// Novosibirsk Shape from RoooFit RooNovosibirsk

Double_t e_p2e_nsb(Double_t *x, Double_t *par) {
  // Put the formula for your PDF's value here. Use the pre-computed
  // value of _norm to normalize the result.

  // par[0]=norm, par[1]=peak, par[2]=width, par[3]=tail
  // par[0]=4.64856e+04 par[1]= 6.67889e+01 
  // par[2]=4.04685e+00 par[3]=-2.17974e-01 

  double qa=0,qb=0,qc=0,qx=0,qy=0;

  if(TMath::Abs(par[3]) < 1.e-7) 
    qc = 0.5*TMath::Power(((x[0]-par[1])/par[2]),2);
  else {
    qa = par[3]*sqrt(log(4.));
    qb = sinh(qa)/qa;
    qx = (x[0]-par[1])/par[2]*qb;
    qy = 1.+par[3]*qx;
  
    //---- Cutting curve from right side

    if( qy > 1.E-7) 
      qc = 0.5*(TMath::Power((log(qy)/par[3]),2) + par[3]*par[3]);
    else
      qc = 15.0;
  }

  //---- Normalize the result
  return par[0]*exp(-qc);
}