Setting Parameter Limits, Fixing Parameters and Their effects on fitting

ROOT
10

Here is a link you answered somebody else before: Integral Error in Signal - #6 by Wile_E_Coyote

Here is the adapted version of YOUR code below to get background:
As you know as soon as you say somewhere “fiz the parameter”, yo -u get ZERO error as normal.

{
60Codata.txt (17.3 KB)

const Bool_t use_partial_covariance = kFALSE; // kFALSE or kTRUE

const char *filename = “60Codata.txt”;

TGraph *g0 = new TGraph(filename, “%lg %lg %*lg”); // X axis = channels

g0->SetTitle(“g0;Channel number;Energy”); // energy calibration

TGraph *g1 = new TGraph(filename, “%lg %*lg %lg”); // X axis = channels

g1->SetTitle(“g1;Channel number;Counts”);

TGraph *g2 = new TGraph(filename, “%*lg %lg %lg”); // X axis = energies

g2->SetTitle(“g2;Energy;Counts”);

if ((g1->GetN() < 2) || (g2->GetN() < 2)) return; // just a precaution

TF1 *fg = new TF1(“fg”, “gaus”);

Double_t sb = 5.0; // sigma band for the fg integral (e.g. 3 or 5 or 7)

TF1 *bcg = new TF1(“bcg”, “pol1”);

Int_t ch_min = 360, ch_max = 460; // “channel numbers”

TF1 *f1 = new TF1(“f1”, “gaus(0) + pol1(3)”, ch_min, ch_max);

f1->SetParameters(30000., 430., 10., 5000., 0.);

f1->FixParameter(3, 5000); //for pol **************************************
f1->FixParameter(4, 0.); //for pol *******************************************

TFitResultPtr r1 = g1->Fit(f1, “WEMRS”);

TMatrixD c1 = r1->GetCovarianceMatrix();

TMatrixD c1g = c1.GetSub(0, 2, 0, 2);

TMatrixD c_bcg = c1.GetSub(3, 4, 3, 4);

Double_t s1, s1e;

fg->SetParameters(f1->GetParameters());

bcg->SetParameters(f1->GetParameters()+3);

// fg->SetParErrors(f1->GetParErrors());

s1 = bcg->Integral(fg->GetParameter(1) - sb * fg->GetParameter(2),

                fg->GetParameter(1) + sb * fg->GetParameter(2));

s1e = bcg->IntegralError(fg->GetParameter(1) - sb * fg->GetParameter(2),

                      fg->GetParameter(1) + sb * fg->GetParameter(2),

                      bcg->GetParameters(),

                      c_bcg.GetMatrixArray());

std::cout << "gaus peak integral (mean +/- " << sb << " sigma) = "

        << s1 << " +/- " << s1e << std::endl;

}