void tree_example2() {

  TH1F *hist1 = new TH1F("hist1", "hist1", 100, -200., 200.0);
  TH1F *hist2 = new TH1F("hist2", "hist2", 100, -200., 200.0);
  TH1F *hist3 = new TH1F("hist3", "hist3", 100, -200., 200.0);
  TH1F *hist4 = new TH1F("M_Z", "Masse of Z boson", 50, 0., 200.0);
  TH1F *hist5 = new TH1F("P_Z", "Momentum of Z boson", 100, -2000.0, 2000.0);

  // --------
  //first method to read the root file
  
  //TFile *f = TFile::Open("ntuple_array.root");
  // ...
  // --------


  
  // -------
  //second method to read the root file

  /*TFile *f;
  f = new TFile("ntuple_array.root");
  f->ls();  
  TDirectoryFile *dir1 = (TDirectoryFile*)f->Get("ntupleProducer;1");
  TTree *tree = (TTree*)dir1->FindObject("tree;1");*/
  // -------


  
  // -------
  //third method to read the root file
  
  TFile *f = new TFile("ntuple_array.root");
  TTree *tree = (TTree*)f->Get("ntupleProducer/tree;1");
  // -------

  TCanvas c; 

  c.SaveAs("plots.pdf["); // opening pdf   
  
  TCut cut1 = "genParticleCharge != 0";
  
  Float_t genParticlePx, genParticlePy, genParticlePz, PZ, En, MZ, deltaMZ(10.);

  hist1->GetXaxis()->SetTitle("genParticlePx");
  tree->Draw("genParticlePx>>hist1", cut1);
  hist1->Draw("colz");
  c.SaveAs("plot.pdf");
  c.Clear();

  c.SaveAs("plots.pdf]");

  f->Close();  
  
  
  /*tree->SetBranchAddress("genParticlePx",&genParticlePx);
  tree->SetBranchAddress("genParticlePy",&genParticlePy);
  tree->SetBranchAddress("genParticlePz",&genParticlePz);

  Int_t nentries = (Int_t)tree->GetEntries(); 
  
  for (Int_t i = 0; i<nentries; i++) {
    tree->GetEntry(i);

    hist1->Fill(genParticlePx);
    hist1->Fill(genParticlePy);
    hist1->Fill(genParticlePz);
    En = sqrt(genParticlePx*genParticlePx+genParticlePy*genParticlePy+genParticlePz*genParticlePz);
    //PZ = (En+genParticlePx+genParticlePy+genParticlePz)*2;
    MZ = sqrt(En*En+genParticlePx*genParticlePx+genParticlePy*genParticlePy+genParticlePz*genParticlePz);
    hist5->Fill(PZ);    
    //if (abs(MZ-91) <= 2*deltaMZ){
    hist4->Fill(MZ);
    //}
  }
  
  TCanvas *c1 = new TCanvas();
  hist4->GetXaxis()->SetTitle("Masse of Z boson");
  hist4->Draw();
  hist4->SetFillColor(kBlue-9);
  c1->SaveAs("masseZ.pdf");*/

  /*TCanvas *c2 = new TCanvas();
  hist5->GetXaxis()->SetTitle("Momentum of Z boson");
  hist5->Draw();
  hist5->SetFillColor(kBlue-9);
  c2->SaveAs("PZ.pdf");*/

}