void Zllvertex()
{
   //
   // Draws the feynman diagram for the Z-2-lepton vertex.
   //
   TCanvas *c1 = new TCanvas("c1", "A canvas", 1, 1, 350, 300);
   c1->Range(0, 0, 60, 50);
   Int_t linsav = gStyle->GetLineWidth();
   gStyle->SetLineWidth(2);
   TLatex t;
   t.SetTextAlign(22);
   t.SetTextSize(0.1);
   TLine * l;
   //
   // The Wll diagram
   //
   Double_t x0 = 30; Double_t y0 = 25;
   Double_t x1 = x0-20; Double_t x2 = x0+20;
   Double_t y1 = y0-15; Double_t y2 = y0+15;
   l = new TLine(x0, y0, x2, y1); l->Draw();
   l = new TLine(x0, y0, x2, y2); l->Draw();
   //
   // Add arrows
   //
   Double_t xa = (x0+x2)/2.0; Double_t ya = (y0+y1)/2.0;
   a = new TArrow(x2, y1, xa, ya); a->Draw();
   Double_t xa = (x0+x2)/2.0; Double_t ya = (y0+y2)/2.0;
   a = new TArrow(x0, y0, xa, ya); a->Draw();
   l = new TLine(x1, y0, x0, y0); l->SetLineStyle(3); l->Draw();
   Double_t xt = x2+5.0; Double_t yt = y2;
   t.DrawLatex(xt,yt,"l");
   Double_t xt = x2+5.0; Double_t yt = y1;
   t.DrawLatex(xt,yt,"#bar{l}");
   Double_t xt = x1-5.0; Double_t yt = y0;
   t.DrawLatex(xt,yt,"Z^{0}");
   
   c1->Update();
   gStyle->SetLineWidth(linsav);
}