Add ITS monopoles response + data format for monopoles

Author: Marco Giacalone

DataFormats/simulation/include/SimulationDataFormat/MonopoleParticles.h

@@ -0,0 +1,41 @@
+// Copyright 2019-2026 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.
+
+/// \file MonopoleParticles.h
+/// \brief Identification of the BSM magnetic monopoles across the simulation
+/// @author M. Giacalone - September 2026
+///
+/// The monopole PDG codes are defined in O2MCApplication::AddParticles() and in
+/// O2DatabasePDG. They are collected here for better syncing inside the simulation code
+///
+/// To do: add dyons and other monopoles configurations when needed
+
+#ifndef ALICEO2_SIMULATION_MONOPOLEPARTICLES_H_
+#define ALICEO2_SIMULATION_MONOPOLEPARTICLES_H_
+
+namespace o2::sim
+{
+
+/// monopole carrying equal electric and magnetic charge
+constexpr int MonopolePdgSymm = 4110000;
+/// monopole carrying opposite electric and magnetic charge
+constexpr int MonopolePdgAsymm = 4120000;
+
+/// true for the monopole and antimonopole of both species
+constexpr bool isMonopole(int pdg) noexcept
+{
+  const int abspdg = pdg < 0 ? -pdg : pdg;
+  return abspdg == MonopolePdgSymm || abspdg == MonopolePdgAsymm;
+}
+
+} // namespace o2::sim
+
+#endif

Detectors/ITSMFT/ITS/simulation/src/Detector.cxx

@@ -24,6 +24,7 @@
 
 #include "DetectorsBase/Stack.h"
 #include "SimulationDataFormat/TrackReference.h"
+#include "SimulationDataFormat/MonopoleParticles.h"
 #include "fairlogger/Logger.h" // for LOG, LOG_IF
 
 // FairRoot includes
@@ -318,7 +319,11 @@ void Detector::InitializeO2Detector()
 Bool_t Detector::ProcessHits(FairVolume* vol)
 {
   // This method is called from the MC stepping
-  if (!(fMC->TrackCharge())) {
+  // Electrically neutral magnetic monopoles deposit energy in the
+  // silicon through G4mplIonisation (Ahlen stopping power), so they must not be
+  // rejected by the electric-charge gate
+  const bool isMonopole = o2::sim::isMonopole(fMC->TrackPid());
+  if (!(fMC->TrackCharge()) && !isMonopole) {
     return kFALSE;
   }
 
@@ -386,6 +391,12 @@ Bool_t Detector::ProcessHits(FairVolume* vol)
     mTrackData.mHitStarted = true;
   }
   if (stopHit) {
+    // A monopole reaches this point even when no ionisation process is attached to
+    // it (G4.monopole=0), in which case it crosses the sensor depositing nothing.
+    // Storing such empty hits would only inflate the hit file, so they are skipped
+    if (isMonopole && mTrackData.mEnergyLoss <= 0.) {
+      return kFALSE;
+    }
     TLorentzVector positionStop;
     fMC->TrackPosition(positionStop);
     // Retrieve the indices with the volume path

Detectors/TPC/simulation/src/Detector.cxx

@@ -17,6 +17,7 @@
 
 #include "DetectorsBase/Stack.h"
 #include "SimulationDataFormat/TrackReference.h"
+#include "SimulationDataFormat/MonopoleParticles.h"
 
 #include "FairVolume.h" // for FairVolume
 
@@ -115,7 +116,7 @@ Bool_t Detector::ProcessHits(FairVolume* vol)
   // Magnetic monopoles have zero electric charge but ionise the gas through
   // their magnetic charge energy loss (G4mplIonisation).
   const int trackPdg = fMC->TrackPid();
-  const bool isMonopole = (TMath::Abs(trackPdg) == 4110000 || TMath::Abs(trackPdg) == 4120000);
+  const bool isMonopole = o2::sim::isMonopole(trackPdg);
   if (static_cast<int>(trackCharge) == 0) {
     // Fall through only for monopoles when ionisation is enabled.
     // The behaviour for the other neutral particles remains as before.

Detectors/gconfig/src/O2MonopolePhysics.cxx

@@ -30,6 +30,7 @@
 
 #include "SimSetup/O2MonopolePhysics.h"
 #include "CommonUtils/ConfigurableParam.h"
+#include "SimulationDataFormat/MonopoleParticles.h"
 
 #include <boost/property_tree/ptree.hpp> // needed to instantiate getValueAs<>
 
@@ -79,13 +80,10 @@ namespace
 // O2MCApplication::AddParticles() and O2DatabasePDG:
 //   +-4110000 : "symmetric"  monopoles
 //   +-4120000 : "asymmetric" monopoles
-constexpr std::array<int, 4> gMonopolePDGs = {4110000, -4110000, 4120000, -4120000};
+constexpr std::array<int, 4> gMonopolePDGs = {o2::sim::MonopolePdgSymm, -o2::sim::MonopolePdgSymm,
+                                              o2::sim::MonopolePdgAsymm, -o2::sim::MonopolePdgAsymm};
 
-inline bool isMonopolePDG(int pdg)
-{
-  const int abspdg = std::abs(pdg);
-  return abspdg == 4110000 || abspdg == 4120000;
-}
+inline bool isMonopolePDG(int pdg) { return o2::sim::isMonopole(pdg); }
 
 /// GEANT4 only queries the magnetic field when a particle has non-zero electric
 /// charge or non-zero magnetic moment (μ) and the monopole has neither. So this is a workaround