Dear Jonas,
I have tried to rerun the fit following the instructions you suggested. Now I have obtained more reasonable uncertainties for the parameters, and the fit converges as well.
However, I still want to confirm one thing related to the limits being set. For the fit results I obtained, I set a range limit of [-0.01,0.01] on ~ each parameter. When I tried a larger range of [-1,1], the fit failed to converge.
I am asking what test will prove that the obtained uncertainties of the considered parameters are reliable, given that we expect the central value of the parameters to be around 0.
Here is the output of the fit that converges with the limits set to [-0.01,0.01].
Info in : MnHesse Done after 235.67 ms
Info in : VariableMetricBuilder After Hessian
Info in : VariableMetricBuilder 5 - FCN = -1423678.473 Edm = 1.999534532e-07 NCalls = 88
Info in : VariableMetricBuilder Stop iterating after 633.734 ms
Minuit2Minimizer : Valid minimum - status = 0
FVAL = -1423678.47254971345
Edm = 1.99953453196305225e-07
Nfcn = 88
deltaAy = 0.0001 +/- 0.0167701 (limited)
deltaAz = 0.000507999 +/- 0.0017295 (limited)
deltaBz = 0.000999998 +/- 0.001263 (limited)
I have attached as well the code used for this
NLL_Version_2.txt (2.4 KB)
Thanks!