[PWGDQ] add code to emulate a shift of the dipole

The shift of the dipole position is emulated by appropriately shifting
the MCH clusters in the trac refit, and the tracks starting point when
extrapolating through the spectrometer.

Author: aferrero2707

PWGDQ/Tasks/muonGlobalAlignment.cxx

@@ -150,6 +150,7 @@ struct muonGlobalAlignment {
   Configurable<uint32_t> fMftTracksMultiplicityMax{"cfgMftTracksMultiplicityMax", 0, "Maximum number of MFT tracks to be processed per event (zero means no limit)"};
 
   Configurable<float> fVertexZshift{"cfgVertexZshift", 0.0f, "Correction to the vertex z position"};
+  Configurable<float> fDipoleZshift{"cfgDipoleZshift", 0.0f, "Correction to the dipole z position"};
 
   ////   Variables for MFT alignment corrections
   struct : ConfigurableGroup {
@@ -1200,6 +1201,15 @@ struct muonGlobalAlignment {
     if (mchTrack.getZ() < absBack && z > absFront) {
       // extrapolation through the absorber in the upstream direction
       o2::mch::TrackExtrap::extrapToVertexWithoutBranson(mchTrack, z);
+    } else if (z < absBack) {
+      // extrapolation downstream of the absorber, correct for dipole longitudinal shift if needed
+      if (fDipoleZshift.value != 0) {
+        mchTrack.setZ(mchTrack.getZ() + fDipoleZshift.value);
+        o2::mch::TrackExtrap::extrapToZCov(mchTrack, z + fDipoleZshift.value);
+        mchTrack.setZ(mchTrack.getZ() - fDipoleZshift.value);
+      } else {
+        o2::mch::TrackExtrap::extrapToZCov(mchTrack, z);
+      }
     } else {
       // all other cases
       o2::mch::TrackExtrap::extrapToZCov(mchTrack, z);
@@ -1389,7 +1399,24 @@ struct muonGlobalAlignment {
       o2::mch::TrackExtrap::extrapToZ(mftTrackProp, -466.f);
       UpdateTrackMomentum(mftTrackProp, mchTrackPar);
     }
-    o2::mch::TrackExtrap::extrapToZ(mftTrackProp, z);
+
+    if (fDipoleZshift.value != 0) {
+      // extrapolate to the back of the absorber, taking into account the dipole shift,
+      // to avoid that the correction bring the track starting point back into the absorber
+      if (fDipoleZshift.value < 0) {
+        o2::mch::TrackExtrap::extrapToZ(mftTrackProp, -505.f);
+      } else if (fDipoleZshift.value > 0) {
+        o2::mch::TrackExtrap::extrapToZ(mftTrackProp, -505.f - fDipoleZshift.value);
+      }
+      // shift the track starting point
+      mftTrackProp.setZ(mftTrackProp.getZ() + fDipoleZshift.value);
+      // extrapolate to the final z, corrected for the dipole shift
+      o2::mch::TrackExtrap::extrapToZ(mftTrackProp, z + fDipoleZshift.value);
+      // remove the shift from the extrapolated track
+      mftTrackProp.setZ(mftTrackProp.getZ() - fDipoleZshift.value);
+    } else {
+      o2::mch::TrackExtrap::extrapToZ(mftTrackProp, z);
+    }
 
     return MCHtoFwd(mftTrackProp);
   }
@@ -1595,6 +1622,11 @@ struct muonGlobalAlignment {
         transformNew[cluster.deId()].LocalToMaster(local, master);
       }
 
+      // shift the clusters to correct the longitudinal shift of the dipole
+      if (fDipoleZshift.value != 0) {
+        master.SetZ(master.z() + fDipoleZshift.value);
+      }
+
       if (applyCorrections) {
         auto correctionsIt = mMchAlignmentCorrections.find(cluster.deId());
         if (correctionsIt != mMchAlignmentCorrections.end()) {
@@ -1628,6 +1660,12 @@ struct muonGlobalAlignment {
       LOGF(fatal, "Muon track %d has no associated clusters.", mchTrack.globalIndex());
     }
 
+    // subtract the longitudinal shift of the dipole from the track z
+    if (fDipoleZshift.value != 0) {
+      auto& trackParam = *(convertedTrack.begin());
+      trackParam.setZ(trackParam.getZ() - fDipoleZshift.value);
+    }
+
     return !removable;
   }