Flag to remove chi_0c and chi_1c (#2035)

Author: jackal1-66

MC/config/examples/epos4/generator_EPOS4.C

@@ -9,6 +9,38 @@ class GeneratorEPOS4 : public o2::eventgen::GeneratorHepMC
         GeneratorEPOS4() = default;
         ~GeneratorEPOS4() = default;
 
+        bool importParticles() override {
+            bool status = GeneratorHepMC::importParticles();
+            if (!status) {
+                LOG(error) << "Failed to import particles from HepMC event!";
+                return false;
+            }
+            // Remove charmonia chi_0c and chi_1c from the particles list (incompatible with default G4 physics list)
+            // These are not decayed by EPOS4 (no daughters)
+            if (!mEnChi)
+            {
+                for (int a = 0; a < mParticles.size(); ++a) {
+                    if (mParticles[a].GetPdgCode() == 10441 || mParticles[a].GetPdgCode() == 20443)
+                    {
+                        LOG(debug) << "Removing charmonium state " << mParticles[a].GetPdgCode() << " from particles list";
+                        mParticles.erase(mParticles.begin() + a);
+                        --a; // Adjust index after erasing
+                    }
+                }
+            }
+            return true;
+        }
+
+        void setChiFlag(bool &flag) {
+            mEnChi = flag;
+            if (!flag) {
+                LOG(info) << "Charmonium states chi_0c and chi_1c will be removed from the particles list";
+            }
+        }
+
+    private:
+        bool mEnChi = false; // Switch to enable chi_0c and chi_1c mesons
+
 };
 
 // Next function takes the optns file as argument and edits the maximum number of events to be generated.
@@ -19,7 +51,7 @@ class GeneratorEPOS4 : public o2::eventgen::GeneratorHepMC
 // type for the nfull parameter. Might be changed in the future.
 // When running locally, or on the GRID (not in hyperloop), the default parameters provided in the .ini file of the
 // external generation can be overwritten using the confKeyValues option (or similar depending on the tool used).
-FairGenerator* generateEPOS4(const std::string &name, const int& nEvents)
+FairGenerator* generateEPOS4(const std::string &name, const int& nEvents, bool enableChi = false)
 {
     // check if the file exists
     auto filename = gSystem->ExpandPathName(name.c_str());
@@ -44,6 +76,8 @@ FairGenerator* generateEPOS4(const std::string &name, const int& nEvents)
     }
     file.close();
     auto gen = new GeneratorEPOS4();
+    // Set the chi flag
+    gen->setChiFlag(enableChi);
     auto &param0 = o2::eventgen::GeneratorFileOrCmdParam::Instance();
     auto &param = o2::eventgen::GeneratorHepMCParam::Instance();
     auto &conf = o2::conf::SimConfig::Instance();