void fitSignal(){
// style options
 gStyle->SetOptStat(0);
 gStyle->SetOptFit(1111);

// prepare histogram to be fitted
 TH1F *h= new TH1F("h",  " Inv. Mass Distribution", 100,0.,3.);
 TF1 * Bg = new TF1("Bg","expo",0,3);
 Bg->SetParameters(1,-2.); 
 TF1 * Sig = new TF1("Sig","gaus",1.5,2.5);
 Sig->SetParameters(1,2.,0.1);
 h->FillRandom("Bg",30000);
 h->FillRandom("Sig",10000);

//Now fit: pre-set initial value of the parameters for total
h->Fit(Bg,"RQ");
h->Fit(Sig,"RQ");
Double_t par[5],dpar[5];
Bg->GetParameters(&par[0]);
Sig->GetParameters(&par[2]);

//define total and fit histo
TF1 *total = new TF1("total","expo(0)+gaus(2)",0,3);
total->SetParameters(par); 
h->Fit(total,"RQ");
h->GetFunction("total")->SetLineColor(3);
cout << "Fit Chisquare: "<< h->GetFunction("total")->GetChisquare()/h->GetFunction("total")->GetNDF()<<endl;
cout << "Gaussian mean: "<<h->GetFunction("total")->GetParameter(3)<<" +/-"<<h->GetFunction("total")->GetParError(3)<<endl;

h->SetMaximum(5.E5);
h->GetXaxis()->SetTitle("M_inv (GeV/c^2)");
h->GetYaxis()->SetTitle("Events");
h->SetMarkerStyle(20);
TCanvas *cFit = new TCanvas("cFit","Fit Example",200,10,600,400);
cFit->SetLogy();
h->Draw("e,p");
Sig->Draw("same");
}