How to add experimental resolution to a 1D Histogram in root

I am quite new to root.
Can someone tell me how to smear my theoritical mass spectra with my experimental mass resolution ?
The experimental mass resolution is 3.5 amu. I want to incorporate this mass resolution in the mass spectrum which I obtain from my theory calculation and want to see how the width changes.

Thanking you in advance!

Welcome to the ROOT forum.
Have you looked at the ROOT tutorials ? the ROOT forum ?

Yes, i have gone through the root tutorials and I had made a macro to do this. But It has some problems. i am sharing the marco, unfortunately I cant upload the root files.

void MassResolution()

TH1D*h2_Mass_resolution = new TH1D("h2_Mass_resolution","h2_Mass_resolution",101.,39.5,140.5); // Defined a histogram

TFile *file=TFile::Open("QnmFF_ROOT_Au_158.root");
TTree *tree=(TTree*)file->Get("DITY");

TH1D*h1 = new TH1D("h1","Os_187.46_MeV",101.,39.5,140.5);
tree->Draw("Aff1>>h1"); // retrived histogram fom the TTree

Double_t x1,y1,z;
Double_t content=0;
Double_t sigma=3.5;

const UInt_t nX = h1->GetNbinsX();

	for(UInt_t i=1;i<=nX;++i)
		x1 = h1->GetXaxis()->GetBinCenter(i);
                y1 = h1->GetBinContent(i);
                z = h1->GetBinWidth(i);

TF1 *f1 = new TF1("f1","TMath::Gaus(x,[1],[2])",40,140);

h2_Mass_resolution->Fill(x1, f1->GetRandom()); 


which problems ?

that would help though …

Assuming that “Aff1” is in “amu” units and you want to add a Gaussian smearing with “3.5 amu” sigma, you could try:

tree->Draw("Aff1 + 3.5 * TMath::Sqrt2() * TMath::ErfInverse(2. * rndm() - 1.) >> h1"); // inverse Gaussian CDF transform sampling


The problem as you can see from the image, I am not getting the smeared histogram.
Red is my Theoritical data, to which I want to add my mass resolution of 3.5 amu. and when I run my macro I get the blue histogram which is not correct.

Since I am a new user of the root forum I am not able to upload my root file.

The method I proposed makes the “h1” histogram smeared (no need for any “h2” gymnastics).

I was just going through what you proposed. It is working, but I need to understand how you did it.

Red is the theoretical data and blue is the theoretical data smeared to 3.5 amu.

Inside my ttree I have all the theoretical masses for which I wanted to add my masss resolution of 3.5 amu. For that I declared a tree branch and variable “Aff1_Res_amu” and did

Aff1_Res_amu = Aff1 + 3.5 * TMath::Sqrt2() * TMath::ErfInverse(2. * rndm() - 1.);

When I do this I get the following error :

Could you please help me resolve it?

In a C++ code, instead of “rndm()” try to use “gRandom->Rndm(1)”.

Thank you for the prompt response !

So I replaced rndm(), like you told :slight_smile:
Aff1_Res_amu = Aff1 + 3.5 * TMath::Sqrt2() * TMath::ErfInverse(2. * gRandom->Rndm(1) - 1.);

but I get this error

I have used the header file " #include<TRandom.h>"

It seems you have an extra “g” in line 221 (column 1).

That was right, there was g. But there is still some problem with gRandom.

Try to add somewhere in your code:
if (!gRandom) gRandom = new TRandom3();

Thats is working.

Once again thanks a lot!

Actually, in a C++ code you can simply use:
Aff1_Res_amu = gRandom->Gaus(Aff1, 3.5);

Yes, it works. Thank You.