How to plot background subtraction in ROOFIT?

Roofit and RooStats
1

When i finish a fit by “ROOFIT”, how can i draw the background subtracted
data points and the fit line (signal PDF) on another pad? The data points
of signal have minus values after background subtraction.

2

Hi,

You can substract events by making a weighted dataset. Suppose you start with two datasets, one with all data and one with the bkg data only to be substracted.

RooDataSet *allData, *bkgData ;

You do it as follows

// Assign a weight variable to both datasets
RooRealVar w(“w”,“w”,-1,1) ;
w.setVal(1) ; allData->addColumn(“w”,kFALSE) ;
w.setVal(-1) ; bkgData->addColumn(“w”,kFALSE) ;

// Append datasets and interpret w as event weight
allData->append(*bkgData) ;
allData->setWeightVar(w) ;

Now you can plot allData on your frame

allData->plotOn(frame,DataError(RooAbsData::SumW2)) ;

Note that you need to select sum-of-weight error bars for the histogram now as the default Poisson error bars are not applicable to weighted events.

See $ROOTSYS/tutorials/roofit/rf403_weightedevts.C for some more info on dealing with weighted events.

Wouter

3

Because the event number is very large, i have to use RooDataHist.
I use these lines to do the subtraction, but when i define the New RooDataHist, the minus values bins of the subtracted hist are all
set to zero.
How can i convert the new Th1F to RooDataHist with minus bin content and draw the fit curve on it?
eg is the data histogram
bkg is the bkg pdf.

TH1D subbkg=(TH1D)bkg.createHistogram(“subbkg”,energy);
eg->Add(subbkg,-double(nbkg.getVal()));
RooDataHist* egsub = new RooDataHist(“egsub”, “Photon Energy(GeV)”, energy, eg);

4

hi,

how am i able to do that nowadays since Exception:
ERROR: RooDataSet::setWeightVar() is deprecated. Specify weight variable in RooDataSet constructor instead using WeightVar()

I split my signal and background slices using the reduce method… then i cant really give the weight in the contructor argument.

thanks,
kazu.