Subtract a ROOchebychev function from a histogram in ROOFIT

[SimN]

Hi,

In root we can subtract a function from a histogram by using:

h->Add(func, -1)

I was wondering if we can do the same in Roofit ? So basically I would like to subtract “bkg” from the histogram “h”.

you can find my code down below.

RooRealVar x("x","MM_{(#pi #pi)} GeV/c^{2}",3.04,3.17) ;
RooDataHist dh("dh","dataset",x,h);
RooPlot* frame = x.frame(Title("")) ;
dh.plotOn(frame,Name("dh")) ;
RooRealVar mean1("#mu","mean of gaussians",3.095,3.099);
RooRealVar sigma1("#sigma","width of gaussians",0.0025,0.006);

RooGaussian sig1("sig1", "sig1", x, mean1, sigma1);

RooRealVar a0("a0","a0",-2.,1.) ;

RooChebychev bkg("bkg","background p.d.f.",x,RooArgList(a0));

RooRealVar nsig("N_{SIG}","signal events",0,1000000);
RooRealVar nbkg("N_{BKG}","signal background even0ts",0,100000000);

RooExtendPdf esig("esig","esig",sig1,nsig) ;

RooExtendPdf ebkg("ebkg","ebkg",bkg,nbkg) ;
RooAddPdf all("all","all",RooArgList(esig,ebkg)) ;

RooRealVar sig1frac("sig1frac","fraction of component 1 in signal",0.8,0.,1.) ;        //sig +bcg
RooAddPdf all("all","model",RooArgList(sig1,bkg),RooArgList(nsig,nbkg));

RooFitResult* r = all.fitTo(dh,Extended(kTRUE),Save()) ;

all.paramOn(frame,Layout(0.5,0.90,0.55));
all.plotOn(frame,Name("all"));
all.plotOn(frame,Components(bkg),LineStyle(kDashed));

r->Print();
frame->Draw();

[StephanH]

Hi,

we discussed this here:

Subtraction of two roodatahists

Hi again, you cannot do this directly. You can, however, create a standard ROOT histogram from a PDF: https://root.cern.ch/doc/master/classRooAbsReal.html#a6659d2c301e5cd65b83ee8c9422c2553 The options for the generation of the histogram are also listed in the documentation I linked above. When you have the TH1x, subtraction of the histograms should be easy.

but it’s a bit long to read.

The gist is:
You can create a standard ROOT histogram from a PDF:
RooAbsReal::createHistogram

The options for the generation of the histogram are also listed in the documentation I linked above.
When you have the TH1x, subtraction of the histograms should be easy.

Note:

• You have to match the binning of the histograms, so something like Binning(200, 0., 500.) is what you should use when you ask for a histogram.
• You likely have to adjust the normalisation. Without doing anything, RooFit will always normalise the PDF to unity. If you need custom scaling, you can either scale the histogram manually hxx->Add(hyy, -1. * nbkg.getVal()) , or you can have it scaled when creating the histogram. Have a look at the documentation again:
  RooAbsPdf::createHistogram docs
  Extended() should enable automatic scaling (hopefully to the value of nbkg).