@@ -0,0 +1,123 @@
+
+#if !defined(__CLING__) || defined(__ROOTCLING__)
+#include "FairGenerator.h"
+#include "FairPrimaryGenerator.h"
+#include "Generators/GeneratorPythia8.h"
+#include "TRandom3.h"
+#include "TParticlePDG.h"
+#include "TDatabasePDG.h"
+#include "TMath.h"
+#include <fairlogger/Logger.h>
+#include <cmath>
+#include <vector>
+#endif
+
+#include "Pythia8/Pythia.h"
+
+class GeneratorPythia8Box : public o2::eventgen::GeneratorPythia8
+{
+public:
+
+  GeneratorPythia8Box(std::vector<int> pdgList, int nInject = 3,
+                      float ptMin = 0.1, float ptMax = 50.0,
+                      float etaMin = -0.8, float etaMax = 0.8)
+    : mPdgList(pdgList), nParticles(nInject),
+      genMinPt(ptMin), genMaxPt(ptMax),
+      genMinEta(etaMin), genMaxEta(etaMax)
+  {
+    // Initialize dedicated Pythia instance for injection/decay only
+    // Reuses same config as main Pythia; beam settings are ignored by moreDecays()
+    auto& param = o2::eventgen::GeneratorPythia8Param::Instance();
+    std::string cfg = gSystem->ExpandPathName(param.config.c_str());
+    if (!mPythiaGun.readFile(cfg, true)) {
+      LOG(fatal) << "Failed to read config for mPythiaGun: " << cfg;
+    }
+    if (!mPythiaGun.init()) {
+      LOG(fatal) << "Failed to init mPythiaGun";
+    }
+  }
+
+  ~GeneratorPythia8Box() = default;
+
+  bool Init() override
+  {
+    return o2::eventgen::GeneratorPythia8::Init();
+  }
+
+protected:
+
+  // ── generateEvent: MB + injection ──
+  Bool_t generateEvent() override
+  {
+    bool hasElectron = false;
+
+    while (!hasElectron) {
+      // ===== 1. Generate minimum-bias event =====
+      if (!mPythia.next()) continue;
+
+      // ===== 2. Inject π⁰/η into separate gun, decay only them =====
+      mPythiaGun.event.reset();
+      for (int i = 0; i < nParticles; ++i) {
+        int pdg = mPdgList[gRandom->Integer(mPdgList.size())];
+        auto* pPDG = TDatabasePDG::Instance()->GetParticle(pdg);
+        if (!pPDG) continue;
+        double mass = pPDG->Mass();
+
+        double pt  = gRandom->Uniform(genMinPt, genMaxPt);
+        double eta = gRandom->Uniform(genMinEta, genMaxEta);
+        double phi = gRandom->Uniform(0., TMath::TwoPi());
+        double px = pt * cos(phi);
+        double py = pt * sin(phi);
+        double pz = pt * sinh(eta);
+        double e  = sqrt(px * px + py * py + pz * pz + mass * mass);
+
+        mPythiaGun.event.append(pdg, 11, 0, 0, px, py, pz, e, mass);
+      }
+
+      mPythiaGun.moreDecays();
+
+      // ===== 3. Check for electron from injection in acceptance =====
+      for (int i = 0; i < mPythiaGun.event.size(); ++i) {
+        auto& part = mPythiaGun.event[i];
+        if (std::abs(part.id()) == 11 && part.isFinal()) {
+          if (part.pT() > 0.1 && std::abs(part.eta()) < 1.2) {
+            hasElectron = true;
+            break;
+          }
+        }
+      }
+    }
+
+    // ===== 4. Copy injection products to MB event with index offset =====
+    int offset = mPythia.event.size();
+
+    for (int i = 0; i < mPythiaGun.event.size(); ++i) {
+      auto part = mPythiaGun.event[i];
+      if (part.id() == 90) continue;            // skip system placeholder
+      if (!part.isFinal() && part.id() != 111 && part.id() != 221) continue;
+
+      if (part.mother1() > 0)   part.mother1(part.mother1()   + offset - 1);
+      if (part.mother2() > 0)   part.mother2(part.mother2()   + offset - 1);
+      if (part.daughter1() > 0) part.daughter1(part.daughter1() + offset - 1);
+      if (part.daughter2() > 0) part.daughter2(part.daughter2() + offset - 1);
+
+      mPythia.event.append(part);
+    }
+
+    return true;
+  }
+
+private:
+  std::vector<int> mPdgList;
+  double genMinPt, genMaxPt;
+  double genMinEta, genMaxEta;
+  int nParticles;
+  
+  Pythia8::Pythia mPythiaGun;
+};
+
+FairGenerator *generatePythia8Box(float ptMin = 0.1, float ptMax = 50.0)
+{
+  std::vector<int> pdgList = {111, 221};
+  return new GeneratorPythia8Box(pdgList, 3, ptMin, ptMax, -0.8, 0.8);
+}