@@ -355,6 +355,7 @@ struct FemtoUniverseProducerTask {
     Configurable<float> confPhiKaonNsigmaTOFfrom055to15{"confPhiKaonNsigmaTOFfrom055to15", 3.0, "Reject if Kaons in 0.55-1.5 are have TOF n sigma above this value."};
     Configurable<float> confPhiKaonNsigmaTPCfrom15{"confPhiKaonNsigmaTPCfrom15", 3.0, "Reject if Kaons above 1.5 are have TPC n sigma above this value."};
     Configurable<float> confPhiKaonNsigmaTOFfrom15{"confPhiKaonNsigmaTOFfrom15", 3.0, "Reject if Kaons above 1.5 are have TOF n sigma above this value."};
+    Configurable<bool> confDoLikeSign{"confDoLikeSign", false, "Enable like-sign (fake Phi) background calculation"};
   } ConfPhiSelection;
 
   // PDG codes for fillMCParticle function
@@ -2229,6 +2230,156 @@ struct FemtoUniverseProducerTask {
     }
   }
 
+  template <bool isMC, typename TrackType, typename CollisionType>
+  void fillLikeSignPhi(CollisionType const& col, TrackType const& tracks)
+  {
+    std::vector<int> childIDs = {0, 0};
+    std::vector<int> tmpIDtrack;
+
+    // Loop over all track combinations using FullIndexPolicy
+    for (const auto& [p1, p2] : combinations(soa::CombinationsFullIndexPolicy(tracks, tracks))) {
+
+      // 1. Check minimal track quality cuts for both tracks
+      if (!trackCuts.isSelectedMinimal(p1) || !trackCuts.isSelectedMinimal(p2)) {
+        continue;
+      }
+
+      if (p1.sign() != p2.sign()) { // K+K- pairs are already handled in fillPhi, so we skip them here, only K+K+ and K-K-
+        continue;
+      }
+
+      // avoid double counting and self-pairing when using FullIndexPolicy
+      if (p1.globalIndex() >= p2.globalIndex()) {
+        continue;
+      }
+
+      // cuts for  daughters
+      if (ConfPhiSelection.confPhiDoLFPID4Kaons) {
+        if (!p1.isGlobalTrackWoDCA() || !p2.isGlobalTrackWoDCA() || !p1.isPVContributor() || !p2.isPVContributor()) {
+          continue;
+        }
+        if (!(isKaonNSigmaLF(p1.pt(), trackCuts.getNsigmaTPC(p1, o2::track::PID::Kaon), trackCuts.getNsigmaTOF(p1, o2::track::PID::Kaon), p1.hasTOF()))) {
+          continue;
+        }
+        if (!(isKaonNSigmaLF(p2.pt(), trackCuts.getNsigmaTPC(p2, o2::track::PID::Kaon), trackCuts.getNsigmaTOF(p2, o2::track::PID::Kaon), p2.hasTOF()))) {
+          continue;
+        }
+      } else {
+        if (!(isKaonNSigma(p1.pt(), trackCuts.getNsigmaTPC(p1, o2::track::PID::Kaon), trackCuts.getNsigmaTOF(p1, o2::track::PID::Kaon)))) {
+          continue;
+        }
+        if (!(isKaonNSigma(p2.pt(), trackCuts.getNsigmaTPC(p2, o2::track::PID::Kaon), trackCuts.getNsigmaTOF(p2, o2::track::PID::Kaon)))) {
+          continue;
+        }
+      }
+
+      // Reject protons and pions contamination
+      if (isKaonRejected(p1.p(), trackCuts.getNsigmaTPC(p1, o2::track::PID::Proton), trackCuts.getNsigmaTOF(p1, o2::track::PID::Proton), trackCuts.getNsigmaTPC(p1, o2::track::PID::Pion), trackCuts.getNsigmaTOF(p1, o2::track::PID::Pion))) {
+        continue;
+      }
+      if (isKaonRejected(p2.p(), trackCuts.getNsigmaTPC(p2, o2::track::PID::Proton), trackCuts.getNsigmaTOF(p2, o2::track::PID::Proton), trackCuts.getNsigmaTPC(p2, o2::track::PID::Pion), trackCuts.getNsigmaTOF(p2, o2::track::PID::Pion))) {
+        continue;
+      }
+
+      const auto mMassOne = o2::constants::physics::MassKPlus;
+      const auto mMassTwo = o2::constants::physics::MassKPlus;
+
+      ROOT::Math::LorentzVector<ROOT::Math::PtEtaPhiM4D<double>> part1Vec(p1.pt(), p1.eta(), p1.phi(), mMassOne);
+      ROOT::Math::LorentzVector<ROOT::Math::PtEtaPhiM4D<double>> part2Vec(p2.pt(), p2.eta(), p2.phi(), mMassTwo);
+
+      ROOT::Math::LorentzVector<ROOT::Math::PtEtaPhiM4D<double>> sumVec(part1Vec);
+      sumVec += part2Vec;
+
+      // kinematic cuts for phi candidate
+      float phiEta = sumVec.Eta();
+      if (std::abs(phiEta) > ConfPhiSelection.confPhiEtaHighLimit.value) {
+        continue;
+      }
+
+      float phiPt = sumVec.Pt();
+      if ((phiPt < ConfPhiSelection.confPhiPtLowLimit.value) || (phiPt > ConfPhiSelection.confPhiPtHighLimit.value)) {
+        continue;
+      }
+
+      float phiPhi = RecoDecay::constrainAngle(sumVec.Phi(), 0);
+      float phiM = sumVec.M();
+
+      if ((phiM < ConfPhiSelection.confPhiInvMassLowLimit) || (phiM > ConfPhiSelection.confPhiInvMassUpLimit)) {
+        continue;
+      }
+
+      phiCuts.fillQA<aod::femtouniverseparticle::ParticleType::kPhi, aod::femtouniverseparticle::ParticleType::kPhiChild>(col, p1, p1, p2, 321, 321);
+
+      int postrackID = p1.globalIndex();
+      int rowInPrimaryTrackTablePos = -1;
+      rowInPrimaryTrackTablePos = getRowDaughters(postrackID, tmpIDtrack);
+      childIDs[0] = rowInPrimaryTrackTablePos;
+      childIDs[1] = 0;
+
+      // first daughter track
+      outputParts(outputCollision.lastIndex(), p1.pt(),
+                  p1.eta(), p1.phi(),
+                  aod::femtouniverseparticle::ParticleType::kPhiChild,
+                  -999,
+                  -999,
+                  p1.dcaXY(),
+                  childIDs,
+                  0,
+                  p1.sign());
+      const int rowOfPosTrack = outputParts.lastIndex();
+      if constexpr (isMC) {
+        fillMCParticle(p1, o2::aod::femtouniverseparticle::ParticleType::kPhiChild);
+      }
+
+      int negtrackID = p2.globalIndex();
+      int rowInPrimaryTrackTableNeg = -1;
+      rowInPrimaryTrackTableNeg = getRowDaughters(negtrackID, tmpIDtrack);
+      childIDs[0] = 0;
+      childIDs[1] = rowInPrimaryTrackTableNeg;
+
+      // second daughter track
+      outputParts(outputCollision.lastIndex(),
+                  p2.pt(),
+                  p2.eta(),
+                  p2.phi(),
+                  aod::femtouniverseparticle::ParticleType::kPhiChild,
+                  -999,
+                  -999,
+                  p2.dcaXY(),
+                  childIDs,
+                  0,
+                  p2.sign());
+      const int rowOfNegTrack = outputParts.lastIndex();
+      if constexpr (isMC) {
+        fillMCParticle(p2, o2::aod::femtouniverseparticle::ParticleType::kPhiChild);
+      }
+
+      std::vector<int> indexChildID = {rowOfPosTrack, rowOfNegTrack};
+
+      // Define like-sign flag: +1 for (K+K+), -1 for (K-K-)
+      int likeSignFlag = p1.sign();
+
+      // Save the Fake Phi candidate from K+K+ or K-K- combination
+      outputParts(outputCollision.lastIndex(),
+                  phiPt,
+                  phiEta,
+                  phiPhi,
+                  aod::femtouniverseparticle::ParticleType::kPhi,
+                  likeSignFlag, // Flag: +1 for K+K+, -1 for K-K-, before it was empty slot for cutcontainer
+                  0,
+                  phiM, // Invariant mass of the fake Phi candidate
+                  indexChildID,
+                  phiM,
+                  0);
+
+      if (confIsDebug) {
+        fillDebugParticle<true, false, false>(p1); // QA for first daughter
+        fillDebugParticle<true, false, false>(p2); // QA for second daughter
+        fillDebugParticle<false, true, false>(p1); // QA for fake Phi mother
+      }
+    }
+  }
+
   template <typename TrackType, bool transientLabels = false, bool resolveDaughs = false>
   void fillParticles(TrackType const& tracks, std::optional<std::reference_wrapper<const std::set<int>>> recoMcIds = std::nullopt)
   {
@@ -2452,6 +2603,9 @@ struct FemtoUniverseProducerTask {
       }
       if (ConfGeneral.confIsActivatePhi) {
         fillPhi<isMC>(col, tracks);
+        if (ConfPhiSelection.confDoLikeSign) {
+          fillLikeSignPhi<isMC>(col, tracks);
+        }
       }
     }
     return colcheck;
@@ -2628,6 +2782,9 @@ struct FemtoUniverseProducerTask {
     if (colcheck) {
       fillTracks<false>(tracks);
       fillPhi<false>(col, tracks);
+      if (ConfPhiSelection.confDoLikeSign) {
+        fillLikeSignPhi<false>(col, tracks);
+      }
     }
   }
   PROCESS_SWITCH(FemtoUniverseProducerTask, processTrackPhiData,
 
@@ -153,6 +153,7 @@ struct FemtoUniversePairTaskTrackPhi {
   Configurable<float> confPhiPtHigh{"confPhiPtHigh", 4.0, "Higher limit of the Phi pT."};
   Configurable<float> confInvMassLowLimitPhi{"confInvMassLowLimitPhi", 1.011, "Lower limit of the Phi invariant mass"}; // change that to do invariant mass cut
   Configurable<float> confInvMassUpLimitPhi{"confInvMassUpLimitPhi", 1.027, "Upper limit of the Phi invariant mass"};
+  Configurable<bool> confFillMinvBackground{"confFillMinvBackground", false, "Fill background histograms for Phi invariant mass."};
 
   /// Partitions for the Phi meson (particle 2)
   Partition<FilteredFemtoFullParticles> partsPhi = (aod::femtouniverseparticle::partType == uint8_t(aod::femtouniverseparticle::ParticleType::kPhi)) &&
@@ -551,37 +552,39 @@ struct FemtoUniversePairTaskTrackPhi {
       sameEventCont.setPair<isMC>(track, phicandidate, multCol, confUse3D, mWeight);
     }
 
-    // Used for better fitting of invariant mass background.
+    if (confFillMinvBackground) {
+      // Used for better fitting of invariant mass background.
 
-    TLorentzVector part1Vec;
-    TLorentzVector part2Vec;
+      TLorentzVector part1Vec;
+      TLorentzVector part2Vec;
 
-    float mMassOne = o2::constants::physics::MassKPlus;
-    float mMassTwo = o2::constants::physics::MassKMinus;
+      float mMassOne = o2::constants::physics::MassKPlus;
+      float mMassTwo = o2::constants::physics::MassKMinus;
 
-    for (auto const& [kaon1, kaon2] : combinations(CombinationsStrictlyUpperIndexPolicy(partsKaons, partsKaons))) {
-      if ((kaon1.mAntiLambda() == 1) && (kaon2.mAntiLambda() == 1)) {
-        part1Vec.SetPtEtaPhiM(kaon1.pt(), kaon1.eta(), kaon1.phi(), mMassOne);
-        part2Vec.SetPtEtaPhiM(kaon2.pt(), kaon2.eta(), kaon2.phi(), mMassOne);
-        TLorentzVector sumVec(part1Vec);
-        sumVec += part2Vec;
-        registryPhiMinvBackground.fill(HIST("InvariantMassKpKp"), sumVec.M());
-      }
-      if ((kaon1.mAntiLambda() == -1) && (kaon2.mAntiLambda() == -1)) {
-        part1Vec.SetPtEtaPhiM(kaon1.pt(), kaon1.eta(), kaon1.phi(), mMassTwo);
-        part2Vec.SetPtEtaPhiM(kaon2.pt(), kaon2.eta(), kaon2.phi(), mMassTwo);
+      for (auto const& [kaon1, kaon2] : combinations(CombinationsStrictlyUpperIndexPolicy(partsKaons, partsKaons))) {
+        if ((kaon1.mAntiLambda() == 1) && (kaon2.mAntiLambda() == 1)) {
+          part1Vec.SetPtEtaPhiM(kaon1.pt(), kaon1.eta(), kaon1.phi(), mMassOne);
+          part2Vec.SetPtEtaPhiM(kaon2.pt(), kaon2.eta(), kaon2.phi(), mMassOne);
+          TLorentzVector sumVec(part1Vec);
+          sumVec += part2Vec;
+          registryPhiMinvBackground.fill(HIST("InvariantMassKpKp"), sumVec.M());
+        }
+        if ((kaon1.mAntiLambda() == -1) && (kaon2.mAntiLambda() == -1)) {
+          part1Vec.SetPtEtaPhiM(kaon1.pt(), kaon1.eta(), kaon1.phi(), mMassTwo);
+          part2Vec.SetPtEtaPhiM(kaon2.pt(), kaon2.eta(), kaon2.phi(), mMassTwo);
 
-        TLorentzVector sumVec(part1Vec);
-        sumVec += part2Vec;
-        registryPhiMinvBackground.fill(HIST("InvariantMassKmKm"), sumVec.M());
-      }
-      if ((kaon1.mAntiLambda() == 1) && (kaon2.mAntiLambda() == -1)) {
-        part1Vec.SetPtEtaPhiM(kaon1.pt(), kaon1.eta(), kaon1.phi(), mMassOne);
-        part2Vec.SetPtEtaPhiM(kaon2.pt(), kaon2.eta(), kaon2.phi(), mMassTwo);
+          TLorentzVector sumVec(part1Vec);
+          sumVec += part2Vec;
+          registryPhiMinvBackground.fill(HIST("InvariantMassKmKm"), sumVec.M());
+        }
+        if ((kaon1.mAntiLambda() == 1) && (kaon2.mAntiLambda() == -1)) {
+          part1Vec.SetPtEtaPhiM(kaon1.pt(), kaon1.eta(), kaon1.phi(), mMassOne);
+          part2Vec.SetPtEtaPhiM(kaon2.pt(), kaon2.eta(), kaon2.phi(), mMassTwo);
 
-        TLorentzVector sumVec(part1Vec);
-        sumVec += part2Vec;
-        registryPhiMinvBackground.fill(HIST("InvariantMassKpKm"), sumVec.M());
+          TLorentzVector sumVec(part1Vec);
+          sumVec += part2Vec;
+          registryPhiMinvBackground.fill(HIST("InvariantMassKpKm"), sumVec.M());
+        }
       }
     }
   }