void calc_eta_2g_eff(){
  //TH1F *h1=(TH1F *)_file0->FindObjectAny("MCBeam");
  TH1F *h1=(TH1F *)_file0->FindObjectAny("MCBeamTagged");
  TH2F *h2=(TH2F *)_file0->FindObjectAny("TwoGammaMass_vs_Ebeam");
  
  ofstream out("eta_2g_eff.dat");
  double eff[45];
  double deff[45];
  double Eg[45];
  for (int i=1;i<=45;i++){
    double numer=h2->Integral(i,i,201,240); // 0.5-0.6 GeV
    double denom=h1->GetBinContent(i)+0.00001;
    eff[i-1]=numer/denom;
    deff[i-1]=eff[i-1]*sqrt(1./numer+1./denom);
    Eg[i-1]=h1->GetBinCenter(i);
    out <<  " " << eff[i-1] << " " << deff[i-1] << endl;
  }
  out.close();

  TCanvas *c1=new TCanvas();
  TH2F *he=new TH2F("he","Efficiency for #eta#rightarrow2#gamma",10,2.8,11.8,10,0,0.5);
  he->SetXTitle("E_{#gamma} [GeV]");
  he->SetYTitle("#varepsilon");
  he->Draw();
  TGraphErrors *g1=new TGraphErrors(45,Eg,eff,0,deff);
  g1->Draw("p");
}