Two macros that open a root file and save a histogram. The other one opens a root file, retrieves a histogram, and fits a function to it.

create-hist.C

{
  float binWeights[12]={180,135,120,105,76,68,60,41,30,35,26,18};

  TH1F kDecayHist("kDecayHist", "Decay length of K^{0} decays;decay length(cm);events",12,0,60);
  for(int i=0; i < kDecayHist.GetNbinsX(); ++i){
    kDecayHist.SetBinContent(i+1,binWeights[i]);
  }

  //recreate overwrites any old files and puts our new stuff inside without warning!
  TFile f("kaondecay.root","RECREATE");
  kDecayHist.Write();
  f.Write();
  f.Close();
}

fit-kaon-data.C

Double_t expDecay(Double_t* v,Double_t *par) {
  return par[0]*TMath::Exp(-v[0]/par[1]);
}

void root_tutorial2(){
  TFile f("kaondecay.root","READ");
  TH1F* hist=f.Get("kDecayHist");
  hist->Sumw2();
  TF1 fitfn("fitfn",expDecay,0,60,2);
  fitfn.SetParameters(200,20);
  fitfn.SetParNames("Norm","Decay_length");
  hist->Fit("fitfn");
  hist->Draw();
  fitfn.Draw("same");
  //L=beta*gamma*c*tau
  //tau=L/(beta*gamma*c)
  Double_t bgc=(5/0.497)*299792458;
  Double_t L=fitfn.GetParameter(1)/100;
  Double_t err=fitfn.GetParError(1);
  Double_t tau=L/bgc;
  cout<<"measured K0 lifetime: "<<tau<<" +/-"<<err/bgc<<endl;
  cout<<"PDG K_L: (5.116+/-0.021)e-8 s"<<endl;
  cout<<"PDG K_S: (8.9564+/-0.0033)e-11 s"<<endl;
}

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