Subtracting minimum of a RooCurve from another RooCurve point by point

Newbie
10

Hi @Natilus! As explained in the thread that was linked by @RENATO_QUAGLIANI, the TGraph::GetMinimum() function simply returns the data member fMinimum which is not always set. Hence you get the default value of -1111 when you try to find the minimum.

Indeed, it’s a reasonable approach here to manipulate the TGraph directly. Building on your code snippet, I have written some example code here that shows how you can do that:

using namespace RooFit;

/// Find the minimum y-value of TGraph within interval `[xMin, xMax]`
double findMinimum(TGraph const& graph, double xMin, double xMax) {
    double yMin = std::numeric_limits<double>::max();

    auto xVals = graph.GetX();
    auto yVals = graph.GetY();

    for(int i = 0; i < graph.GetN(); ++i) {
        if (xVals[i] >= xMin && xVals[i] <= xMax) {
            yMin = std::min(yMin, yVals[i]);
        }
    }

    return yMin;
}

/// Shift the y-values of a TGraph by constant value
void shiftTGraphY(TGraph& graph, double shift) {
    for(int i = 0; i < graph.GetN(); ++i) {
        graph.SetPointY(i, graph.GetPointY(i) + shift);
    }
}

void my_example() {

    const double JPsiMass_PDG = 3.;

    RooRealVar mass("mass","mass", 1.8, 4.8);

    RooRealVar JPsiMean("JPsiMean","Mean of Gaussian",JPsiMass_PDG);
    RooRealVar JPsiSigma("JPsiSigma","Sigma of Gaussian", 0.6);
    RooGaussian JPsiSignal("JPsiSignal","Signal P.D.F.",
                                             mass, JPsiMean, JPsiSigma);

    RooPolynomial JPsiBkg("JPsiBkg","Polynomial Background P.D.F.", mass,
            RooArgSet(
                RooConst(-5.61596639 - 0.004),
                RooConst(5.24971989),
                RooConst(-1.57142857),
                RooConst(0.15406162)
            )
    );

    RooPlot* massFrame = mass.frame();
    auto sigCurve = JPsiSignal.plotOn(massFrame)->getCurve();
    auto bkgCurve = JPsiBkg.plotOn(massFrame)->getCurve();

    // Find the minimum of background curve within desired interval
    double bkgMin = findMinimum(*bkgCurve, mass.getMin(), mass.getMax());

    // Subtract the minimum from both signal and background curve
    shiftTGraphY(*bkgCurve, -bkgMin);
    shiftTGraphY(*sigCurve, -bkgMin);

    massFrame->Draw();
}

The the RooCurve directly inherits from TGraph, so the TGraph documentation can be quite useful here.

I hope my code example solves the problem for you or at least helps you to get started. If not, please let me know!

Cheers,
Jonas