## main141

Back to index.

    // main141.cc is a part of the PYTHIA event generator.
    // Copyright (C) 2026 Torbjorn Sjostrand.
    // PYTHIA is licenced under the GNU GPL v2 or later, see COPYING for details.
    // Please respect the MCnet Guidelines, see GUIDELINES for details.

    // Authors:
    //            Dag Gillberg

    // Keywords:
    //            Root
    //            Event display

    // This is a program that uses ROOT to visualize the particles produced by
    // Pythia. Particles are drawn in (y, phi)-space to depict the E/p flow.
    // A pdf file is produced with multiple pages showing WH->qqbb events.

    #include "Pythia8/Pythia.h"
    #include "TCanvas.h"
    #include "TStyle.h"
    #include "TString.h"
    #include "TH2D.h"
    #include "TMath.h"
    #include "TLine.h"
    #include "TMarker.h"
    #include "TLatex.h"

    using namespace Pythia8;

    //==========================================================================

    // Draws text to the canvas using non-direct coordinates:
    // (x, y)=(0, 0) -> lower-left, (1, 1) -> upper-right.

    void drawText(double x, double y, TString txt, int col= kBlack,
      double tsize = 0.032, int align = 11) {
      static auto tex = new TLatex();
      tex->SetTextColor(col);
      tex->SetTextSize(tsize);
      tex->SetTextFont(42);
      tex->SetNDC();
      tex->SetTextAlign(align);
      tex->DrawLatex(x, y, txt);
    }

    //==========================================================================

    // Draws right-justified text, as above.

    void drawTextR(double x, double y, TString txt, int col = kBlack,
      double tsize = 0.032) {
      drawText(x, y, txt, col, tsize, 31);
    }

    //==========================================================================

    // Draw a particle in (y, phi)-space with a symbol and label.

    void drawParticle(double x, double y, TString txt, int col = kGray + 1,
      double tsize = 0.03) {
      static auto m = new TMarker();
      m->SetMarkerColor(col);
      m->SetMarkerStyle(20);
      m->SetMarkerSize(1.2);
      m->DrawMarker(x, y);

      // Modify Pythia's default particle label to suit ROOT's TLatex format.
      if (txt.Contains("bar")) txt = "#bar{" + txt.ReplaceAll("bar", "") + "}";
      txt.ReplaceAll("pi", "#pi");
      txt.ReplaceAll("+", "^{+}");
      txt.ReplaceAll("-", "^{-}");
      txt.ReplaceAll("0", "^{0}");
      txt.ReplaceAll("_L", "_{L}");
      txt.ReplaceAll("gamma", "#gamma");
      txt.ReplaceAll("Lambda", "#Lambda");
      txt.ReplaceAll("Delta", "#Delta");
      txt.ReplaceAll("Sigma", "#Sigma");
      txt.ReplaceAll("rho", "#rho");
      txt.ReplaceAll("omega", "#omega");
      txt.ReplaceAll("eta", "#eta");
      txt.ReplaceAll("_S", "_{S}");
      static auto tex = new TLatex();
      tex->SetTextColor(col);
      tex->SetTextSize(tsize);
      tex->SetTextFont(42);
      tex->SetTextAlign(22);
      tex->DrawLatex(x, y - 0.2, txt);
    }

    //==========================================================================

    // Draw a single particle.

    void drawParticle(const Particle &p) {
      int col =
        // System.
        p.idAbs() == 90 ? kGray    :
        // Light quarks.
        p.idAbs() <=  4 ? kBlue    :
        // b quarks.
        p.idAbs() ==  5 ? kGreen + 2 :
        p.idAbs() == 21 ? kGray + 1  :
        p.isHadron()    ? kBlue    : kRed;
      drawParticle(p.y(), p.phi(), p.name().c_str(), col);
    }

    //==========================================================================

    // Draw a line.

    void drawLine(double x1, double y1, double x2, double y2, int col, int style) {
      static auto line = new TLine();
      line->SetLineColor(col);
      line->SetLineStyle(style);
      line->DrawLine(x1, y1, x2, y2);
    }

    //==========================================================================

    // Example main program to draw an event display.

    int main() {

      // Adjust ROOTs display options.
      gStyle->SetOptTitle(0);
      gStyle->SetOptStat(0);
      gStyle->SetPadTickX(1);
      // Tick marks on top and RHS.
      gStyle->SetPadTickY(1);
      gStyle->SetTickLength(0.02, "x");
      gStyle->SetTickLength(0.02, "y");

      // Define the canvas.
      auto can = new TCanvas();
      double x = 0.06, y = 0.96;
      // left-right-bottom-top.
      can->SetMargin(x, 0.02, 0.08, 0.05);

      // To draw axis frame, I find it easiest to create an empty histogram.
      double yMax = 5, phiMax = 3.4;
      // Adding a bit of margin around pi
      auto axis = new TH2D( "", ";Rapidity #it{y};Azimuth #it{#phi}", 1, -yMax,
        yMax, 1, -phiMax, phiMax);
      axis->GetYaxis()->SetTitleOffset(0.8);

      // Name of output pdf file.
      TString pdf = "fig141.pdf";
      can->Print(pdf + "[");
      // Turn off loads of TCanvas::Print Info messages.
      // Current canvas added to pdf file.
      gErrorIgnoreLevel = 4000;

      // Configure and describe the process.
      // Description uses ROOT's TLatex notation
      TString desc = "#it{pp} #rightarrow #it{WH} "
        "#rightarrow #it{q#bar{q}b#bar{b}},  #sqrt{#it{s}} = 13.6 TeV";

      // Generator.
      Pythia pythia;

      // PYTHIA setup. Process selection. LHC initialization.
      pythia.readString("Beams:eCM = 13600.");
      pythia.readString("HiggsSM:ffbar2HW = on");
      // Force H->bb decays and hadronic W decays.
      pythia.readString("25:onMode = off");
      pythia.readString("25:onIfAny = 5");
      pythia.readString("24:onMode = off");
      pythia.readString("24:onIfAny = 1 2 3 4 5");

      // If Pythia fails to initialize, exit with error.
      if (!pythia.init()) return 1;

      // List of status codes to draw with associated descriptions.
      // Descriptions from "Particle Properties" in the Pythia8 manual.
      std::map<int, TString> statusCodeMap;
      //statusCodeMap[11] = "Beam particles";
      statusCodeMap[21] = "Hardest subprocess";
      statusCodeMap[31] = "Particles of subsequent subprocesses";
      statusCodeMap[41] = "Initial-state radiation";
      statusCodeMap[51] = "Final-state radiation";
      statusCodeMap[61] = "Beam remnant treatment";
      statusCodeMap[71] = "Preparation of hadronization";
      statusCodeMap[81] = "Primary hadrons";
      statusCodeMap[91] = "Decay products";

      // Event loop.
      for (int iEvent = 0; iEvent < 5; ++iEvent) {

        // Generate event. (Skip to next if pythia.next() returns false = error.)
        if (!pythia.next()) continue;

        // Clear and draw an empty canvas to paint on.
        axis->Reset();
        axis->Draw();
        drawLine( -yMax, TMath::Pi(), yMax, TMath::Pi(), kGray, 7);
        drawLine( -yMax, -TMath::Pi(), yMax, -TMath::Pi(), kGray, 7);

        // 1. Draw the hard process.
        for (int i = 0; i < pythia.process.size(); ++i) {
          auto &p = pythia.process[i];
          drawParticle(p);
          // printf("(id,pT,y,phi,status) = (%3i,%5.1f,%5.2f,%5.2f,%i)\n", p.id(),
          //   p.pT(), p.y(), p.phi(), p.status());
        }
        // Text.
        drawText( x, y, desc);
        drawTextR( 0.98, y, "Hard process");

        // Redraw the axis to make sure they are not covered by points.
        axis->Draw("axis same");
        can->Print(pdf);

        // 2. Draw intermediate particles.
        for (auto statusCode:statusCodeMap) {
          // Clear and re-draw the axis. Draw dashed-lines at +/- pi.
          axis->Reset();
          axis->Draw();
          drawLine( -yMax, TMath::Pi(), yMax, TMath::Pi(), kGray, 7);
          drawLine( -yMax, -TMath::Pi(), yMax, -TMath::Pi(), kGray, 7);
          int max = -statusCode.first, min = max-8;
          // printf("Range [%i,%i] %s\n", min, max, statusCode.second.Data());
          for (int i = 0; i < pythia.event.size(); ++i) {
            auto &p = pythia.event[i];
            // Impose desired status code(s) and rapidity intervals.
            if ( p.status() < min || p.status() > max) continue;
            if ( std::abs(p.y()) > yMax ) continue;
            drawParticle(p);
          }
          // Text.
          drawText( x, y, desc);
          drawTextR( 0.98, y, statusCode.second + Form(", status #in  [%.i,%.i]",
              min, max));
          // Redraw the axis to make sure they are not covered by points.
          axis->Draw("axis same");
          can->Print(pdf);
        }

        // 3. Draw the final hadrons.
        axis->Reset();
        axis->Draw();
        drawLine( -yMax, TMath::Pi(), yMax, TMath::Pi(), kGray, 7);
        drawLine( -yMax, -TMath::Pi(), yMax, -TMath::Pi(), kGray, 7);
        for (int i = 0; i < pythia.event.size(); ++i) {
          auto &p = pythia.event[i];
          if (not p.isFinal()) continue;
          if (std::abs(p.y())>yMax) continue;
          drawParticle(p);
        }
        drawText( x, y, desc);
        drawTextR( 0.98, y, "Stable particles");
        axis->Draw("axis same");
        can->Print(pdf);
      }

      // Close the pdf
      can->Print(pdf + "]");
      printf( "\nProduced %s\n\n", pdf.Data());

      // Done.
      return 0;
    }