/afs/cern.ch/work/a/aaltunda/public/www/CMSSW_6_2_5/src/ElectroWeakAnalysis/Utilities/src/FSRWeightProducer.cc

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#include <memory>

#include "FWCore/Framework/interface/Frameworkfwd.h"
#include "FWCore/Framework/interface/EDProducer.h"

#include "FWCore/Framework/interface/Event.h"
#include "FWCore/Framework/interface/MakerMacros.h"

#include "FWCore/ParameterSet/interface/ParameterSet.h"

#include "DataFormats/HepMCCandidate/interface/GenParticle.h"
#include "DataFormats/Candidate/interface/CompositeCandidate.h"
#include "CommonTools/CandUtils/interface/AddFourMomenta.h"
#include "CommonTools/CandUtils/interface/Booster.h"
#include <Math/VectorUtil.h>

//
// class declaration
//
class FSRWeightProducer : public edm::EDProducer {
   public:
      explicit FSRWeightProducer(const edm::ParameterSet&);
      ~FSRWeightProducer();

   private:
      virtual void beginJob() ;
      virtual void produce(edm::Event&, const edm::EventSetup&) override;
      virtual void endJob() ;
      double alphaRatio(double) ;

      edm::InputTag genTag_;
};


#include "DataFormats/HepMCCandidate/interface/GenParticle.h"
FSRWeightProducer::FSRWeightProducer(const edm::ParameterSet& pset) {
      genTag_ = pset.getUntrackedParameter<edm::InputTag> ("GenTag", edm::InputTag("genParticles"));

      produces<double>();
} 

FSRWeightProducer::~FSRWeightProducer(){}

void FSRWeightProducer::beginJob() {}

void FSRWeightProducer::endJob(){}

void FSRWeightProducer::produce(edm::Event& iEvent, const edm::EventSetup&) {

      if (iEvent.isRealData()) return;

      edm::Handle<reco::GenParticleCollection> genParticles;
      iEvent.getByLabel(genTag_, genParticles);

      std::auto_ptr<double> weight (new double);

      // Set a default weight to start with
      (*weight) = 1.;

      unsigned int gensize = genParticles->size();
      for (unsigned int i = 0; i<gensize; ++i) {
            const reco::GenParticle& lepton = (*genParticles)[i];
            if (lepton.status()!=3) continue;
            int leptonId = lepton.pdgId();
            if (abs(leptonId)!=11 && abs(leptonId)!=13 && abs(leptonId)!=15) continue;
            if (lepton.numberOfMothers()!=1) continue;
            const reco::Candidate * boson = lepton.mother();
            int bosonId = abs(boson->pdgId());
            if (bosonId!=23  && bosonId!=24) continue;
            double bosonMass = boson->mass();
            double leptonMass = lepton.mass();
            double leptonEnergy = lepton.energy();
            double cosLeptonTheta = cos(lepton.theta());
            double sinLeptonTheta = sin(lepton.theta());
            double leptonPhi = lepton.phi();

            int trueKey = i;
            if (lepton.numberOfDaughters()==0) { 
                  continue;
            } else if (lepton.numberOfDaughters()==1) { 
                  int otherleptonKey = lepton.daughterRef(0).key();
                  const reco::GenParticle& otherlepton = (*genParticles)[otherleptonKey];
                  if (otherlepton.pdgId()!=leptonId) continue;
                  if (otherlepton.numberOfDaughters()<=1) continue;
                  trueKey = otherleptonKey;
            }

            const reco::GenParticle& trueLepton = (*genParticles)[trueKey];
            unsigned int nDaughters = trueLepton.numberOfDaughters();

            for (unsigned int j = 0; j<nDaughters; ++j) {
                  const reco::Candidate * photon = trueLepton.daughter(j);
                  if (photon->pdgId()!=22) continue;
                  double photonEnergy = photon->energy();
                  double cosPhotonTheta = cos(photon->theta());
                  double sinPhotonTheta = sin(photon->theta());
                  double photonPhi = photon->phi();
                  double costheta = sinLeptonTheta*sinPhotonTheta*cos(leptonPhi-photonPhi)
                                  + cosLeptonTheta*cosPhotonTheta;
                  // Missing O(alpha) terms in soft-collinear approach
                  // Only for W, from hep-ph/0303260
                  if (bosonId==24) {
                        double betaLepton = sqrt(1-pow(leptonMass/leptonEnergy,2));
                        double delta = - 8*photonEnergy *(1-betaLepton*costheta)
                          / pow(bosonMass,3) 
                          / (1-pow(leptonMass/bosonMass,2))
                          / (4-pow(leptonMass/bosonMass,2))
                          * leptonEnergy * (pow(leptonMass,2)/bosonMass+2*photonEnergy);
                        (*weight) *= (1 + delta);
                  }
                  // Missing NLO QED orders in QED parton shower approach
                  // Change coupling scale from 0 to kT to estimate this effect
                  (*weight) *= alphaRatio(photonEnergy*sqrt(1-pow(costheta,2)));
            }
      }


      iEvent.put(weight);
}

double FSRWeightProducer::alphaRatio(double pt) {

      double pigaga = 0.;

      // Leptonic contribution (just one loop, precise at < 0.3% level)
      const double alphapi = 1/137.036/M_PI;
      const double mass_e = 0.0005;
      const double mass_mu = 0.106;
      const double mass_tau = 1.777;
      const double mass_Z = 91.2;
      if (pt>mass_e) pigaga += alphapi * (2*log(pt/mass_e)/3.-5./9.);
      if (pt>mass_mu) pigaga += alphapi * (2*log(pt/mass_mu)/3.-5./9.);
      if (pt>mass_tau) pigaga += alphapi * (2*log(pt/mass_tau)/3.-5./9.);

      // Hadronic vaccum contribution
      // Using simple effective parametrization from Physics Letters B 513 (2001) 46.
      // Top contribution neglected
      double A = 0.; 
      double B = 0.; 
      double C = 0.; 
      if (pt<0.7) {
            A = 0.0; B = 0.0023092; C = 3.9925370;
      } else if (pt<2.0) {
            A = 0.0; B = 0.0022333; C = 4.2191779;
      } else if (pt<4.0) {
            A = 0.0; B = 0.0024402; C = 3.2496684;
      } else if (pt<10.0) {
            A = 0.0; B = 0.0027340; C = 2.0995092;
      } else if (pt<mass_Z) {
            A = 0.0010485; B = 0.0029431; C = 1.0;
      } else if (pt<10000.) {
            A = 0.0012234; B = 0.0029237; C = 1.0;
      } else {
            A = 0.0016894; B = 0.0028984; C = 1.0;
      }
      pigaga += A + B*log(1.+C*pt*pt);

      // Done
      return 1./(1.-pigaga);
}

DEFINE_FWK_MODULE(FSRWeightProducer);