MEzCalculator.cc

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#include "TopQuarkAnalysis/TopTools/interface/MEzCalculator.h"
#include "TMath.h"

MEzCalculator::MEzCalculator() 
{
  isComplex_ = false;
  isMuon_ = true;
}

MEzCalculator::~MEzCalculator() 
{
}

double
MEzCalculator::Calculate(int type) 
{
  if(type<0 || type>3)
    throw cms::Exception("UnimplementedFeature") << "Type " << type << " not supported in MEzCalculator.\n";

  double M_W  = 80.4;
  double M_mu =  0.10566;
  double M_e = 0.511e-3;
  double M_lepton = M_mu;
  if (! isMuon_ ) M_lepton = M_e;
  
  double emu = lepton_.energy();
  double pxmu = lepton_.px();
  double pymu = lepton_.py();
  double pzmu = lepton_.pz();
  double pxnu = MET_.px();
  double pynu = MET_.py();
  double pznu = 0.;

  // use pznu = - B/2*A +/- sqrt(B*B-4*A*C)/(2*A)
  
  double a = M_W*M_W - M_lepton*M_lepton + 2.0*(pxmu*pxnu + pymu*pynu);
  double A = 4.0*(emu*emu - pzmu*pzmu);
  double B = -4.0*a*pzmu;
  double C = 4.0*emu*emu*(pxnu*pxnu + pynu*pynu) - a*a;
  
  double tmproot = B*B - 4.0*A*C;
  
  if (tmproot<0) {
    isComplex_= true;
    pznu = - B/(2*A); // take real part of complex roots
  }
  else {
    isComplex_ = false;
    double tmpsol1 = (-B + TMath::Sqrt(tmproot))/(2.0*A);
    double tmpsol2 = (-B - TMath::Sqrt(tmproot))/(2.0*A);
    
    if (type == 0 ) {
      // two real roots, pick the one closest to pz of muon
      if (TMath::Abs(tmpsol2-pzmu) < TMath::Abs(tmpsol1-pzmu)) { pznu = tmpsol2;}
      else pznu = tmpsol1;
      // if pznu is > 300 pick the most central root
      if ( pznu > 300. ) {
    if (TMath::Abs(tmpsol1)<TMath::Abs(tmpsol2) ) pznu = tmpsol1;
    else pznu = tmpsol2;
      }
    }
    if (type == 1 ) {
      // two real roots, pick the one closest to pz of muon
      if (TMath::Abs(tmpsol2-pzmu) < TMath::Abs(tmpsol1-pzmu)) { pznu = tmpsol2;}
      else pznu = tmpsol1;
    }
    if (type == 2 ) {
      // pick the most central root.
      if (TMath::Abs(tmpsol1)<TMath::Abs(tmpsol2) ) pznu = tmpsol1;
      else pznu = tmpsol2;
    }
    if (type == 3 ) {
      // pick the largest value of the cosine
      TVector3 p3w, p3mu;
      p3w.SetXYZ(pxmu+pxnu, pymu+pynu, pzmu+ tmpsol1);
      p3mu.SetXYZ(pxmu, pymu, pzmu );
      
      double sinthcm1 = 2.*(p3mu.Perp(p3w))/M_W;
      p3w.SetXYZ(pxmu+pxnu, pymu+pynu, pzmu+ tmpsol2);
      double sinthcm2 = 2.*(p3mu.Perp(p3w))/M_W;
      
      double costhcm1 = TMath::Sqrt(1. - sinthcm1*sinthcm1);
      double costhcm2 = TMath::Sqrt(1. - sinthcm2*sinthcm2);
      
      if ( costhcm1 > costhcm2 ) pznu = tmpsol1;
      else pznu = tmpsol2;
    }   

  }
  
  //Particle neutrino;
  //neutrino.setP4( LorentzVector(pxnu, pynu, pznu, TMath::Sqrt(pxnu*pxnu + pynu*pynu + pznu*pznu ))) ;
  
  return pznu;
}