double myFunc(double *x, double *par){
  double result = 
    par[0]*exp(-((x[0]-par[1])/(2*par[2]))*((x[0]-par[1])/(2*par[2]))) + par[3];
  return result;
}

void fitFunzione() {

  TH1F histo("pippo","Segnale con Fondo",100,-5.,5.);
  TF1 miaFunc("miaFunc",myFunc,-10.,10.,4);
  miaFunc.SetParLimits(2,0,100);
  miaFunc.SetParameter(0,100);
  miaFunc.SetParameter(1,0.5);
  miaFunc.SetParameter(2,0.5);
  miaFunc.SetParameter(3,10);
  miaFunc.SetParName(0,"Const");
  miaFunc.SetParName(1,"Mean");
  miaFunc.SetParName(2,"Sigma");
  miaFunc.SetParName(3,"Bkgnd");
  miaFunc.DrawCopy();
  histo.FillRandom("miaFunc",10000);
  histo.Fit("miaFunc");
  gStyle->SetOptFit(11111);
  histo.DrawCopy();
}