About normalization of RooAbsPdf in multi-range fit

[yanqiw]

Dear experts,

When I use RooFit to do blind fit, I constructed a RooAddPdf called ‘modelSB’ (RIGHT in following plots) in the form of [ nbkg * bkg_pdf + nsig * sig_pdf ] . And I add some constrains to form a RooProdPdf called ‘model’ (LEFT in following plots) using the former RooAddPdf. However, when I try to do blind fit, the nbkg is not the expected number in full range.

In ROOT v6.30, when I use range (0,40) and (60,100) to fit 5000 entries, I get nbkg ~ 5000 for RooAddPdf and nbkg ~ 4000 for RooProdPdf. My expectation is they both give ~ 5000 nbkg.

While in ROOT v6.24, both results are ~ 5000 nbkg.

What should I do if I want to get expected entry number in full range in new ROOT version, when do fit with RooProdPdf?

Thanks a lot!

[yanqiw]

Also I see a similar behaviour in roofit tutorial rf204b.

ROOT v6.24

ROOT v6.30

The middle plot N has changed.

Those code (XMLAnaWorkspaceBuilder) is written when ROOT is in old version, and this update (if properly) in ROOT change the behaviour in it.

[Danilo]

Hi Yanqi,

Welcome to the ROOT Community!

This requires some knowledge of @jonas . In the meantime could you perhaps share a minimal reproducer?

Best,
Danilo

[yanqiw]

Hi Danilo,

Thanks for your reply.
This is a reproducer in C++ code. I have run it on lxplus. The system says that I can’t attach link to posts because I am a new user, so I just attach the code after the message.

Here the pdf construction lines are as follows, for which I expect they give the same fit result:
RooAddPdf pdfadd(“pdfadd”, “NBkg*background”, {model}, {Nbkg});
RooProdPdf pdfprod(“pdfprod”, “pdfprod”, RooArgSet(pdfadd));

In ROOT v6.24\06, the fit results are the same.

And in ROOT v6.30\02, it gives different Nbkg result.

Best regards,
Yanqi

#include "RooRealVar.h"
#include "RooExponential.h"
#include "RooGaussian.h"
#include "RooAddPdf.h"
#include "RooProdPdf.h"
#include "RooDataSet.h"
#include "RooPlot.h"
#include "RooExtendPdf.h"
#include "RooFitResult.h"
#include "TCanvas.h"

int RooFitMacro()
{

    using namespace RooFit;
    
    RooRealVar x("x", "x", 0, 100);
    RooRealVar alpha("alpha", "alpha", -0.04, -0.1, -0.0);
    RooExponential model("model", "Exponential model", x, alpha);
    
    // Define side band regions and full range

    x.setRange("LEFT", 0, 20);
    x.setRange("RIGHT", 60, 100);
    x.setRange("BLIND",20,60);
    x.setRange("FULL", 0, 100);
    std::unique_ptr<RooDataSet> data{model.generate(x, 10000)};
    
    RooRealVar Nsig("Nsig", "Number of signal events", 1000, 0, 2000);
    RooRealVar Nbkg("Nbkg", "Number of background events", 10000, 0, 20000);
    
    RooRealVar mean("mean", "Mean of signal model", 40.);
    RooRealVar width("width", "Width of signal model", 5.);
    RooGaussian sig("sig", "Signal model", x, mean, width);
    
    // RooAddPdf pdfadd("pdfadd", "NSig*signal + NBkg*background", {sig, model}, {Nsig, Nbkg});
    RooAddPdf pdfadd("pdfadd", "NBkg*background", {model}, {Nbkg});
    RooProdPdf pdfprod("pdfprod", "pdfprod", RooArgSet(pdfadd));
    
    pdfadd.Print();
    pdfprod.Print();

    TCanvas* c = new TCanvas("c", "c", 1400, 1000);
    c->Divide(2,2);

    c->cd(1);
    RooPlot* frame1 = x.frame();
    pdfadd.plotOn(frame1);
    pdfadd.paramOn(frame1,Label("RooAddPdf"));
    frame1->Draw();

    c->cd(2);
    RooPlot* frame2 = x.frame();
    pdfprod.plotOn(frame2);
    pdfprod.paramOn(frame2,Label("RooProdPdf"));
    frame2->Draw();

    c->cd(3);
    RooPlot* frame3 = x.frame();
    RooFitResult* status3 = pdfadd.fitTo(*data,Range("LEFT,RIGHT"),Save());
    status3->Print();
    data->plotOn(frame3);
    pdfadd.plotOn(frame3);
    pdfadd.paramOn(frame3,Label("RooAddPdf_fitted"));
    frame3->Draw();

    c->cd(4);
    RooPlot* frame4 = x.frame();
    RooFitResult* status4 = pdfprod.fitTo(*data,Range("LEFT,RIGHT"),Save());
    status4->Print();
    data->plotOn(frame4);
    pdfprod.plotOn(frame4);
    pdfprod.paramOn(frame4,Label("RooProdPdf_fitted"));
    frame4->Draw();

    c->Draw();
    c->SaveAs("RooFitMacro.png");

    pdfadd.Print();
    status3->Print();
    pdfprod.Print();
    status4->Print();

    return 0;
}

[yanqiw]

Just to give a more detailed information about the change. I tested different ROOT versions. And the change is not found in v6_26/08 and exists from v6_28/00.

[yanqiw]

I just moved this topic to Git issue, which may be more proper to place the topic.