/data/refman/pasoursint/CMSSW_4_1_8_patch9/src/RecoVertex/BeamSpotProducer/src/PVFitter.cc

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#include "RecoVertex/BeamSpotProducer/interface/PVFitter.h"
#include "FWCore/ServiceRegistry/interface/Service.h"
#include "CondCore/DBOutputService/interface/PoolDBOutputService.h"
#include "CondFormats/BeamSpotObjects/interface/BeamSpotObjects.h"

#include "FWCore/Utilities/interface/InputTag.h"
#include "DataFormats/Common/interface/View.h"

#include "DataFormats/TrackCandidate/interface/TrackCandidate.h"
#include "DataFormats/TrackCandidate/interface/TrackCandidateCollection.h"
#include "DataFormats/TrackReco/interface/Track.h"
#include "DataFormats/TrackReco/interface/TrackFwd.h"
#include "DataFormats/TrackReco/interface/HitPattern.h"
#include "DataFormats/VertexReco/interface/Vertex.h"
#include "DataFormats/VertexReco/interface/VertexFwd.h"

#include "TFitterMinuit.h"
#include "Minuit2/FCNBase.h"
#include "RecoVertex/BeamSpotProducer/interface/FcnBeamSpotFitPV.h"

#include "TF1.h"

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

// ----------------------------------------------------------------------
// Useful function:
// ----------------------------------------------------------------------

// static char * formatTime(const std::time_t & t)  {
//   struct std::tm * ptm;
//   ptm = gmtime(&t);
//   static char ts[32];
//   strftime(ts,sizeof(ts),"%Y.%m.%d %H:%M:%S %Z",ptm);
//   return ts;
// }

PVFitter::PVFitter(const edm::ParameterSet& iConfig): ftree_(0)
{

  debug_             = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<bool>("Debug");
  vertexLabel_     = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<edm::InputTag>("VertexCollection", edm::InputTag("offlinePrimaryVertices"));
  do3DFit_           = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<bool>("Apply3DFit");
  //writeTxt_          = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<bool>("WriteAscii");
  //outputTxt_         = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<std::string>("AsciiFileName");

  maxNrVertices_     = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<unsigned int>("maxNrStoredVertices");
  minNrVertices_     = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<unsigned int>("minNrVerticesForFit");
  minVtxNdf_         = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("minVertexNdf");
  maxVtxNormChi2_    = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("maxVertexNormChi2");
  minVtxTracks_      = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<unsigned int>("minVertexNTracks");
  minVtxWgt_         = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("minVertexMeanWeight");
  maxVtxR_           = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("maxVertexR");
  maxVtxZ_           = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("maxVertexZ");
  errorScale_        = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("errorScale");
  sigmaCut_          = iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<double>("nSigmaCut");
  fFitPerBunchCrossing=iConfig.getParameter<edm::ParameterSet>("PVFitter").getUntrackedParameter<bool>("FitPerBunchCrossing");

  // preset quality cut to "infinite"
  dynamicQualityCut_ = 1.e30;

  hPVx = new TH2F("hPVx","PVx vs PVz distribution",200,-maxVtxR_, maxVtxR_, 200, -maxVtxZ_, maxVtxZ_);
  hPVy = new TH2F("hPVy","PVy vs PVz distribution",200,-maxVtxR_, maxVtxR_, 200, -maxVtxZ_, maxVtxZ_);
}

PVFitter::~PVFitter() {

}


void PVFitter::readEvent(const edm::Event& iEvent)
{

//   frun = iEvent.id().run();
//   const edm::TimeValue_t ftimestamp = iEvent.time().value();
//   const std::time_t ftmptime = ftimestamp >> 32;

//   if (fbeginLumiOfFit == -1) freftime[0] = freftime[1] = ftmptime;
//   if (freftime[0] == 0 || ftmptime < freftime[0]) freftime[0] = ftmptime;
//   const char* fbeginTime = formatTime(freftime[0]);
//   sprintf(fbeginTimeOfFit,"%s",fbeginTime);

//   if (freftime[1] == 0 || ftmptime > freftime[1]) freftime[1] = ftmptime;
//   const char* fendTime = formatTime(freftime[1]);
//   sprintf(fendTimeOfFit,"%s",fendTime);

//   flumi = iEvent.luminosityBlock();
//   frunFit = frun;

//   if (fbeginLumiOfFit == -1 || fbeginLumiOfFit > flumi) fbeginLumiOfFit = flumi;
//   if (fendLumiOfFit == -1 || fendLumiOfFit < flumi) fendLumiOfFit = flumi;
//   std::cout << "flumi = " <<flumi<<"; fbeginLumiOfFit = " << fbeginLumiOfFit <<"; fendLumiOfFit = "<<fendLumiOfFit<<std::endl;

  //------ Primary Vertices
  edm::Handle< reco::VertexCollection > PVCollection;
  bool hasPVs = false;
  //edm::View<reco::Vertex> vertices;
  //const reco::VertexCollection & vertices = 0;

  if ( iEvent.getByLabel(vertexLabel_, PVCollection ) ) {
      //pv = *PVCollection;
      //vertices = *PVCollection;
      hasPVs = true;
  }
  //------

  //------ Beam Spot in current event
  edm::Handle<reco::BeamSpot> recoBeamSpotHandle;
  const reco::BeamSpot *refBS =  0;
  if ( iEvent.getByLabel("offlineBeamSpot",recoBeamSpotHandle) )
      refBS = recoBeamSpotHandle.product();
  //-------


  if ( hasPVs ) {

      for (reco::VertexCollection::const_iterator pv = PVCollection->begin(); pv != PVCollection->end(); ++pv ) {


           //for ( size_t ipv=0; ipv != pv.size(); ++ipv ) {

          //--- vertex selection
          if ( pv->isFake() || pv->tracksSize()==0 )  continue;
          if ( pv->ndof() < minVtxNdf_ || (pv->ndof()+3.)/pv->tracksSize()<2*minVtxWgt_ )  continue;
          //---

          hPVx->Fill( pv->x(), pv->z() );
          hPVy->Fill( pv->y(), pv->z() );

          //
          // 3D fit section
          //
          // apply additional quality cut
          if ( pvQuality(*pv)>dynamicQualityCut_ )  continue;
          // if store exceeds max. size: reduce size and apply new quality cut
          if ( pvStore_.size()>=maxNrVertices_ ) {
             compressStore();
             if ( pvQuality(*pv)>dynamicQualityCut_ )  continue;
          }
          //
          // copy PV to store
          //
          int bx = iEvent.bunchCrossing();
          BeamSpotFitPVData pvData;
          pvData.bunchCrossing = bx;
          pvData.position[0] = pv->x();
          pvData.position[1] = pv->y();
          pvData.position[2] = pv->z();
          pvData.posError[0] = pv->xError();
          pvData.posError[1] = pv->yError();
          pvData.posError[2] = pv->zError();
          pvData.posCorr[0] = pv->covariance(0,1)/pv->xError()/pv->yError();
          pvData.posCorr[1] = pv->covariance(0,2)/pv->xError()/pv->zError();
          pvData.posCorr[2] = pv->covariance(1,2)/pv->yError()/pv->zError();
          pvStore_.push_back(pvData);

          if(ftree_ != 0){
            theBeamSpotTreeData_.run(iEvent.id().run());
            theBeamSpotTreeData_.lumi(iEvent.luminosityBlock());
            theBeamSpotTreeData_.bunchCrossing(bx);
            theBeamSpotTreeData_.pvData(pvData);
            ftree_->Fill();
          }

          if (fFitPerBunchCrossing) bxMap_[bx].push_back(pvData);

      }

  }




}

void PVFitter::setTree(TTree* tree){
  ftree_ = tree;
  theBeamSpotTreeData_.branch(ftree_);
}

bool PVFitter::runBXFitter() {

  edm::LogInfo("PVFitter") << " Number of bunch crossings: " << bxMap_.size() << std::endl;

  bool fit_ok = true;

  for ( std::map<int,std::vector<BeamSpotFitPVData> >::const_iterator pvStore = bxMap_.begin();
        pvStore!=bxMap_.end(); ++pvStore) {

    // first set null beam spot in case
    // fit fails
    fbspotMap[pvStore->first] = reco::BeamSpot();

    edm::LogInfo("PVFitter") << " Number of PVs collected for PVFitter: " << (pvStore->second).size() << " in bx: " << pvStore->first << std::endl;

    if ( (pvStore->second).size() <= minNrVertices_ ) {
        edm::LogWarning("PVFitter") << " not enough PVs, continue" << std::endl;
        fit_ok = false;
      continue;
    }

    //bool fit_ok = false;
    edm::LogInfo("PVFitter") << "Calculating beam spot with PVs ..." << std::endl;

    //
    // LL function and fitter
    //
    FcnBeamSpotFitPV* fcn = new FcnBeamSpotFitPV(pvStore->second);
    TFitterMinuit minuitx;
    minuitx.SetMinuitFCN(fcn);
    //
    // fit parameters: positions, widths, x-y correlations, tilts in xz and yz
    //
    minuitx.SetParameter(0,"x",0.,0.02,-10.,10.);
    minuitx.SetParameter(1,"y",0.,0.02,-10.,10.);
    minuitx.SetParameter(2,"z",0.,0.20,-30.,30.);
    minuitx.SetParameter(3,"ex",0.015,0.01,0.,10.);
    minuitx.SetParameter(4,"corrxy",0.,0.02,-1.,1.);
    minuitx.SetParameter(5,"ey",0.015,0.01,0.,10.);
    minuitx.SetParameter(6,"dxdz",0.,0.0002,-0.1,0.1);
    minuitx.SetParameter(7,"dydz",0.,0.0002,-0.1,0.1);
    minuitx.SetParameter(8,"ez",1.,0.1,0.,30.);
    minuitx.SetParameter(9,"scale",errorScale_,errorScale_/10.,errorScale_/2.,errorScale_*2.);
    //
    // first iteration without correlations
    //
    int ierr(0);
    minuitx.FixParameter(4);
    minuitx.FixParameter(6);
    minuitx.FixParameter(7);
    minuitx.FixParameter(9);
    minuitx.SetMaxIterations(100);
    //       minuitx.SetPrintLevel(3);
    minuitx.SetPrintLevel(0);
    minuitx.CreateMinimizer();
    ierr = minuitx.Minimize();
    if ( ierr ) {
        edm::LogInfo("PVFitter") << "3D beam spot fit failed in 1st iteration" << std::endl;
        fit_ok = false;
      continue;
    }
    //
    // refit with harder selection on vertices
    //
    fcn->setLimits(minuitx.GetParameter(0)-sigmaCut_*minuitx.GetParameter(3),
                   minuitx.GetParameter(0)+sigmaCut_*minuitx.GetParameter(3),
                   minuitx.GetParameter(1)-sigmaCut_*minuitx.GetParameter(5),
                   minuitx.GetParameter(1)+sigmaCut_*minuitx.GetParameter(5),
                   minuitx.GetParameter(2)-sigmaCut_*minuitx.GetParameter(8),
                   minuitx.GetParameter(2)+sigmaCut_*minuitx.GetParameter(8));
    ierr = minuitx.Minimize();
    if ( ierr ) {
      edm::LogInfo("PVFitter") << "3D beam spot fit failed in 2nd iteration" << std::endl;
      fit_ok = false;
      continue;
    }
    //
    // refit with correlations
    //
    minuitx.ReleaseParameter(4);
    minuitx.ReleaseParameter(6);
    minuitx.ReleaseParameter(7);

    ierr = minuitx.Minimize();
    if ( ierr ) {
        edm::LogInfo("PVFitter") << "3D beam spot fit failed in 3rd iteration" << std::endl;
        fit_ok = false;
      continue;
    }
    // refit with floating scale factor
    //   minuitx.ReleaseParameter(9);
    //   minuitx.Minimize();

    //minuitx.PrintResults(0,0);

    fwidthX = minuitx.GetParameter(3);
    fwidthY = minuitx.GetParameter(5);
    fwidthZ = minuitx.GetParameter(8);
    fwidthXerr = minuitx.GetParError(3);
    fwidthYerr = minuitx.GetParError(5);
    fwidthZerr = minuitx.GetParError(8);

    reco::BeamSpot::CovarianceMatrix matrix;
    // need to get the full cov matrix
    matrix(0,0) = pow( minuitx.GetParError(0), 2);
    matrix(1,1) = pow( minuitx.GetParError(1), 2);
    matrix(2,2) = pow( minuitx.GetParError(2), 2);
    matrix(3,3) = fwidthZerr * fwidthZerr;
    matrix(4,4) = pow( minuitx.GetParError(6), 2);
    matrix(5,5) = pow( minuitx.GetParError(7), 2);
    matrix(6,6) = fwidthXerr * fwidthXerr;

    fbeamspot = reco::BeamSpot( reco::BeamSpot::Point(minuitx.GetParameter(0),
                                                      minuitx.GetParameter(1),
                                                      minuitx.GetParameter(2) ),
                                fwidthZ,
                                minuitx.GetParameter(6), minuitx.GetParameter(7),
                                fwidthX,
                                matrix );
    fbeamspot.setBeamWidthX( fwidthX );
    fbeamspot.setBeamWidthY( fwidthY );
    fbeamspot.setType( reco::BeamSpot::Tracker );

    fbspotMap[pvStore->first] = fbeamspot;
    edm::LogInfo("PVFitter") << "3D PV fit done for this bunch crossing."<<std::endl;
    minuitx.Clear();
    //delete fcn;
    fit_ok = fit_ok & true;
  }

  return fit_ok;
}


bool PVFitter::runFitter() {

    edm::LogInfo("PVFitter") << " Number of PVs collected for PVFitter: " << pvStore_.size() << std::endl;

    if ( pvStore_.size() <= minNrVertices_ ) return false;

    //bool fit_ok = false;

    if ( ! do3DFit_ ) {
      TH1F *h1PVx = (TH1F*) hPVx->ProjectionX("h1PVx", 0, -1, "e");
      TH1F *h1PVy = (TH1F*) hPVy->ProjectionX("h1PVy", 0, -1, "e");
      TH1F *h1PVz = (TH1F*) hPVx->ProjectionY("h1PVz", 0, -1, "e");

      h1PVx->Fit("gaus","QLM0");
      h1PVy->Fit("gaus","QLM0");
      h1PVz->Fit("gaus","QLM0");

      TF1 *gausx = h1PVx->GetFunction("gaus");
      TF1 *gausy = h1PVy->GetFunction("gaus");
      TF1 *gausz = h1PVz->GetFunction("gaus");

      fwidthX = gausx->GetParameter(2);
      fwidthY = gausy->GetParameter(2);
      fwidthZ = gausz->GetParameter(2);
      fwidthXerr = gausx->GetParError(2);
      fwidthYerr = gausy->GetParError(2);
      fwidthZerr = gausz->GetParError(2);

      reco::BeamSpot::CovarianceMatrix matrix;
      matrix(2,2) = gausz->GetParError(1) * gausz->GetParError(1);
      matrix(3,3) = fwidthZerr * fwidthZerr;
      matrix(6,6) = fwidthXerr * fwidthXerr;

      fbeamspot = reco::BeamSpot( reco::BeamSpot::Point(gausx->GetParameter(1),
                                                        gausy->GetParameter(1),
                                                        gausz->GetParameter(1) ),
                                  fwidthZ,
                                  0., 0.,
                                  fwidthX,
                                  matrix );
      fbeamspot.setBeamWidthX( fwidthX );
      fbeamspot.setBeamWidthY( fwidthY );
      fbeamspot.setType(reco::BeamSpot::Tracker);

    }
    else { // do 3D fit
      //
      // LL function and fitter
      //
      FcnBeamSpotFitPV* fcn = new FcnBeamSpotFitPV(pvStore_);
      TFitterMinuit minuitx;
      minuitx.SetMinuitFCN(fcn);
      //
      // fit parameters: positions, widths, x-y correlations, tilts in xz and yz
      //
      minuitx.SetParameter(0,"x",0.,0.02,-10.,10.);
      minuitx.SetParameter(1,"y",0.,0.02,-10.,10.);
      minuitx.SetParameter(2,"z",0.,0.20,-30.,30.);
      minuitx.SetParameter(3,"ex",0.015,0.01,0.,10.);
      minuitx.SetParameter(4,"corrxy",0.,0.02,-1.,1.);
      minuitx.SetParameter(5,"ey",0.015,0.01,0.,10.);
      minuitx.SetParameter(6,"dxdz",0.,0.0002,-0.1,0.1);
      minuitx.SetParameter(7,"dydz",0.,0.0002,-0.1,0.1);
      minuitx.SetParameter(8,"ez",1.,0.1,0.,30.);
      minuitx.SetParameter(9,"scale",errorScale_,errorScale_/10.,errorScale_/2.,errorScale_*2.);
      //
      // first iteration without correlations
      //
      int ierr(0);
      minuitx.FixParameter(4);
      minuitx.FixParameter(6);
      minuitx.FixParameter(7);
      minuitx.FixParameter(9);
      minuitx.SetMaxIterations(100);
//       minuitx.SetPrintLevel(3);
      minuitx.SetPrintLevel(0);
      minuitx.CreateMinimizer();
      ierr = minuitx.Minimize();
      if ( ierr ) {
          edm::LogWarning("PVFitter") << "3D beam spot fit failed in 1st iteration" << std::endl;
          return false;
      }
      //
      // refit with harder selection on vertices
      //
      fcn->setLimits(minuitx.GetParameter(0)-sigmaCut_*minuitx.GetParameter(3),
                     minuitx.GetParameter(0)+sigmaCut_*minuitx.GetParameter(3),
                     minuitx.GetParameter(1)-sigmaCut_*minuitx.GetParameter(5),
                     minuitx.GetParameter(1)+sigmaCut_*minuitx.GetParameter(5),
                     minuitx.GetParameter(2)-sigmaCut_*minuitx.GetParameter(8),
                     minuitx.GetParameter(2)+sigmaCut_*minuitx.GetParameter(8));
      ierr = minuitx.Minimize();
      if ( ierr ) {
          edm::LogWarning("PVFitter") << "3D beam spot fit failed in 2nd iteration" << std::endl;
          return false;
      }
      //
      // refit with correlations
      //
      minuitx.ReleaseParameter(4);
      minuitx.ReleaseParameter(6);
      minuitx.ReleaseParameter(7);
      ierr = minuitx.Minimize();
      if ( ierr ) {
          edm::LogWarning("PVFitter") << "3D beam spot fit failed in 3rd iteration" << std::endl;
          return false;
      }
      // refit with floating scale factor
      //   minuitx.ReleaseParameter(9);
      //   minuitx.Minimize();

      //minuitx.PrintResults(0,0);

      fwidthX = minuitx.GetParameter(3);
      fwidthY = minuitx.GetParameter(5);
      fwidthZ = minuitx.GetParameter(8);
      fwidthXerr = minuitx.GetParError(3);
      fwidthYerr = minuitx.GetParError(5);
      fwidthZerr = minuitx.GetParError(8);

      reco::BeamSpot::CovarianceMatrix matrix;
      // need to get the full cov matrix
      matrix(0,0) = pow( minuitx.GetParError(0), 2);
      matrix(1,1) = pow( minuitx.GetParError(1), 2);
      matrix(2,2) = pow( minuitx.GetParError(2), 2);
      matrix(3,3) = fwidthZerr * fwidthZerr;
      matrix(6,6) = fwidthXerr * fwidthXerr;

      fbeamspot = reco::BeamSpot( reco::BeamSpot::Point(minuitx.GetParameter(0),
                                                        minuitx.GetParameter(1),
                                                        minuitx.GetParameter(2) ),
                                  fwidthZ,
                                  minuitx.GetParameter(6), minuitx.GetParameter(7),
                                  fwidthX,
                                  matrix );
      fbeamspot.setBeamWidthX( fwidthX );
      fbeamspot.setBeamWidthY( fwidthY );
      fbeamspot.setType(reco::BeamSpot::Tracker);
    }
    return true; //FIXME: Need to add quality test for the fit results!
}

void PVFitter::dumpTxtFile(){
/*
  fasciiFile << "Runnumber " << frun << std::endl;
  fasciiFile << "BeginTimeOfFit " << fbeginTimeOfFit << std::endl;
  fasciiFile << "EndTimeOfFit " << fendTimeOfFit << std::endl;
  fasciiFile << "LumiRange " << fbeginLumiOfFit << " - " << fendLumiOfFit << std::endl;
  fasciiFile << "Type " << fbeamspot.type() << std::endl;
  fasciiFile << "X0 " << fbeamspot.x0() << std::endl;
  fasciiFile << "Y0 " << fbeamspot.y0() << std::endl;
  fasciiFile << "Z0 " << fbeamspot.z0() << std::endl;
  fasciiFile << "sigmaZ0 " << fbeamspot.sigmaZ() << std::endl;
  fasciiFile << "dxdz " << fbeamspot.dxdz() << std::endl;
  fasciiFile << "dydz " << fbeamspot.dydz() << std::endl;
  if (inputBeamWidth_ > 0 ) {
    fasciiFile << "BeamWidthX " << inputBeamWidth_ << std::endl;
    fasciiFile << "BeamWidthY " << inputBeamWidth_ << std::endl;
  } else {
    fasciiFile << "BeamWidthX " << fbeamspot.BeamWidthX() << std::endl;
    fasciiFile << "BeamWidthY " << fbeamspot.BeamWidthY() << std::endl;
  }

  for (int i = 0; i<6; ++i) {
    fasciiFile << "Cov("<<i<<",j) ";
    for (int j=0; j<7; ++j) {
      fasciiFile << fbeamspot.covariance(i,j) << " ";
    }
    fasciiFile << std::endl;
  }
  // beam width error
  if (inputBeamWidth_ > 0 ) {
    fasciiFile << "Cov(6,j) 0 0 0 0 0 0 " << "1e-4" << std::endl;
  } else {
    fasciiFile << "Cov(6,j) 0 0 0 0 0 0 " << fbeamspot.covariance(6,6) << std::endl;
  }
  fasciiFile << "EmittanceX " << fbeamspot.emittanceX() << std::endl;
  fasciiFile << "EmittanceY " << fbeamspot.emittanceY() << std::endl;
  fasciiFile << "BetaStar " << fbeamspot.betaStar() << std::endl;

*/
}


void
PVFitter::compressStore ()
{
  //
  // fill vertex qualities
  //
  pvQualities_.resize(pvStore_.size());
  for ( unsigned int i=0; i<pvStore_.size(); ++i )  pvQualities_[i] = pvQuality(pvStore_[i]);
  sort(pvQualities_.begin(),pvQualities_.end());
  //
  // Set new quality cut to median. This cut will be used to reduce the
  // number of vertices in the store and also apply to all new vertices
  // until the next reset
  //
  dynamicQualityCut_ = pvQualities_[pvQualities_.size()/2];
  //
  // remove all vertices failing the cut from the store
  //   (to be moved to a more efficient memory management!)
  //
  unsigned int iwrite(0);
  for ( unsigned int i=0; i<pvStore_.size(); ++i ) {
    if ( pvQuality(pvStore_[i])>dynamicQualityCut_ )  continue;
    if ( i!=iwrite )  pvStore_[iwrite] = pvStore_[i];
    ++iwrite;
  }
  pvStore_.resize(iwrite);
  edm::LogInfo("PVFitter") << "Reduced primary vertex store size to "
                           << pvStore_.size() << " ; new dynamic quality cut = "
                           << dynamicQualityCut_ << std::endl;

}

double
PVFitter::pvQuality (const reco::Vertex& pv) const
{
  //
  // determinant of the transverse part of the PV covariance matrix
  //
  return
    pv.covariance(0,0)*pv.covariance(1,1)-
    pv.covariance(0,1)*pv.covariance(0,1);
}

double
PVFitter::pvQuality (const BeamSpotFitPVData& pv) const
{
  //
  // determinant of the transverse part of the PV covariance matrix
  //
  double ex = pv.posError[0];
  double ey = pv.posError[1];
  return ex*ex*ey*ey*(1-pv.posCorr[0]*pv.posCorr[0]);
}