[PWGDQ] improved momentum rescaling for global forward tracks

The MCH tracks are extrapolated to the first measured MFT point using
TrackExtrap::extrapToVertex() instead of TrackExtrap::extrapToVertexWithoutBranson(),
which cures large inaccuracies in the MCH momentum estimation at the MFT
front for some tracks. Those inaccuracies were contributing to high-mass
tails in the di-muon invariant mass distribution reconstructed from global
forward tracks with rescaled momentum.

Author: aferrero2707

PWGDQ/Tasks/muonGlobalAlignment.cxx

@@ -1301,8 +1301,8 @@ struct muonGlobalAlignment { // o2-linter: disable=name/workflow-file,name/struc
                                 collision.posX(),
                                 collision.posY(),
                                 collision.posZ(),
-                                collision.covXX(),
-                                collision.covYY());
+                                std::sqrt(collision.covXX()),
+                                std::sqrt(collision.covYY()));
   }
 
   template <class TMFT>
@@ -1449,24 +1449,20 @@ struct muonGlobalAlignment { // o2-linter: disable=name/workflow-file,name/struc
   {
     // extrapolation with MCH tools
     auto mchTrackAtMFT = FwdtoMCH(mchTrackPar);
-    o2::mch::TrackExtrap::extrapToVertexWithoutBranson(mchTrackAtMFT, mftTrackPar.getZ());
-
-    auto mftTrackProp = FwdtoMCH(mftTrackPar);
+    o2::mch::TrackExtrap::extrapToVertex(mchTrackAtMFT,
+                                         mftTrackPar.getX(),
+                                         mftTrackPar.getY(),
+                                         mftTrackPar.getZ(),
+                                         std::sqrt(mftTrackPar.getSigma2X()),
+                                         std::sqrt(mftTrackPar.getSigma2Y()));
 
-    // update global track momentum from the MCH track
-    double pRatio = mftTrackProp.p() / mchTrackAtMFT.p();
-    double newInvBendMom = mftTrackProp.getInverseBendingMomentum() * pRatio;
-    mftTrackProp.setInverseBendingMomentum(newInvBendMom);
-    mftTrackProp.setCharge(mchTrackAtMFT.getCharge());
+    auto fwdTrackProp = fwdtrackutils::refitGlobalMuonCov(MCHtoFwd(mchTrackAtMFT), mftTrackPar);
 
-    o2::mch::TrackExtrap::extrapToVertex(mftTrackProp,
-                                         collision.posX(),
-                                         collision.posY(),
-                                         collision.posZ(),
-                                         collision.covXX(),
-                                         collision.covYY());
+    auto geoMan = o2::base::GeometryManager::meanMaterialBudget(fwdTrackProp.getX(), fwdTrackProp.getY(), fwdTrackProp.getZ(), collision.posX(), collision.posY(), collision.posZ());
+    auto x2x0 = static_cast<float>(geoMan.meanX2X0);
+    fwdTrackProp.propagateToVtxhelixWithMCS(collision.posZ(), {collision.posX(), collision.posY()}, {collision.covXX(), collision.covYY()}, mBzAtMftCenter, x2x0);
 
-    return MCHtoFwd(mftTrackProp);
+    return fwdTrackProp;
   }
 
   void getMuonPairs(const CollisionInfo& collisionInfo,

PWGDQ/Tasks/qaMatching.cxx

@@ -17,6 +17,7 @@
 #include "PWGDQ/Core/VarManager.h"
 
 #include "Common/CCDB/RCTSelectionFlags.h"
+#include "Common/Core/fwdtrackUtilities.h"
 #include "Common/DataModel/Centrality.h"
 #include "Common/DataModel/CollisionAssociationTables.h"
 #include "Common/DataModel/EventSelection.h"
@@ -1640,8 +1641,8 @@ struct QaMatching {
                                 collision.posX(),
                                 collision.posY(),
                                 collision.posZ(),
-                                collision.covXX(),
-                                collision.covYY());
+                                std::sqrt(collision.covXX()),
+                                std::sqrt(collision.covYY()));
   }
 
   o2::dataformats::GlobalFwdTrack propagateToVertexMft(o2::dataformats::GlobalFwdTrack muon,
@@ -1667,8 +1668,8 @@ struct QaMatching {
                                 collision.posX(),
                                 collision.posY(),
                                 collision.posZ(),
-                                collision.covXX(),
-                                collision.covYY());
+                                std::sqrt(collision.covXX()),
+                                std::sqrt(collision.covYY()));
   }
 
   template <typename TMCH, typename TMFT, class C>
@@ -1678,24 +1679,19 @@ struct QaMatching {
   {
     // extrapolation with MCH tools
     auto mchTrackAtMFT = mExtrap.FwdtoMCH(fwdToTrackPar(mchTrack));
-    o2::mch::TrackExtrap::extrapToVertexWithoutBranson(mchTrackAtMFT, mftTrack.z());
+    o2::mch::TrackExtrap::extrapToVertex(mchTrackAtMFT,
+                                         mftTrack.x(),
+                                         mftTrack.y(),
+                                         mftTrack.z(),
+                                         0, 0);
 
-    auto mftTrackProp = mExtrap.FwdtoMCH(fwdToTrackPar(mftTrack));
+    auto fwdTrackProp = fwdtrackutils::refitGlobalMuonCov(mExtrap.MCHtoFwd(mchTrackAtMFT), fwdToTrackPar(mftTrack));
 
-    // update global track momentum from the MCH track
-    double pRatio = mftTrackProp.p() / mchTrackAtMFT.p();
-    double newInvBendMom = mftTrackProp.getInverseBendingMomentum() * pRatio;
-    mftTrackProp.setInverseBendingMomentum(newInvBendMom);
-    mftTrackProp.setCharge(mchTrackAtMFT.getCharge());
-
-    o2::mch::TrackExtrap::extrapToVertex(mftTrackProp,
-                                         collision.posX(),
-                                         collision.posY(),
-                                         collision.posZ(),
-                                         collision.covXX(),
-                                         collision.covYY());
+    auto geoMan = o2::base::GeometryManager::meanMaterialBudget(fwdTrackProp.getX(), fwdTrackProp.getY(), fwdTrackProp.getZ(), collision.posX(), collision.posY(), collision.posZ());
+    auto x2x0 = static_cast<float>(geoMan.meanX2X0);
+    fwdTrackProp.propagateToVtxhelixWithMCS(collision.posZ(), {collision.posX(), collision.posY()}, {collision.covXX(), collision.covYY()}, mBzAtMftCenter, x2x0);
 
-    return mExtrap.MCHtoFwd(mftTrackProp);
+    return fwdTrackProp;
   }
 
   template <class MCP>