How can i fitting more accurately?

[HyungJoe_Kwon]

Hi expert!
I have a problem when i was fitting histogram.
Here is my code.

TFile *f = new TFile("NaI37_bdtcut.root");
TH1F *N_new = new TH1F("N_new","N_new",19,1,20);
Double_t xbins[100];
TH1F *N_37 = (TH1F*)f->Get("N_37");
N_37->Draw("E");

Double_t par1[13];

double r1 = 1;
double r2 = 8;
double r3 = 2;
double r4 = 4;

TF1 *fp = new TF1("fp","pol0",r1,r2);
N_37->Fit(fp,"R");

TF1 *fg = new TF1("fg","gaus",r3,r4);
fg->FixParameter(1, 3.5);
fg->FixParameter(2, 6.76268e-1);
N_37->Fit(fg,"B","R+");



TF1 *f1 = new TF1("f1","pol0(0)+gaus(1)",r1,r2);
f1->Draw("same");
f1->SetLineColor(kRed);
fp->GetParameters(&par1[0]);
fg->GetParameters(&par1[1]);
f1->SetParameters(par1);
f1->FixParameter(2, f1->GetParameter(2));
f1->FixParameter(3, f1->GetParameter(3));

f1->SetLineColor(kRed);
N_37->Fit(f1,"B","R+",1,8);


TF1 *f2 = new TF1("f2","pol0",r1,r2);
f2->Draw("same");
f2->SetLineColor(kBlue);
f2->SetParameters(par1);


N_37->Fit(f2,"B","R+",1,8);

auto fitresult1 = N_37->Fit(f1,"S","",1,8);
auto fitresult2 = N_37->Fit(f2,"S+","",1,8);

and result is below picture.

This is not the fitting result i expected.
I want the graph fitted to a peak of 3.5.
And also want to fitting the pol0 graph and pol0+gaus graph in unison.

What should i do?

[moneta]

Hi,
I think the fit is correct, given your limited data points. Maybe an unbinned fit will be better, if you have the original data. You can also try to use option “I” (integral of the functgion in the bin).
If I need to look in more detail, I would need the input ROOT file to be posted as well.

Cheers

Lorenzo