<path to contrib installation> 
CONTRIB_INC=$CONTRIB_DIR/include
CONTRIB_LIB="-Wl,-rpath $CONTRIB_DIR -L$CONTRIB_DIR -lNsubjettiness -lSoftKiller"

and add CONTRIB_INC and CONTRIB_LIB into CXXFLAGS and {{{LDFLAGS`:

CXXFLAGS="$FASTJET_INC -I$ROOT_INC -I$DELPHES_DIR -I$DELPHES_DIR/external -I$CONTRIB_INC"
LDFLAGS="$FASTJET_LIB $ROOT_LIB $DELPHES_LIB $CONTRIB_LIB"

Compile:

g++ $CXXFLAGS $LDFLAGS examples/ExternalFastJetBasic.cpp -o examples/ExternalFastJetBasic
g++ $CXXFLAGS $LDFLAGS examples/ExternalFastJetHepMC.cpp -o examples/ExternalFastJetHepMC

If you need a test HepMC file, you can take it from:

curl -O http://cp3.irmp.ucl.ac.be/~demin/test.hepmc.gz
gunzip test.hepmc.gz

Then run one of the two examples ExternalFastJetBasic or ExternalFastJetHepMC (for ExternalFastJetHepMC you need an event file in HepMC format as input):

./examples/ExternalFastJetBasic cards/delphes_card_CMS_NoFastJet.tcl
./examples/ExternalFastJetHepMC cards/delphes_card_CMS_NoFastJet.tcl test.hepmc