void plot_xsec_from_exp(){
  double Eg[14],dEg[14];
  for (int i=0;i<14;i++){
    Eg[i]=2.9+0.2*i;
    dEg[i]=0.1;
  }
  double scale[14];
  double dscale[14];
  double tslope[14];
  double dtslope[14];

  ifstream in_file("phi_parms.dat");
  for (int i=0;i<14;i++){
    double dummy;
    in_file >> dummy;
    in_file >> scale[i];
    in_file >> dscale[i];
    in_file >> tslope[i];
    in_file >> dtslope[i];
  }

  double dsdt[14],ddsdt[14];
  double xsec[14];
  double dxsec[14];

  double m_p_sq=0.9837*0.93827;
  double m_phi_sq=1.0195*1.0195;
  for (int i=0;i<14;i++){
    dsdt[i]=exp(scale[i]);
    ddsdt[i]=dsdt[i]*dscale[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);
    cout << t0 << " " << t1 << " " << xsec[i] << " " << dxsec[i] << endl;
  }
		     

  TH2F *h=new TH2F("h","cross section",15,2.6,5.6,10,0,1);
  h->SetXTitle("E_{#gamma} [GeV]");
  h->SetYTitle("#sigma [#mub]");
  h->Draw();
  TGraphErrors *g1=new TGraphErrors(14,Eg,xsec,0,dxsec);
  g1->SetMarkerColor(2);
  g1->SetMarkerStyle(20);
  g1->SetLineColor(2);
  g1->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,"our measurements","p");
  legend->AddEntry(g2,"Ballam 1973","p");
  legend->Draw();

}