// Crystal Ball Shape (CBShape from RooFit (RooCBShape)

Double_t e_p2e_cb(Double_t *x, Double_t *par) {

  // par[0]=norm, par[2]=peak, par[2]=width, par[3]=tail, par[4]=exponent
  // par[0]=4.96496e+04 par[1]=6.62343e+01 par[2]=3.49003e+00
  // par[3]= 1.08457e+00 par[4]=7.34376e+00
  // good starting parameters: peak=66.2, width=3.49, tail=1.19, exp=3.1 (no cuts), exp=7.3 (CsIOut<5)

  Double_t t = (x[0]-par[1])/par[2];
  if (par[3] < 0) t = -t;

  Double_t absAlpha = fabs(par[3]);

  if (t >= -absAlpha) {
    return par[0]*exp(-0.5*t*t);
  }
  else {
    Double_t a =  TMath::Power(par[4]/absAlpha,par[4])*exp(-0.5*absAlpha*absAlpha);
    Double_t b= par[4]/absAlpha - absAlpha; 

    return par[0]*a/TMath::Power(b - t, par[4]);
  }
}