void muon_lifetime() {

  // Begin by loading the libraries
  gSystem->CompileMacro("Calibrate.C");
  gSystem->CompileMacro("LikelihoodCurve.C");

  // first we open the data file and fill an histogram
  TFile *_file0 = TFile::Open("data.root");
  // the binning is chosen very fine
  TH1F* h = new TH1F("histo","histo",1000,0,4096);
  data->Draw("dt>>histo");
  // we prepare the fit function
  TF1* f1=new TF1("user","[0]*exp(-(x)/[1])+[2]*0",0,4000);
  // with such a small binning, we must do a likelihood fit (we also set the verbose flag to get all details)
  user->SetParameters(40,100);
  user->FixParameter(2,1);
  histo->Fit("user","LV");
  // fitting over the full range, we obtain a rather bad chi2
  std::cout << "chi2/ndf for a fit over the full range: " << user->GetChisquare()/user->GetNDF() << std::endl;
  std::cout << "the corresponding probability is " << TMath::Prob(user->GetChisquare(),user->GetNDF())<< std::endl; 
  // fitting over a limited range, the result still looks rather bad.
  // the normalized chi2 is ~1.2, but the NDOF is large and we would expect something
  // much better. TMath::Prob gives ~1E-4
  histo->Fit("user","LV","",500,3500);
  float nchi2 = user->GetChisquare()/user->GetNDF();
  std::cout << "chi2/ndf for a fit over [500,3500]: " << nchi2 << std::endl;
  std::cout << "the corresponding probability is " << TMath::Prob(user->GetChisquare(),user->GetNDF())<< std::endl; 

  // to understand better if the chi2 probability makes sense, we can calibrate the cdf in our case.
  int nToyMc = 10000;
  TH1F* chisqu = Calibrate(histo,user,nToyMc,500,3500);
  // the distribution obtained is centered at ~1.2 !
  // and the calibrated probability is ~70% ! The fit is actually excellent.
  std::cout << "calibrated fit probability is " << (1-(chisqu->Integral(0,chisqu->FindBin(nchi2))/float(nToyMc)))<< std::endl;
  
  // here, we plot the likelihood curves
  histo->Fit("user","LQ0","",500,3500);
  user->SetParLimits(2,1,2); // that parameter is just to let minuit compute something
  LikelihoodCurve(histo,user);
  
  // now, if we rebin, we can fit with a chi2. 
  // We then see the fit is ok:
  // TMath::Prob(lastFitFunc->GetChisquare(),lastFitFunc->GetNDF()) ~0.5
  // but note that we lost the details of the beginning of the curve. We don't see there is a problem there anymore.

}