macro8.C

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void format_line(TAttLine* line,int col,int sty){
    line->SetLineWidth(5); line->SetLineColor(col);
    line->SetLineStyle(sty);}
 
double the_gausppar(double* vars, double* pars){
    return pars[0]*TMath::Gaus(vars[0],pars[1],pars[2])+
        pars[3]+pars[4]*vars[0]+pars[5]*vars[0]*vars[0];}
 
int macro8(){
    gStyle->SetOptTitle(0); gStyle->SetOptStat(0);
    gStyle->SetOptFit(1111); gStyle->SetStatBorderSize(0);
    gStyle->SetStatX(.89); gStyle->SetStatY(.89);
 
    TF1 parabola("parabola","[0]+[1]*x+[2]*x**2",0,20);
    format_line(&parabola,kBlue,2);
 
    TF1 gaussian("gaussian","[0]*TMath::Gaus(x,[1],[2])",0,20);
    format_line(&gaussian,kRed,2);
 
    TF1 gausppar("gausppar",the_gausppar,-0,20,6);
    double a=15; double b=-1.2; double c=.03;
    double norm=4; double mean=7; double sigma=1;
    gausppar.SetParameters(norm,mean,sigma,a,b,c);
    gausppar.SetParNames("Norm","Mean","Sigma","a","b","c");
    format_line(&gausppar,kBlue,1);
 
    TH1F histo("histo","Signal plus background;X vals;Y Vals",50,0,20);
    histo.SetMarkerStyle(8);
 
    // Fake the data
    for (int i=1;i<=5000;++i) histo.Fill(gausppar.GetRandom());
 
    // Reset the parameters before the fit and set
    // by eye a peak at 6 with an area of more or less 50
    gausppar.SetParameter(0,50);
    gausppar.SetParameter(1,6);
    int npar=gausppar.GetNpar();
    for (int ipar=2;ipar<npar;++ipar) gausppar.SetParameter(ipar,1);
 
    // perform fit ...
    auto fitResPtr = histo.Fit(&gausppar, "S");
    // ... and retrieve fit results
    fitResPtr->Print(); // print fit results
    // get covariance Matrix an print it
    TMatrixDSym covMatrix (fitResPtr->GetCovarianceMatrix());
    covMatrix.Print();
 
    // Set the values of the gaussian and parabola
    for (int ipar=0;ipar<3;ipar++){
        gaussian.SetParameter(ipar,gausppar.GetParameter(ipar));
        parabola.SetParameter(ipar,gausppar.GetParameter(ipar+3));
    }
 
    histo.GetYaxis()->SetRangeUser(0,250);
    histo.DrawClone("PE");
    parabola.DrawClone("Same"); gaussian.DrawClone("Same");
    TLatex latex(2,220,"#splitline{Signal Peak over}{background}");
    latex.DrawClone("Same");
    return 0;
}