Skip a few bins in the range while fitting

ROOT
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2

Hi @tangophysics,

this is not so easy to implement in the standard ROOT fitting. You could have a look at RooFit, which is very widely used for more advanced fits.

Fitting a polynomial PDF to two ranges would look like this:

void script() {

    RooRealVar x{"x", "x", 0, 0, 10};

    x.setRange("r1", 2, 4);
    x.setRange("r2", 5, 7);

    RooRealVar p0{"p0", "p0", 20, -100, 100};
    RooRealVar p1{"p1", "p1", 0.6, -100, 100};
    RooRealVar p2{"p2", "p2", 1.2, -100, 100};
    RooRealVar p3{"p3", "p3", -0.12, -100, 100};

    RooArgList params{p0, p1, p2, p3};

    RooPolynomial poly{"poly", "poly", x, params, 0};

    // Generating toy data set to demonstrate, but you would have to create a
    // RooDataHist from your TH1.
    std::unique_ptr<RooDataSet> data{poly.generate(x, 10000)};

    using namespace RooFit;
    std::unique_ptr<RooFitResult> result{poly.fitTo(*data, Range("r1,r2"), PrintLevel(-1), Save())};
    result->Print();

    auto c1 = new TCanvas("c1");

    auto frame = x.frame();

    frame->Draw();

    data->plotOn(frame);
    poly.plotOn(frame);

    frame->Draw();

    c1->SaveAs("plot.png");
}

plot
plot696×472 8.4 KB

There is also a dedicated tutorial on fitting subranges:

You can easily construct a RooFit histogram to use as the data for the fit with the Import() command in the RooDataHist constructor.

Could this be a solution for you?

Cheers,
Jonas