void plot_xsec_comp(){
  double Eg[15],dEg[15];
  for (int i=0;i<15;i++){
    Eg[i]=2.9+0.2*i;
    dEg[i]=0.1;
  }
  double dsdt[15];
  double ddsdt[15];
  double tslope[15];
  double dtslope[15];

  ifstream in_file("phi_parms.dat");
  for (int i=0;i<15;i++){
    double dummy;
    in_file >> dummy;
    in_file >> dsdt[i];
    in_file >> ddsdt[i];
    in_file >> tslope[i];
    in_file >> dtslope[i];
  }

  double xsec[15];
  double dxsec[15]; 

  double dsdt1[15];
  double ddsdt1[15];
  double tslope1[15];
  double dtslope1[15];

  ifstream in_file2("phi_parms_method1.dat");
  for (int i=0;i<15;i++){
    double dummy;
    in_file2 >> dummy;
    in_file2 >> dsdt1[i];
    in_file2 >> ddsdt1[i];
    in_file2 >> tslope1[i];
    in_file2 >> dtslope1[i];
  }

  double xsec1[15];
  double dxsec1[15];


  double m_p_sq=0.9837*0.93827;
  double m_phi_sq=1.0195*1.0195;
  for (int i=0;i<15;i++){

    double s=m_p_sq+2.*0.93827*Eg[i];
    double t0=(m_phi_sq*m_phi_sq-pow(s-m_p_sq-sqrt(pow(s-m_p_sq,2)-2*m_phi_sq*(s+m_p_sq)+m_phi_sq*m_phi_sq),2))/(4.*s);
    double t1=(m_phi_sq*m_phi_sq-pow(s-m_p_sq+sqrt(pow(s-m_p_sq,2)-2*m_phi_sq*(s+m_p_sq)+m_phi_sq*m_phi_sq),2))/(4.*s);
    double mytslope=fabs(tslope[i]);
    xsec[i]=dsdt[i]/mytslope*(exp(mytslope*t0)-exp(mytslope*t1));
    
    double sigB=ddsdt[i]/dsdt[i]*xsec[i];
    double sigA=dsdt[i]/mytslope*((t0-1./mytslope)*exp(mytslope*t0)
				   -(t1-1./mytslope)*exp(mytslope*t1))*dtslope[i];
    dxsec[i]=fabs(sigB)+fabs(sigA); 

    double mytslope1=fabs(tslope1[i]);
    xsec1[i]=dsdt1[i]/mytslope1*(exp(mytslope*t0)-exp(mytslope1*t1));
    
    double sigB1=ddsdt1[i]/dsdt1[i]*xsec1[i];
    double sigA1=dsdt1[i]/mytslope1*((t0-1./mytslope1)*exp(mytslope1*t0)
				   -(t1-1./mytslope1)*exp(mytslope1*t1))*dtslope1[i];
    dxsec1[i]=fabs(sigB1)+fabs(sigA1);
    
  }
  

  TH2F *h=new TH2F("h","cross section",15,2.6,5.8,10,0,0.6);
  h->SetXTitle("E_{#gamma} [GeV]");
  h->SetYTitle("#sigma [#mub]");
  h->Draw();
  TGraphErrors *g1=new TGraphErrors(15,Eg,xsec,0,dxsec);
  g1->SetMarkerColor(4);
  g1->SetMarkerStyle(21);
  g1->SetLineColor(4);
  g1->Draw("p"); 
  TGraphErrors *g3=new TGraphErrors(15,Eg,xsec1,0,dxsec1);
  g3->SetMarkerColor(2);
  g3->SetMarkerStyle(20);
  g3->SetLineColor(2);
  g3->Draw("p");



  double ballamE[2]={2.8,4.7};
  double ballamdE[2]={0.5,0.6};
  double ballamx[2]={0.40,0.41};
  double ballamdx[2]={0.1,0.09};
  TGraphErrors *g2=new TGraphErrors(2,ballamE,ballamx,ballamdE,ballamdx);
  g2->SetLineColor(8);
  g2->SetMarkerColor(8);
  g2->SetMarkerStyle(22);
  g2->Draw("p");

  TLegend *legend =new TLegend(0.15,0.65,0.45,0.85);
  legend->AddEntry(g1,"Method 2","p"); 
  legend->AddEntry(g3,"Method 1","p");
  legend->AddEntry(g2,"Ballam 1973","p");
  legend->Draw();

}