Bugfix for empty event mixing histograms

- do not add vertex/centrality subbins, if event mixing histogram is empty
- store number of triggers
- subtract from all triggers for correct normalization

diff --git a/PWGCF/EBYE/BalanceFunctions/AliBalancePsi.cxx b/PWGCF/EBYE/BalanceFunctions/AliBalancePsi.cxx
index 87f14fdb7f9b64b48f0e1c88bda03ccba58d06c7..41fa4ca4a76956286c4d5b48bab9b14a86f6f145 100644 (file)
--- a/PWGCF/EBYE/BalanceFunctions/AliBalancePsi.cxx
+++ b/PWGCF/EBYE/BalanceFunctions/AliBalancePsi.cxx
@@ -1918,6 +1918,10 @@ TH2D *AliBalancePsi::GetCorrelationFunction(TString type,
   Double_t binVertexLowEdge = 0.;
   Double_t binVertexUpEdge  = 1000.;
 
+  // if event mixing is empty, we can not add that sub bin
+  // need to record the number of triggers for correct normalization
+  Double_t nTrigSubBinEmpty = 0.;
+
   // loop over all bins
   for(Int_t iBinPsi = binPsiMin; iBinPsi <= binPsiMax; iBinPsi++){
     for(Int_t iBinVertex = binVertexMin; iBinVertex <= binVertexMax; iBinVertex++){
@@ -1981,8 +1985,10 @@ TH2D *AliBalancePsi::GetCorrelationFunction(TString type,
 
       if(fSame && fMixed){
        // then get the correlation function (divide fSame/fmixed)
-       fSame->Divide(fMixed);
-       
+
+       if(fMixed->Integral()>0)
+         fSame->Divide(fMixed);
+
        // averaging with number of triggers:
        // average over number of triggers in each sub-bin
        Double_t NTrigSubBin = 0;
@@ -1994,13 +2000,23 @@ TH2D *AliBalancePsi::GetCorrelationFunction(TString type,
          NTrigSubBin = (Double_t)(fHistN->Project(0,1)->Integral() + fHistP->Project(0,1)->Integral());
        fSame->Scale(NTrigSubBin);
        
-       // for the first: clone
-       if( (iBinPsi == binPsiMin && iBinVertex == binVertexMin) || !gHist ){
-         gHist = (TH2D*)fSame->Clone();
+       // only if event mixing has enough statistics
+       if(fMixed->Integral()>0){
+         
+         // for the first: clone
+         if( (iBinPsi == binPsiMin && iBinVertex == binVertexMin) || !gHist ){
+           gHist = (TH2D*)fSame->Clone();
+         }
+         else{  // otherwise: add for averaging
+           gHist->Add(fSame);
+         }
        }
-       else{  // otherwise: add for averaging
-         gHist->Add(fSame);
+
+       // otherwise record the number of triggers for correct normalization
+       else{
+         nTrigSubBinEmpty += NTrigSubBin;
        }
+
       }
     }
   }
@@ -2031,8 +2047,11 @@ TH2D *AliBalancePsi::GetCorrelationFunction(TString type,
       fHistP->GetGrid(0)->GetGrid()->GetAxis(1)->SetRangeUser(ptTriggerMin,ptTriggerMax-0.00001);
       NTrigAll = (Double_t)(fHistN->Project(0,1)->Integral() + fHistP->Project(0,1)->Integral());
     }
+
+    // subtract number of triggers with empty sub bins for correct normalization
+    NTrigAll -= nTrigSubBinEmpty;
+
     gHist->Scale(1./NTrigAll);
-   
   }
   
   return gHist;