fithist.C File Reference

Detailed Description

Example of fit where the model is histogram + function

 
 FCN=7.84156 FROM MIGRAD    STATUS=CONVERGED     183 CALLS         184 TOTAL
                     EDM=4.73985e-09    STRATEGY= 1      ERROR MATRIX ACCURATE 
  EXT PARAMETER                                   STEP         FIRST   
  NO.   NAME      VALUE            ERROR          SIZE      DERIVATIVE 
   1  p0           1.56629e+02   5.91047e+00   7.83834e-05   3.68508e-06
   2  p1           5.98184e+00   1.89049e-02   2.72632e-05  -3.85176e-03
   3  p2          -5.06680e-01   1.80489e-02   2.01697e-05  -3.36498e-03
   4  p3           1.00029e+00   1.12955e-02   1.52093e-05   1.64149e-03

 
#include <TF1.h>
#include <TFile.h>
#include <TH1F.h>
 
TH1F *background;
void histgen() {
   //generate the histogram background and save it to a file
   //background taken as linearly decreasing
 
   TF1 f1("f1","pol1",0,10);
   f1.SetParameters(5,-0.5);
   TH1F h("background","linear background",100,0,10);
   h.FillRandom("f1",10000);
   TFile f("background.root","recreate");
   //save the background histogram
   h.Write();
   //superimpose a Gaussian signal to the background histogram
   TF1 f2("f2","gaus",0,10);
   f2.SetParameters(1,6,0.5);
   h.FillRandom("f2",2000);
   h.SetName("result");
   h.Write();
}
 
double ftotal(double *x, double *par) {
   double xx = x[0];
   int bin = background->GetXaxis()->FindBin(xx);
   double br = par[3]*background->GetBinContent(bin);
   double arg = (xx-par[1])/par[2];
   double sr = par[0]*TMath::Exp(-0.5*arg*arg);
   return sr + br;
}
void fithist() {
   //fit function ftotal to signal + background
 
   histgen();
 
   TFile *f = new TFile("background.root");
   background = (TH1F*)f->Get("background"); //pointer used in ftotal
   TH1F *result = (TH1F*)f->Get("result");
 
   TF1 *ftot = new TF1("ftot",ftotal,0,10,4);
   double norm = result->GetMaximum();
   ftot->SetParameters(0.5*norm,5,.2,norm);
   ftot->SetParLimits(0,.3*norm,norm);
 
   result->Fit("ftot","b");
}

Author

Rene Brun

Definition in file fithist.C.