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

  Double_t zz  = x[0];
  Double_t xx  = x[1];
  Double_t E0  = par[0];
  Double_t Z0  = par[1];
  Double_t X0  = par[2];

  Double_t f = (X0*X0/xx) * TMath::Exp(-zz*zz/(2*Z0*Z0)) * (1.0-TMath::Exp(-xx*xx/(2*X0*X0)));
           
  return f;
}




void testHisto2D() {

  gROOT->Reset();
  
  Double_t E0 = 1;
  Double_t Z0 = 0.1;
  Double_t X0 = 0.1;

  TF2 *fErBeam = new TF2("fErBeam",ErGauss,-5*Z0,5*Z0,-30*X0,30*X0,3);
  fErBeam->SetParameters(E0,Z0,X0);
  fErBeam->SetParNames("E0","Z0","X0");
  fErBeam->SetTitle("");
  fErBeam->GetXaxis()->SetTitle("z");
  fErBeam->GetYaxis()->SetTitle("x");

  gStyle->SetPalette(55);
  fErBeam->SetContour(128);


  fErBeam->Draw("colz");

  Double_t Max =  0.02;
  Double_t Min = -0.02;

  fErBeam->SetMaximum(Max);
  fErBeam->SetMinimum(Min);

  fErBeam->Draw("colz");
  
  
}