// Gaussian fit, units mm

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

  // par[0]=norm, par[2]=peak, par[2]=rms
  // par[0]=6343, par[1]=1.506, par[2]=1.683

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

  return par[0]*TMath::Exp(-t*t);

  }