[PWGLF] Update KF cascade building in strangenessBuilderHelper

PWGLF/Utils/strangenessBuilderHelper.h

@@ -390,14 +390,14 @@
     negativeTrackParam.getPxPyPzGlo(v0.negativeMomentum);
     positiveTrackParam.getXYZGlo(v0.positivePosition);
     negativeTrackParam.getXYZGlo(v0.negativePosition);
     for (int i = 0; i < 3; i++) {
       // avoids misuse if mixed with KF particle use
       v0.momentum[i] = v0.positiveMomentum[i] + v0.negativeMomentum[i];
     }
 
     // get decay vertex coordinates
     const auto& vtx = fitter.getPCACandidate();
     for (int i = 0; i < 3; i++) {
       v0.position[i] = vtx[i];
     }
 
@@ -470,7 +470,7 @@
       positiveTrackParam.getCovXYZPxPyPzGlo(covTpositive);
       negativeTrackParam.getCovXYZPxPyPzGlo(covTnegative);
       constexpr int MomInd[6] = {9, 13, 14, 18, 19, 20}; // cov matrix elements for momentum component
       for (int i = 0; i < 6; i++) {
         v0.momentumCovariance[i] = covTpositive[MomInd[i]] + covTnegative[MomInd[i]];
       }
     }
@@ -568,7 +568,7 @@
       return false;
     }
 
     if (kfConstrainedMassValue > -1e-4) {
       // photon constraint: this one's got no mass
       KFV0.SetNonlinearMassConstraint(kfConstrainedMassValue);
     }
@@ -597,7 +597,7 @@
     }
 
     // check radius
     for (int i = 0; i < 3; i++) {
       v0.position[i] = xyz_decay[i];
     }
     if (std::hypot(v0.position[0], v0.position[1]) < v0selections.v0radius) {
@@ -773,7 +773,7 @@
     // Set up covariance matrices (should in fact be optional)
     std::array<float, 21> covV = {0.};
     constexpr int MomInd[6] = {9, 13, 14, 18, 19, 20}; // cov matrix elements for momentum component
     for (int i = 0; i < 6; i++) {
       covV[MomInd[i]] = v0input.momentumCovariance[i];
       covV[i] = v0input.positionCovariance[i];
     }
@@ -811,7 +811,7 @@
     lBachelorTrack.getPxPyPzGlo(cascade.bachelorMomentum);
     // get decay vertex coordinates
     const auto& vtx = fitter.getPCACandidate();
     for (int i = 0; i < 3; i++) {
       cascade.cascadePosition[i] = vtx[i];
     }
     if (std::hypot(cascade.cascadePosition[0], cascade.cascadePosition[1]) < cascadeselections.cascradius) {
@@ -901,10 +901,10 @@
       lV0Track.getCovXYZPxPyPzGlo(covTv0);
       lBachelorTrack.getCovXYZPxPyPzGlo(covTbachelor);
       constexpr int MomInd[6] = {9, 13, 14, 18, 19, 20}; // cov matrix elements for momentum component
       for (int i = 0; i < 21; i++) {
         cascade.covariance[i] = 0.0f;
       }
       for (int i = 0; i < 6; i++) {
         cascade.covariance[i] = positionCovariance[i];
         cascade.covariance[MomInd[i]] = covTv0[MomInd[i]] + covTbachelor[MomInd[i]];
       }
@@ -1059,6 +1059,11 @@
       return false;
     }
 
+    // Calculate V0 mass before mass constraint
+    float MLambda, SigmaLambda;
+    KFV0.GetMass(MLambda, SigmaLambda);
+    cascade.kfMLambda = MLambda;
+
     if (kfUseV0MassConstraint) {
       KFV0.SetNonlinearMassConstraint(o2::constants::physics::MassLambda);
     }
@@ -1117,9 +1122,20 @@
       cascade = {};
       return false;
     }
+
+    // Calculate masses a priori before potentially applying mass constraint
+    // --> this is the invariant mass of the decay products, not the mass-constrained value
+    float MXi, SigmaXi, MOmega, SigmaOmega;
+    KFXi.GetMass(MXi, SigmaXi);
+    KFOmega.GetMass(MOmega, SigmaOmega);
+    cascade.massXi = MXi;
+    cascade.massOmega = MOmega;
+
     if (kfUseCascadeMassConstraint) {
       // set mass constraint if requested
       // WARNING: this is only adequate for decay chains, i.e. XiC -> Xi or OmegaC -> Omega
+      // WARNING: be aware of the fact that if enabled, the stored particle momentum and covariance matrix will be mass-constrained
+      // while the stored mass will be the invariant mass of the decay products (i.e. not mass-constrained)
       KFXi.SetNonlinearMassConstraint(o2::constants::physics::MassXiMinus);
       KFOmega.SetNonlinearMassConstraint(o2::constants::physics::MassOmegaMinus);
     }
@@ -1215,18 +1231,9 @@
     }
     cascade.pointingAngle = TMath::ACos(cosPA);
 
-    // Calculate masses a priori
-    float MLambda, SigmaLambda, MXi, SigmaXi, MOmega, SigmaOmega;
-    KFV0.GetMass(MLambda, SigmaLambda);
-    KFXi.GetMass(MXi, SigmaXi);
-    KFOmega.GetMass(MOmega, SigmaOmega);
-    cascade.kfMLambda = MLambda;
-    cascade.massXi = MXi;
-    cascade.massOmega = MOmega;
-
     // KF Cascade covariance matrix
     std::array<float, 21> covCascKF;
     for (int i = 0; i < 21; i++) { // get covariance matrix elements (lower triangle)
       covCascKF[i] = KFXi.GetCovariance(i);
       cascade.covariance[i] = covCascKF[i];
     }