Computing integral errors

From: Alberto Pulvirenti <alberto.pulvirenti_at_ct.infn.it>
Date: Fri, 28 May 2010 13:41:08 +0200

Dear ROOTers,

I have the following problem, to which I don't see an easy solution. I have a histogram, which contains a peak signal plus some background.

I want to extract the signal counts in the peak.

Then, I do the following:

1) define a function which should reproduce the background
2) create a TF1 whose expression is Background + Gaussian
3) fit the whole histogram with that TF1
4) create two new TF1 objects containing the Background only and the 

Gaussian only

5) compute the integral of the Background TF1 in the peak
6) compute the integral of the histogram in the peak
7) obtain my counts from the subtraction of value (6) from value (7).
8) as a cross-check, compute the integral of the Gaussian in the peak.

Now, for what concerns computing the integrals, I have no problem. Problems appear if I want to compute the error on those integrals. In fact, I have seen that apparently the computation of an integral error works only if I work with the TF1 object which I have used for the real fitting, but if I 'split' that global TF1 into its two components, I cannot obtain an integral error even if I initialize these components TF1 with the same parameters (with their errors) as I obtained in the fit done at point (3).

Is there a way to overcome this difficulty, and compute the integral error on a component of this function?

Hope I explained clearly the problem.

Cheers

        Alberto Received on Fri May 28 2010 - 13:41:13 CEST