/afs/cern.ch/work/a/aaltunda/public/www/CMSSW_5_3_14/src/EgammaAnalysis/ElectronTools/src/ElectronEPcombinator.cc

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#include "EgammaAnalysis/ElectronTools/interface/ElectronEPcombinator.h"
#include <iostream>

//Accessor to the combination results
void ElectronEPcombinator::combine(SimpleElectron & electron)
{
        electron_ = electron;
        computeEPcombination();
        electron.setCombinedMomentum(combinedMomentum_);
        electron.setCombinedMomentumError(combinedMomentumError_);
}

//Core code to compute the EP combination
void ElectronEPcombinator::computeEPcombination()
{
    if ( mode_==1 )
    {
        scEnergy_ = electron_.getNewEnergy();
        scEnergyError_ = electron_.getNewEnergyError();
    }
    if ( mode_==2 )
    {
        scEnergy_ = electron_.getRegEnergy();
        scEnergyError_ = electron_.getRegEnergyError();
    }
    trackerMomentum_ = electron_.getTrackerMomentum();
    trackerMomentumError_ = electron_.getTrackerMomentumError();
    elClass_ = electron_.getElClass();
    
    combinedMomentum_ = scEnergy_; // initial
    combinedMomentumError_ = 999.;
    
    // first check for large errors
    
    if ( trackerMomentumError_/trackerMomentum_ > 0.5 
        && scEnergyError_/scEnergy_ <= 0.5 ) 
    {
      combinedMomentum_ = scEnergy_;    
      combinedMomentumError_ = scEnergyError_;
    }
    else if ( trackerMomentumError_/trackerMomentum_ <= 0.5 
            && scEnergyError_/scEnergy_ > 0.5 )
    {
      combinedMomentum_ = trackerMomentum_;  
      combinedMomentumError_ = trackerMomentumError_;
    }
    else if ( trackerMomentumError_/trackerMomentum_ > 0.5 
            && scEnergyError_/scEnergy_ > 0.5 )
    {
        if ( trackerMomentumError_/trackerMomentum_ < scEnergyError_/scEnergy_ ) 
        {
            combinedMomentum_ = trackerMomentum_; 
            combinedMomentumError_ = trackerMomentumError_;
        }
        else
        {
            combinedMomentum_ = scEnergy_; 
            combinedMomentumError_ = scEnergyError_;
        }
    }
    
    // then apply the combination algorithm
    else 
    {
        // calculate E/p and corresponding error
        float eOverP = scEnergy_ / trackerMomentum_;
        float errorEOverP = sqrt(
                             (scEnergyError_/trackerMomentum_)*(scEnergyError_/trackerMomentum_) +
                             (scEnergy_*trackerMomentumError_/trackerMomentum_/trackerMomentum_) *
                             (scEnergy_*trackerMomentumError_/trackerMomentum_/trackerMomentum_) );
        
        bool eleIsNotInCombination = false;
        if ( ( eOverP  > 1 + 2.5*errorEOverP ) 
                 || ( eOverP  < 1 - 2.5*errorEOverP ) 
                 || ( eOverP < 0.8 ) || ( eOverP > 1.3 ) )
        { 
            eleIsNotInCombination = true; 
        }

        if ( eleIsNotInCombination )
        {
            if ( eOverP > 1 )
            { 
                combinedMomentum_ = scEnergy_; 
                combinedMomentumError_ = scEnergyError_; 
            }
            else
            {
                if ( elClass_ == 0 ) // == reco::GsfElectron::GOLDEN)
                { 
                    combinedMomentum_ = scEnergy_; 
                    combinedMomentumError_ = scEnergyError_; 
                }
                if ( elClass_ == 1 ) //reco::GsfElectron::BIGBREM)
                {
                    if ( scEnergy_ < 36 )
                    { 
                        combinedMomentum_ = trackerMomentum_; 
                        combinedMomentumError_ = trackerMomentumError_; 
                    } else
                    { 
                        combinedMomentum_ = scEnergy_; 
                        combinedMomentumError_ = scEnergyError_; 
                    }
                }
                if ( elClass_ == 2 ) // == reco::GsfElectron::BADTRACK)
                { 
                    combinedMomentum_ = scEnergy_; 
                    combinedMomentumError_ = scEnergyError_; 
                }
                if ( elClass_ == 3 ) //reco::GsfElectron::SHOWERING)
                {
                    if ( scEnergy_ < 30 )
                    { 
                        combinedMomentum_ = trackerMomentum_; 
                        combinedMomentumError_ = trackerMomentumError_; 
                    } else
                    { 
                        combinedMomentum_ = scEnergy_; 
                        combinedMomentumError_ = scEnergyError_;
                    }
                }
                if ( elClass_ == 4 ) //reco::GsfElectron::GAP)
                {
                    if ( scEnergy_ < 60 )
                    { 
                        combinedMomentum_ = trackerMomentum_; 
                        combinedMomentumError_ = trackerMomentumError_; 
                    } else
                    { 
                        combinedMomentum_ = scEnergy_; 
                        combinedMomentumError_ = scEnergyError_; 
                    }
                }
            }
        } else
        {
         // combination
         combinedMomentum_ = (scEnergy_/scEnergyError_/scEnergyError_ 
                            + trackerMomentum_/trackerMomentumError_/trackerMomentumError_) /
                            (1/scEnergyError_/scEnergyError_ + 1/trackerMomentumError_/trackerMomentumError_);
         float combinedMomentum_Variance = 1 / (1/scEnergyError_/scEnergyError_ + 1/trackerMomentumError_/trackerMomentumError_);
         combinedMomentumError_ = sqrt(combinedMomentum_Variance);
        }
    } 
}