expo()
{
  gSystem->Load("libRooFit");
  using namespace RooFit;

  RooRealVar pid("pid","pid",-600,600) ;
  RooRealVar dectime("dectime","dectime",0,15) ;

  // Build Gaussian PDF
  RooRealVar x("x","x",0,15) ;
  RooRealVar tau("tau","",-0.714286,-1,-0.5) ;
  RooExponential expo("expo","expo PDF",x,tau) ;  
  
  // Generate a toy MC set
  //  RooDataSet* data = expo.generate(x,500) ;  
  RooDataSet* data = RooDataSet::read("piddectime.txt",RooArgList(pid,dectime));  

  // Fit pdf to toy
  tau.setConstant(kFALSE) ;

  expo.fitTo(*data,Range(0.1,8),Minos(false)) ;
  
  // Plot PDF and toy data overlaid
  RooPlot* xframe2 = dectime.frame() ;
  data->plotOn(xframe2) ;
  expo.plotOn(xframe2) ;
  xframe2->Draw() ;

  // Print final value of parameters
  tau.Print() ;
}