Minimize using Minuit and measured values from a Tree

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Hi Lorenzo, thanks a lot!

I ended also trying to just doing it without creating these arrays (std::vector) by doing everything inside my Loop. In part because some of the variables I needed were not exactly a measured value, but a calculated value from measured values (after applying some selections).

Here is how it looks. Just a final doubt for setting some limits at the end.

//Before this part I have defined jte1, jte2, cost, sig1,sig2,gam
 
auto myFCN = [&](const double * par) {
     double chi2 = 0;  
     double k1 = par[0];
     double k2 = par[1];
     double M12 = sqrt(2.*k1*k2*(1.-cost));
     chi2 = (jte1-k1)*(jte1-k1)/sig1/sig1 + (jte2-k2)*(jte2-k2)/sig2/sig2 + (M12-MW)*(M12-MW)/gam ;
     return chi2; 
  };
  ROOT::Math::Functor f(myFCN,2);
     //-------------------------------------------
     //-----------------------------------------------------------
     ROOT::Math::Minimizer* minimum =
        ROOT::Math::Factory::CreateMinimizer("Minuit", "Migrad");
  
     double step[2] = {0.001,0.001};
     // starting point
     double variable[2] = { jte[iq1],jte[iq2]}; //same as measured value

     minimum->SetFunction(f);

     // Set the free variables to be minimized !
     //minimum->SetVariable(0,"jte1",variable[0], step[0]);
     //minimum->SetVariableLowerLimit(0, 0.);

     minimum->SetLowerLimitedVariable(0,"k1",variable[0], step[0],0); //Lower value is energy=0
     minimum->SetLowerLimitedVariable(1,"k2",variable[1], step[1],0);
     
     // do the minimization
     minimum->Minimize();

     const double *xs = minimum->X();
     cout << "Minimum: f(" << xs[0] << "," << xs[1] << "): "
              << minimum->MinValue()  << std::endl;

I tried to set some limits on my fit variables, but it is giving me a warning which I don’t understand.

I tried to set the lower limit of k1 to 0, as I can’t get negative values of energy, but it returns the following:

Warning in <TMinuitMinimizer::SetLowerLimitedVariable>: not implemented - use as upper limit 1.E7 instead of +if

Not sure why it says anything about upper limit (and +inf) if I’m trying to set a lower limit, and I could’t find some implementation of this online. Neither the SetVariableLowerLimit works. So, any help is appreciated.

Thanks!