[PWGEM,Photon] Add option to correct eta for vertex shift in event mi…

PWGEM/PhotonMeson/Tasks/taskPi0FlowEMC.cxx

@@ -247,6 +247,8 @@ struct TaskPi0FlowEMC {
     Configurable<bool> cfgApplySPresolution{"cfgApplySPresolution", false, "Apply resolution correction"};
     Configurable<bool> doEMCalCalib{"doEMCalCalib", false, "Produce output for EMCal calibration"};
     Configurable<bool> cfgEnableNonLin{"cfgEnableNonLin", false, "flag to turn extra non linear energy calibration on/off"};
+    Configurable<float> cfgEmcalEffRadius{"cfgEmcalEffRadius", 430.f, "effective EMCal radius (cm) used for mixed-event vertex-swap correction"};
+    Configurable<bool> cfgCorrectMixedVtxEta{"cfgCorrectMixedVtxEta", true, "re-project cluster2 eta onto collision1's vertex in mixed event"};
   } correctionConfig;
 
   SliceCache cache;
@@ -528,6 +530,18 @@ struct TaskPi0FlowEMC {
     registry.fill(HIST(HistTypes[histType]), mass, pt, cent);
   }
 
+  /// \brief eta a cluster would have if its own vertex vzOld is swapped for vzNew,
+  /// assuming a nominal cylindrical EMCal surface at transverse radius emcalR (cm).
+  /// phi is untouched: transverse vertex spread is negligible next to emcalR.
+  /// \param etaOld old eta value
+  /// \param vzOld old primary vertex z position
+  /// \param vzNew new primary vertex z position
+  /// \param emcalR radius of the EMCal
+  static float correctEtaForVertexShift(float etaOld, float vzOld, float vzNew, float emcalR)
+  {
+    return std::asinh(std::sinh(etaOld) + (vzOld - vzNew) / emcalR);
+  }
+
   /// Get the centrality
   /// \param collision is the collision with the centrality information
   template <o2::soa::is_iterator TCollision>
@@ -1202,7 +1216,13 @@ struct TaskPi0FlowEMC {
           }
         }
         ROOT::Math::PtEtaPhiMVector v1(g1.corrPt(), g1.eta(), g1.phi(), 0.);
-        ROOT::Math::PtEtaPhiMVector v2(g2.corrPt(), g2.eta(), g2.phi(), 0.);
+
+        // changing the eta position of cluster 2 from collision 2 to match the z-vertex position of collision 1
+        float eta2 = g2.eta();
+        if (correctionConfig.cfgCorrectMixedVtxEta.value) {
+          eta2 = correctEtaForVertexShift(g2.eta(), c2.posZ(), c1.posZ(), correctionConfig.cfgEmcalEffRadius.value);
+        }
+        ROOT::Math::PtEtaPhiMVector v2(g2.corrPt(), eta2, g2.phi(), 0.);
         ROOT::Math::PtEtaPhiMVector vMeson = v1 + v2;
 
         float dTheta = v1.Theta() - v2.Theta();