main81

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// main81.cc is a part of the PYTHIA event generator.
// Copyright (C) 2020 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  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 ("<