Fitting problems

Hello,

Very recently I started using root to fit data on energy losses in thin materials.

Using the predefined landau function works very well on the data I use. Currently I face the problem that I do not know the exact definitions of the three obtained fit parameters.

The MPV (=most probable value) I assume is the most probable energy loss of the particle.
The sigma is going to be the width of the function, but in which relation does it stand to FWHM?
What is the third parameter, the constant?

Is it also possible to obtain the maximum, e.g. corresponding to the maximum energy loss of a particle in the material from the fit?

Could someone help me finding the exact definitions or point me to the dataset where I might find them? My current searches on the root hompage didn’t give any useable results.

Thanks!

[quote]The MPV (=most probable value) I assume is the most probable energy loss of the particle.
[/quote]

Yes

[quote]The sigma is going to be the width of the function, but in which relation does it stand to FWHM?
[/quote]
sigma is approximatively 0.5*FWHM

[quote]What is the third parameter, the constant?
[/quote]

Yes, the normalisation constant

see
root.cern.ch/root/htmldoc///TFormula.html
section "special cases for normalized gaussian or landau distributions"
in TFormula::Analyze
see also:
root.cern.ch/root/htmldoc////TMa … ath:Landau

[quote]Is it also possible to obtain the maximum, e.g. corresponding to the maximum energy loss of a particle in the material from the fit?
[/quote]

If you have a TF1 object, call TF1::GetMaximum, GetMaximumX
depending if you want the Y or/X value of the maximum.
see:
root.cern.ch/root/htmldoc///TF1. … GetMaximum
root.cern.ch/root/htmldoc///TF1. … etMaximumX

Rene

Thank you for your quick reply.