analyze_eta_dalitz.C

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//TITLE Analyze the eta Dalitz output file
 
{
    // This macro uses the output of 
    // * complete_eta.C
    // * parse_eta.C
    // * hello_world.C (can have an filter)
 
    TFile *f = new TFile("eta_dalitz.root");
    TTree *Reaction = (TTree*)gDirectory->Get("data");
    TClonesArray *evt=new TClonesArray("PParticle", 100);
    Reaction->SetBranchAddress("Particles", &evt);
    PParticle *ep, *em;
 
    TH1F *hf1= new TH1F("hf2", "Di-Lepton mass", 100, 0., 0.6);
    Int_t nentries = Reaction->GetEntries();
    if (nentries > 1000) 
    nentries = 1000; //limit number of events
    
    for (Int_t i=0; i<nentries; i++) {
    ep = NULL;
    em = NULL;
    Reaction->GetEntry(i);
 
    for (int j=0; j<evt->GetEntriesFast(); j++) {
        PParticle *current = (PParticle*)evt->At(j);
        //current->Print();
        if (current->Is("e+")) ep = current;
        if (current->Is("e-")) em = current;
    }
    
    if (ep && em) {
        //particles found
 
        //It is very important to parse the PParticle
        //to a TLorentzVector, because the "+" operator
        //is reserved for a reaction (i.e. a compound
        //particle is created
        
        TLorentzVector dilepton =
        (*(TLorentzVector *) ep) + (*(TLorentzVector *) em);
        hf1->Fill(dilepton.M());
    }
    }
 
    hf1->Draw();
}