[ROOT] problem fitting landau distribution

From: Vladimir Morozov (vlmrz@sseos.lbl.gov)
Date: Sun Mar 17 2002 - 09:56:49 MET 

I'm having trouble fitting a histogram to Landau distribution in a
limited variable range.
Here's a macro, which will build a histogram, fill it and try to fit it
to Landau. The fit always fails (cannot find an appropriate value for
fit parameters), and I don't know what's wrong. I've tried helping the
fitter by providing a reasonable starting point for the fit, but it
still doesn't work. Any idea what I might be doing wrong? I'm running
Root4STAR 3.02/07

Thanx,
Vladimir.

_______________________________________-

#include <TMath.h>

Double_t fitFunction(Double_t *x, Double_t *par) {
  Double_t xx = x[0];
  return &par[0]*TMath::Landau(xx,.02,.005);
}

void macro() {

   TH1 *histo = new TH1F("histo","pPt",19,-0.002,0.19);
   histo->SetEntries(174);
   histo->SetDirectory(0);
   histo->SetBinContent(1,12);
   histo->SetBinContent(2,22);
   histo->SetBinContent(3,18);
   histo->SetBinContent(4,10);
   histo->SetBinContent(5,8);
   histo->SetBinContent(6,8);
   histo->SetBinContent(7,3);
   histo->SetBinContent(8,3);
   histo->SetBinContent(9,2);
   histo->SetBinContent(10,3);
   histo->SetBinContent(11,1);
   histo->SetBinContent(12,1);
   histo->SetBinContent(13,1);
   histo->SetBinContent(14,1);
   histo->SetBinContent(15,7);
   histo->SetBinContent(16,3);
   histo->SetBinContent(17,8);
   histo->SetBinContent(19,4);

   histo->SetMarkerStyle(20);
   histo->Draw("");

   TF1 *fun = new TF1("fun",fitFunction,0.03,.12,1);
   fun->SetParameter(0,140.0);
   histo->Fit("fun","R");
}

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