Exponential fit doesn't work

ROOT
1

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Hi Rooters!

Well, I’m working on TGraphError im going to show you bellow, but the exponential (gaussian fit) doesn’t work.

{
   c1 = new TCanvas("c1","Pure Z/ Cross section vs Energy  ",200,10,700,500);
   c1->SetFillColor(42);
   c1->SetGrid();
   c1->GetFrame()->SetFillColor(21);
   c1->GetFrame()->SetBorderSize(12);
   const Int_t n = 15;
   Double_t x[n]  = {20, 30, 34.4, 40, 50, 60, 70, 80, 85, 90, 91, 95, 100, 110, 120};
   Double_t y[n]  = {7.19E-02,1.85E-01,2.63E-01,4.00E-01,8.27E-01,1.78E+00,4.51E+00,1.77E+01,5.72E+01,7.38E+02,1.40E+03,2.93E+02,1.02E+02,3.78E+01,2.13E+01};
   Double_t ex[n] = {.0,.0,.0,.0,.0,.0,.0,.0,.0,.0,.0,.0,.0,.0,.0};
   Double_t ey[n] = {3.58E-05,9.20E-05,1.30E-04,1.98E-04,4.19E-04,9.06E-04,2.30E-03,9.35E-03,3.72E-02,3.98E-01,7.50E-01,1.34E-01,5.56E-02,2.07E-02,1.17E-02};
   gr = new TGraphErrors(n,x,y,ex,ey);
   gr->SetTitle("Cross Section (pb) vs Energy (GeV)");
   gr->SetMarkerColor(4);
   gr->SetMarkerStyle(21);
   gr->Draw("ALP");

    
   TF1 *f1 = new TF1("f1","pol1(0)",20,75);
   TF1 *f2 = new TF1("f2","[1400]*exp(-[1]*((x-[91])**2)/([2]*([0.5]**2)))",75,120); 
   f2->SetParameters(75,80,85,90,91,95,10,110,120);
   gr->Fit("f1","R"); 
   gr->Fit("f2","B");
  
 gr->Draw(); 
}

It runs the commands without any problem but the fitting won’t show in the graph. I’m new to all this thing so, I hope I’m not disregarding something important.

Thank you in adnance!

2

Try:

   TF1 *f2 = new TF1("f2", "[0]+[1]*x+[2]*exp(-[3]*(pow((x-[4]),2))/([5]*(pow([6],2))))", 10., 130.);
   f2->Print();
   f2->SetParameters(-0.3, 0.02, 1400., 0.1, 91., 1., 1.);
   f2->SetNpx(1000);
   gr->Fit("f2", "");

BTW. When you post “source code” or “output” here, do remember to enclose them into two lines which contain just three characters ``` (see how your post has been edited above).

3

Yes,I’m sorry. I’ll have it in mind.
It actually worked now!

Thanks a lot!!!

4

Please note that, in your “f2” function, parameters “[5]” and “[6]” are completely redundant / superfluous. They both should actually go into your parameter “[3]” (i.e. you should have just one parameter “[3]” = “[3]/([5]*(pow([6],2)))”).

5

Yes, right- put it this way to have it in the full gaussian form vissualy, but there’s no need, indeed.