I encountered a very difficult problem when using ROOT::Minuit2::Minuit2Minimizer. I would really appreciate it if friends who have encountered and understood this problem could explain it to me.
First I use ROOT::Minuit2::Minuit2Minimizer. After calling the Minimize() function for the first time for fitting, it will give me an error sigma1 of a certain important parameter. When I call Hesse() or Minimize() again after this line, Minuit2Minimizer will give me a new error sigma2 for the same parameter. Although it doesn’t always happen, sigma2 will occasionally be significantly smaller than sigma1. But I scanned this parameter and found that the error sigma1 actually corresponds to Delta chi ^2=1, which is what I want. Does anyone know why the Hesse(), which can calculate the error more accurately, does not give me an error that corresponds to Delta chi^2 =1? Does sigma2 make any sense in this case?
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_ROOT Version: 6.28.06
_Compiler: Apple clang version 15.0.0 (clang-1500.0.40.1)