[question] residual plot and chi2test

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1

resi
resi540×716 8.15 KB 

{
  TCanvas *c2 = new TCanvas("c2", "My Canavas", 1200, 450);
  c2->Divide(2,1);
  c2->cd(1);

  hQtot_Bi->SetNormFactor(1);
  hQtot_Bi->SetLineColor(kBlue);
  hQtot_Bi->SetLineWidth(2);
  hQtot_Bi->Draw("HIST");
  hQtot_Bi->SetStats(0);
  hQtot_Bi_d->Add(hQtot_Bi_g,-1);
  hQtot_Bi_d->SetNormFactor(1);
  hQtot_Bi_d->SetMarkerStyle(20);
  hQtot_Bi_d->SetMarkerSize(1);
  hQtot_Bi_d->Draw("PEsame");
  c2->Update();

  TCanvas *c3 = new TCanvas("c3", "residual", 1200, 900);
  hQtot_Bi->SetNormFactor(1);
  hQtot_Bi_d->SetNormFactor(1);
  TH1D *h1 = (TH1D*)hQtot_Bi->Clone("h1");
  h1->SetLineColor(kBlack);
  h1->SetLineWidth(2);
  h1->Sumw2();
  h1->SetStats(0);
  h1->Divide(hQtot_Bi_d);
  //h1->SetMinimum(0);
  //h1->SetMaximum(10000);
  hQtot_Bi->Chi2Test(hQtot_Bi_d, "WW P");
  h1->Draw("E0");
}

Hello Rooters.

I am attempting a residual plot of the spectrum obtained through MC simulation (blue line) and experiments (black dots). The experimental result is the spectrum excluding the background. The residual plot was done as shown in the figure below and the chi2 value was obtained. The chi2 value was 10576, the NDF value was 39. I am wondering if this code and the result obtained is correct. Please add a comment.

2

It looks fine. may be @moneta can comment.

3

If you didn’t make any coding or analysis design mistakes then the result is correct :slight_smile: That said, the probability of ending up with chi2==10576 with NDF 39 is extremely small:

root [0] TMath::Prob(10576, 39)
(double) 0.0000000

That suggests that either the histogram uncertainties are incorrect (most likely) or there are other physics effects causing a discrepancy. I cannot see the uncertainties of hQtot_Cs; I don’t know the relation of hQtot_Cs or hQtot_Cs_d with hQtot_Bi and hQtot_Bi_d. But “is the uncertainty what I should expect” is a really good question to discuss with your physics group or supervisor.

Cheers, Axel.

4

Sorry for the confusion.
hQtot_Cs and hQtot_Cs_d are not included in the figure.
Cs and Bi are seperate.

Thank you.