TPC VDrift: hold refVdrift constant

Author: matthias-kleiner

DataFormats/Detectors/TPC/include/DataFormatsTPC/VDriftCorrFact.h

@@ -42,17 +42,12 @@ struct VDriftCorrFact {
   float getTimeOffset() const { return refTimeOffset + timeOffsetCorr; }
 
   // renormalize VDrift reference and correction either to provided new reference (if >0) or to correction 1 wrt current reference
-  void normalize(float newVRef = 0.f, float tp = 0.f)
+  void normalize(float newVRef = 0.f)
   {
     float normVDrift = newVRef;
     if (newVRef == 0.f) {
       normVDrift = refVDrift * corrFact;
       newVRef = normVDrift;
-      if ((tp > 0) && (refTP > 0)) {
-        // linear scaling based on relative change of T/P
-        normVDrift *= refTP / tp;
-        refTP = tp; // update reference T/P
-      }
     }
     float fact = refVDrift / normVDrift;
     refVDrift = newVRef;
@@ -74,6 +69,18 @@ struct VDriftCorrFact {
     }
   }
 
+  // scale the drift velocity with the relative change of T/P wrt the reference T/P
+  // The scaling is folded into the correction factor, keeping refVDrift constant
+  void normalizeTP(float tp)
+  {
+    if ((tp > 0) && (refTP > 0)) {
+      const float scale = tp / refTP;
+      corrFact *= scale;
+      corrFactErr *= scale;
+      refTP = tp;
+    }
+  }
+
   ClassDefNV(VDriftCorrFact, 3);
 };
 

Detectors/TPC/calibration/src/VDriftHelper.cxx

@@ -165,7 +165,7 @@ void VDriftHelper::extractCCDBInputs(ProcessingContext& pc, bool laser, bool its
       }
       if (mIsTPScalingPossible) {
         mUpdated = true;
-        vd.normalize(0, tp);
+        vd.normalizeTP(tp); // keep refVDrift constant, fold the T/P scaling into the correction factor
         if (vd.creationTime == saveVD.creationTime) {
           LOGP(info, "VDriftHelper: Scaling VDrift from {} to {} with T/P from {} to {}", saveVD.getVDrift(), vd.getVDrift(), saveVD.refTP, vd.refTP);
         } else {