[PWGDQ] Fix for event-selected MCsignal in processMCGen (#11801)

Co-authored-by: Maurice Coquet <mcoquet@lxplus994.cern.ch>

Author: mcoquet642

PWGDQ/Tasks/dqEfficiency_withAssoc.cxx

@@ -2159,11 +2159,16 @@ struct AnalysisSameEventPairing {
         continue;
       }
 
-      auto groupedMCTracks = mcTracks.sliceBy(perReducedMcGenEvent, event.reducedMCeventId());
-      groupedMCTracks.bindInternalIndicesTo(&mcTracks);
-      for (auto& track : groupedMCTracks) {
+      // auto groupedMCTracks = mcTracks.sliceBy(perReducedMcGenEvent, event.reducedMCeventId());
+      // groupedMCTracks.bindInternalIndicesTo(&mcTracks);
+      // for (auto& track : groupedMCTracks) {
+      for (auto& track : mcTracks) {
+        if (track.reducedMCeventId() != event.reducedMCeventId()) {
+          continue;
+        }
         VarManager::FillTrackMC(mcTracks, track);
-        auto track_raw = groupedMCTracks.rawIteratorAt(track.globalIndex());
+        auto track_raw = mcTracks.rawIteratorAt(track.globalIndex());
+        // auto track_raw = groupedMCTracks.rawIteratorAt(track.globalIndex());
         for (auto& sig : fGenMCSignals) {
           if (sig->CheckSignal(true, track_raw)) {
             fHistMan->FillHistClass(Form("MCTruthGenSel_%s", sig->GetName()), VarManager::fgValues);