= Running 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 [source:examples/ExternalFastJet] directory, [source:examples/ExternalFastJet/ExternalFastJetBasic.cpp] and [source:examples/ExternalFastJet/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 [source:cards/delphes_card_CMS_NoFastJet.tcl] configuration detector card is a truncated version of the [source: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 [source:cards/delphes_card_ATLAS.tcl] and [source:cards/delphes_card_FCC_basic.tcl] likewise. 

The input particle collection to jet clustering are specified inside [source:examples/ExternalFastJet/ExternalFastJetBasic.cpp] and [source:examples/ExternalFastJet/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 [wiki:WorkBook/QuickTour here], for !FastJet see [http://fastjet.fr/quickstart.html 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 [http://fastjet.hepforge.org/contrib/ 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="$FASTJET_INC -I$CONTRIB_INC -I$ROOT_INC -I$DELPHES_DIR -I$DELPHES_DIR/external"
LDFLAGS="$FASTJET_LIB $CONTRIB_LIB $ROOT_LIB $DELPHES_LIB"
}}}

Compile:

{{{
g++ $CXXFLAGS $LDFLAGS examples/ExternalFastJet/ExternalFastJetBasic.cpp -o ExternalFastJetBasic
g++ $CXXFLAGS $LDFLAGS examples/ExternalFastJet/ExternalFastJetHepMC.cpp -o 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):

{{{
./ExternalFastJetBasic cards/delphes_card_CMS_NoFastJet.tcl
./ExternalFastJetHepMC cards/delphes_card_CMS_NoFastJet.tcl test.hepmc
}}}