Add a tool that writes the Geant4 particle table

This commit provides a new tool o2-sim-g4-particle-table that writes the Geant4 particle table to a text file.

- One line per particle with a PDG code: pdg, Geant4 name, ctau in mm, short-lived flag, decay-table flag.
- The Geant4 setup is the same as in o2-sim-g4-determine-unknown-pdg-properties.
- Particles that o2-sim adds at run time are not in the table.

Usage: o2-sim-g4-particle-table --output g4_particle_table.dat

https://its.cern.ch/jira/browse/O2-5903

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: sawenzel

run/CMakeLists.txt

@@ -113,6 +113,12 @@ o2_add_executable(g4-determine-unknown-pdg-properties
                   SOURCES g4DetermineUnknownPdgProperties.cxx
                   PUBLIC_LINK_LIBRARIES O2::O2TrivialMC MC::Geant4 MC::Geant4VMC O2::SimConfig)
 
+# Writes the Geant4 particle table to a text file.
+o2_add_executable(g4-particle-table
+                  COMPONENT_NAME sim
+                  SOURCES o2sim_g4_particle_table.cxx
+                  PUBLIC_LINK_LIBRARIES O2::O2TrivialMC MC::Geant4 MC::Geant4VMC O2::SimConfig)
+
 # Reads a geometry file and a field and reports where the two do not fit together.
 # It needs neither the simulation engines nor any detector code -- the medium flags
 # it reasons about are GSTMED parameters stored in the geometry file itself -- so it

run/o2sim_g4_particle_table.cxx

@@ -0,0 +1,77 @@
+// Copyright 2019-2020 CERN and copyright holders of ALICE O2.
+// See https://alice-o2.web.cern.ch/copyright for details of the copyright holders.
+// All rights not expressly granted are reserved.
+//
+// This software is distributed under the terms of the GNU General Public
+// License v3 (GPL Version 3), copied verbatim in the file "COPYING".
+//
+// In applying this license CERN does not waive the privileges and immunities
+// granted to it by virtue of its status as an Intergovernmental Organization
+// or submit itself to any jurisdiction.
+
+// Writes the Geant4 particle table (one line per particle with a PDG code) to a text file.
+
+#include <cstdio>
+#include <iostream>
+#include <string>
+#include <fstream>
+
+#include "G4ParticleTable.hh"
+#include "G4ParticleDefinition.hh"
+#include "G4Version.hh"
+#include "CLHEP/Units/PhysicalConstants.h"
+
+#include "TGeant4.h"
+#include "TG4RunConfiguration.h"
+
+#include "SimConfig/G4Params.h"
+#include "O2TrivialMC/O2TrivialMCApplication.h"
+
+int main(int argc, char** argv)
+{
+  if (argc != 3 || std::string(argv[1]) != "--output") {
+    std::cerr << "Usage: " << argv[0] << " --output <file>\n";
+    return 1;
+  }
+
+  std::ofstream os{argv[2]};
+  if (!os.is_open()) {
+    std::cerr << "Cannot open " << argv[2] << "\n";
+    return 1;
+  }
+
+  // We use our O2 VMC setup to make sure we are in line with our physics definition.
+  new o2::mc::O2TrivialMCApplication();
+  auto& physicsSetup = ::o2::conf::G4Params::Instance().getPhysicsConfigString();
+  auto runConfiguration = new TG4RunConfiguration("geomRoot", physicsSetup);
+  auto vmc = new TGeant4("TGeant4", "The Geant4 Monte Carlo", runConfiguration);
+  // needs to be initialised to have the particle table / physics entirely ready
+  vmc->Init();
+
+  os << "# Geant4 particle table, Geant4 version " << G4VERSION_NUMBER << "\n";
+  os << "# physics setup: " << physicsSetup << "\n";
+  os << "# pdg g4name ctau_mm shortlived hasDecayTable\n";
+
+  auto table = G4ParticleTable::GetParticleTable();
+  auto it = table->GetIterator();
+  it->reset();
+  int nParticles = 0;
+  while ((*it)()) {
+    auto particle = it->value();
+    int pdg = particle->GetPDGEncoding();
+    if (pdg == 0) {
+      continue;
+    }
+    double lifetime = particle->GetPDGLifeTime();
+    // lifetime is in G4 internal units, times c_light gives mm
+    double ctau = lifetime > 0. ? lifetime * CLHEP::c_light : 0.;
+    char buf[64];
+    std::snprintf(buf, sizeof(buf), "%.6g", ctau);
+    os << pdg << " " << particle->GetParticleName() << " " << buf << " "
+       << (particle->IsShortLived() ? 1 : 0) << " "
+       << (particle->GetDecayTable() ? 1 : 0) << "\n";
+    ++nParticles;
+  }
+  std::cout << "Wrote " << nParticles << " particles to " << argv[2] << "\n";
+  return 0;
+}