MassDropTagger.cc

//STARTHEADER
// $Id: MassDropTagger.cc 2209 2011-06-02 19:47:54Z soyez $
//
// Copyright (c) 2005-2011, Matteo Cacciari, Gavin Salam and Gregory Soyez
//
//----------------------------------------------------------------------
// This file is part of FastJet.
//
//  FastJet is free software; you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation; either version 2 of the License, or
//  (at your option) any later version.
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//  The algorithms that underlie FastJet have required considerable
//  development and are described in hep-ph/0512210. If you use
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//  You should have received a copy of the GNU General Public License
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//  Foundation, Inc.:
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//ENDHEADER

#include <fastjet/tools/MassDropTagger.hh>
#include <fastjet/ClusterSequence.hh>
#include <sstream>

FASTJET_BEGIN_NAMESPACE

using namespace std;

//----------------------------------------------------------------------
// MassDropTagger class implementation
//----------------------------------------------------------------------

//------------------------------------------------------------------------
// description of the tagger
string MassDropTagger::description() const{ 
  ostringstream oss;
  oss << "MassDropTagger with mu=" << _mu << " and ycut=" << _ycut;
  return oss.str();
}

//------------------------------------------------------------------------
// the tagging itself
//  - jet   the PseudoJet to tag
PseudoJet MassDropTagger::result(const PseudoJet & jet) const{
  PseudoJet j = jet;
  PseudoJet j1, j2;
  bool had_parents;

  // we just ask that we can "walk" in the cluster sequence.
  // appropriate errors will be thrown automatically if this is not
  // the case
  while ((had_parents = j.has_parents(j1,j2))) {
    // make parent1 the more massive jet
    if (j1.m() < j2.m()) std::swap(j1,j2);

    // if we pass the conditions on the mass drop and its degree of
    // asymmetry (kt_dist/m^2 > rtycut [where kt_dist/m^2 \sim
    // z/(1-z)), then we've found something interesting, so exit the
    // loop
    if ( (j1.m() < _mu*j.m()) && (j1.kt_distance(j2) > _ycut*j.m2()) )
      break;
    else
      j = j1;
  }
    
  if (!had_parents)
    // no Higgs found, return an empty PseudoJet
    return PseudoJet();

  // create the result and its structure
  const JetDefinition::Recombiner *rec
    = jet.associated_cluster_sequence()->jet_def().recombiner();
  PseudoJet result = join<MassDropStructure>(j1,j2,*rec);
  MassDropStructure * s = (MassDropStructure *) result.structure_non_const_ptr();
  s->_mu = (j.m()!=0.0) ? j1.m()/j.m() : 0.0;
  s->_y  = (j1.m2()!=0.0) ? j1.kt_distance(j2)/j.m2() : 0.0;

  return result;
}

FASTJET_END_NAMESPACE