[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>
@@ -1449,7 +1449,12 @@ 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());
+    o2::mch::TrackExtrap::extrapToVertex(mchTrackAtMFT,
+                                         mftTrackPar.getX(),
+                                         mftTrackPar.getY(),
+                                         mftTrackPar.getZ(),
+                                         std::sqrt(mftTrackPar.getSigma2X()),
+                                         std::sqrt(mftTrackPar.getSigma2Y()));
 
     auto mftTrackProp = FwdtoMCH(mftTrackPar);
 
@@ -1463,8 +1468,8 @@ 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()));
 
     return MCHtoFwd(mftTrackProp);
   }

PWGDQ/Tasks/qaMatching.cxx

@@ -1640,8 +1640,8 @@ 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 +1667,8 @@ 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>
@@ -1678,7 +1678,11 @@ 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));
 
@@ -1692,8 +1696,8 @@ struct QaMatching {
                                          collision.posX(),
                                          collision.posY(),
                                          collision.posZ(),
-                                         collision.covXX(),
-                                         collision.covYY());
+                                         std::sqrt(collision.covXX()),
+                                         std::sqrt(collision.covYY()));
 
     return mExtrap.MCHtoFwd(mftTrackProp);
   }