I am trying to fit a crystal ball function to Z peak. The fit finishes successfully and fit result printout (RooFitResult::Print()) has reasonable error values. However, if you look at the table for the last fitting step (HESSE), error is much larger than expected. At the same time, the parameters themselves have the same values as the results.
** 11 **HESSE 2000
COVARIANCE MATRIX CALCULATED SUCCESSFULLY
FCN=8.72292e-05 FROM HESSE STATUS=OK 31 CALLS 232 TOTAL
EDM=1.34832e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER INTERNAL INTERNAL
NO. NAME VALUE ERROR STEP SIZE VALUE
1 cb_alpha -1.23559e+00 1.13083e+01 7.08829e-04 -1.23876e-01
2 cb_mass 9.07937e+01 1.52140e+01 1.94097e-03 7.94558e-02
3 cb_power 1.30459e+00 8.42296e+00 3.54643e-03 -8.31706e-01
4 cb_sigma 2.69402e+00 9.69808e+01 1.18205e-03 -1.24710e+00
ERR DEF= 0.5
EXTERNAL ERROR MATRIX. NDIM= 25 NPAR= 4 ERR DEF=0.5
4.741e+05 -2.869e+05 5.965e+05 -3.063e+05
-2.869e+05 6.945e+05 -3.088e+05 3.332e+05
5.965e+05 -3.088e+05 9.262e+05 -2.936e+05
-3.063e+05 3.332e+05 -2.936e+05 7.386e+05
PARAMETER CORRELATION COEFFICIENTS
NO. GLOBAL 1 2 3 4
1 0.92946 1.000 -0.500 0.900 -0.518
2 0.56102 -0.500 1.000 -0.385 0.465
3 0.91046 0.900 -0.385 1.000 -0.355
4 0.60990 -0.518 0.465 -0.355 1.000
[#1] INFO:Minization – RooMinimizer::optimizeConst: deactivating const optimization`
RooFitResult: minimized FCN value: 8.72292e-05, estimated distance to minimum: 1.34832e-08
covariance matrix quality: Unknown, matrix was externally provided
Status : MINIMIZE=0 HESSE=0 HESSE=0
Floating Parameter FinalValue +/- Error
cb_alpha -1.2356e+00 +/- 2.21e-02
cb_mass 9.0794e+01 +/- 2.53e-02
cb_power 1.3046e+00 +/- 3.11e-02
cb_sigma 2.6940e+00 +/- 2.60e-02