Plotting individual PDFs from a RooAddPdf

[byates]

I’ve created a RooAddPdf containing a Gaussian and an exponential to fit to an invariant mass. When I perform the fit and plot the RooAddPdf, I see good agreement. However, if I just try to plot the exponential

it sits just above all my side-band data points, and plotting the Gaussian starts at zero and peaks at the RooAddPdf height.

[StephanH]

Did you use the Components() notation as in the tutorial rf201?
If you just plot the PDF, it will try to normalise the background PDF to the integral of signal+background (the data that’s in the plot), and that will obviously not work.
Instead, you should plot the sum, and select a component. This way, the total sum is normalised to the data, and the component is plotted afterwards.

[byates]

I see that works for plotting, but how can I grab the function and evaluate it. I’m trying to implement a simple background subtraction by hand to help debug things.

[StephanH]

You can directly use the function, it also has the correct parameters. What you need to do, though, is to scale it correctly. The scale factor is N_evt * background fraction. For an AddPdf, you either gave a coefficient to the background, so that’s the fraction, or it’s 1 - Sum (coeffs) if you didn’t give it a coefficient.

[byates]

What I wound up doing was loading the model, plotting just the component I wanted, and then using the getCurve function:
auto model = (RooAddPdf*)w->pdf("model");
model->plotOn(frame, RooFit::Components("expo"), RooFit::Binning(60));
auto ex = frame->getCurve();
I could then use ex like a T1F, invoking Eval(mass). This gives a slightly different answer than:
nbkg>setVal(mass);
float bkg = expo->getVal() * nbkg->getVal();