[PWGDQ] add decay length vars for MCtruth pairs to direct task (#15898)

Co-authored-by: ALICE Action Bot <alibuild@cern.ch>

Author: ffionda

PWGDQ/Tasks/dqEfficiency_withAssoc_direct.cxx

@@ -47,6 +47,7 @@
 #include <Framework/AnalysisTask.h>
 #include <Framework/Configurable.h>
 #include <Framework/InitContext.h>
+#include <Framework/O2DatabasePDGPlugin.h>
 #include <Framework/runDataProcessing.h>
 #include <ReconstructionDataFormats/Track.h>
 
@@ -1151,6 +1152,8 @@ struct AnalysisSameEventPairing {
   } fConfigCCDB;
 
   Service<o2::ccdb::BasicCCDBManager> fCCDB;
+  // PDG database
+  Service<o2::framework::O2DatabasePDG> pdgDB;
 
   HistogramManager* fHistMan;
 
@@ -2076,6 +2079,16 @@ struct AnalysisSameEventPairing {
                 mcDecision |= (static_cast<uint32_t>(1) << isig);
                 VarManager::FillPairMC<TPairType>(t1_raw, t2_raw);
                 // cout << "      Filled VarManager for the pair." << endl;
+                // check if t1_raw and t2_raw have same mother to compute decay length related variables
+                if (t1_raw.has_mothers() && t2_raw.has_mothers()) {
+                  auto motherMCParticle_t1 = t1_raw.template mothers_first_as<McParticles>();
+                  auto motherMCParticle_t2 = t2_raw.template mothers_first_as<McParticles>();
+                  if (motherMCParticle_t1 == motherMCParticle_t2) {
+                    auto mcEvent = mcEvents.rawIteratorAt(motherMCParticle_t1.mcCollisionId());
+                    std::array<double, 3> collVtxPos = {mcEvent.posX(), mcEvent.posY(), mcEvent.posZ()};
+                    VarManager::FillTrackCollisionMC<TPairType>(motherMCParticle_t1, collVtxPos, pdgDB->Mass(motherMCParticle_t1.pdgCode()));
+                  }
+                }
                 if (fUseMCGenAccCut) {
                   if (!fMCGenAccCut.IsSelected(VarManager::fgValues)) {
                     continue;