main81
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// main81.cc is a part of the PYTHIA event generator.
// Copyright (C) 2023 Torbjorn Sjostrand.
// PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
// Please respect the MCnet Guidelines, see GUIDELINES for details.
// Authors:
// Stefan Prestel
// Keywords:
// Merging
// Leading order
// It illustrates how to do CKKW-L merging, see the Matrix Element
// Merging page in the online manual. An example command is
// ./main81 main81.cmnd w+_production_lhc_0.lhe histout81.dat
// where main81.cmnd supplies the commands, w+_production_lhc_0.lhe
// provides the input LHE events, and histout81.dat is the output
// file. This example requires FastJet.
#include "Pythia8/Pythia.h"
using namespace Pythia8;
// Functions for histogramming
#include "fastjet/PseudoJet.hh"
#include "fastjet/ClusterSequence.hh"
#include "fastjet/CDFMidPointPlugin.hh"
#include "fastjet/CDFJetCluPlugin.hh"
//==========================================================================
// Find the Durham kT separation of the clustering from
// nJetMin --> nJetMin-1 jets in the input event
double pTfirstJet( const Event& event, int nJetMin, double Rparam) {
double yPartonMax = 4.;
// Fastjet analysis - select algorithm and parameters
fastjet::Strategy strategy = fastjet::Best;
fastjet::RecombinationScheme recombScheme = fastjet::E_scheme;
fastjet::JetDefinition *jetDef = NULL;
// For hadronic collision, use hadronic Durham kT measure
if(event[3].colType() != 0 || event[4].colType() != 0)
jetDef = new fastjet::JetDefinition(fastjet::kt_algorithm, Rparam,
recombScheme, strategy);
// For e+e- collision, use e+e- Durham kT measure
else
jetDef = new fastjet::JetDefinition(fastjet::ee_kt_algorithm,
recombScheme, strategy);
// Fastjet input
std::vector <fastjet::PseudoJet> fjInputs;
// Reset Fastjet input
fjInputs.resize(0);
// Loop over event record to decide what to pass to FastJet
for (int i = 0; i < event.size(); ++i) {
// (Final state && coloured+photons) only!
if ( !event[i].isFinal()
|| event[i].isLepton()
|| event[i].id() == 23
|| abs(event[i].id()) == 24
|| abs(event[i].y()) > yPartonMax)
continue;
// Store as input to Fastjet
fjInputs.push_back( fastjet::PseudoJet (event[i].px(),
event[i].py(), event[i].pz(),event[i].e() ) );
}
// Do nothing for empty input
if (int(fjInputs.size()) == 0) {
delete jetDef;
return 0.0;
}
// Run Fastjet algorithm
fastjet::ClusterSequence clustSeq(fjInputs, *jetDef);
// Extract kT of first clustering
double pTFirst = sqrt(clustSeq.exclusive_dmerge_max(nJetMin-1));
delete jetDef;
// Return kT
return pTFirst;
}
//==========================================================================
// Example main programm to illustrate merging
int main( int argc, char* argv[] ){
// Check that correct number of command-line arguments
if (argc != 4) {
cerr << " Unexpected number of command-line arguments ("<<argc<<"). \n"
<< " You are expected to provide the arguments" << endl
<< " 1. Input file for settings" << endl
<< " 2. Full name of the input LHE file (with path)" << endl
<< " 3. Path for output histogram files" << endl
<< " Program stopped. " << endl;
return 1;
}
Pythia pythia;
// Input parameters:
// 1. Input file for settings
// 2. Path to input LHE file
// 3. Output histogram path
pythia.readFile(argv[1]);
string iPath = string(argv[2]);
string oPath = string(argv[3]);
// Number of events
int nEvent = pythia.mode("Main:numberOfEvents");
// For ISR regularisation off
pythia.settings.forceParm("SpaceShower:pT0Ref",0.);
// Declare histograms
Hist histPTFirst("pT of first jet",100,0.,100.);
Hist histPTSecond("pT of second jet",100,0.,100.);
Hist histPTThird("pT of third jet",100,0.,100.);
// Read in ME configurations
pythia.readString("Beams:frameType = 4");
pythia.readString("Beams:LHEF = " + iPath);
pythia.init();
// Start generation loop
for( int iEvent=0; iEvent<nEvent; ++iEvent ){
// Generate next event
if( ! pythia.next()) continue;
// Get CKKWL weight of current event
double evtweight = pythia.info.weight();
double weight = pythia.info.mergingWeight();
weight *= evtweight;
// Fill bins with CKKWL weight
// Functions use fastjet to get first / second jet
double pTfirst = pTfirstJet(pythia.event,1, 0.4);
double pTsecnd = pTfirstJet(pythia.event,2, 0.4);
double pTthird = pTfirstJet(pythia.event,3, 0.4);
histPTFirst.fill( pTfirst, weight);
histPTSecond.fill( pTsecnd, weight);
histPTThird.fill( pTthird, weight);
if(iEvent%1000 == 0) cout << iEvent << endl;
} // end loop over events to generate
// print cross section, errors
pythia.stat();
// Normalise histograms
double norm = 1.
* pythia.info.sigmaGen()
* 1./ double(nEvent);
histPTFirst *= norm;
histPTSecond *= norm;
histPTThird *= norm;
// Get the number of jets in the LHE file from the file name
string jetsInLHEF = iPath.substr(iPath.size()-5, iPath.size());
jetsInLHEF = jetsInLHEF.substr(0, jetsInLHEF.size()-4);
// Write histograms to dat file. Use "jetsInLHEF" to label the files
// Once all the samples up to the maximal desired jet multiplicity from the
// matrix element are run, add all histograms to produce a
// matrix-element-merged prediction
ofstream write;
stringstream suffix;
suffix << jetsInLHEF << "_wv.dat";
// Write histograms to file
write.open( (char*)(oPath + "PTjet1_" + suffix.str()).c_str());
histPTFirst.table(write);
write.close();
write.open( (char*)(oPath + "PTjet2_" + suffix.str()).c_str());
histPTSecond.table(write);
write.close();
write.open( (char*)(oPath + "PTjet3_" + suffix.str()).c_str());
histPTThird.table(write);
write.close();
// Done
return 0;
}