@@ -30,6 +30,7 @@
 #include "DetectorsCalibration/IntegratedClusterCalibrator.h"
 #include "CommonUtils/TreeStreamRedirector.h"
 #include "MathUtils/Tsallis.h"
+#include "MathUtils/Utils.h"
 #include "ReconstructionDataFormats/TrackTPCITS.h"
 #include "ReconstructionDataFormats/PrimaryVertex.h"
 #include "ReconstructionDataFormats/VtxTrackIndex.h"
@@ -71,7 +72,7 @@ class TPCTimeSeries : public Task
     uint8_t trdPattern = 0;
     uint8_t nTRDTracklets = 0;
     int trackletIndices[6] = {-1, -1, -1, -1, -1, -1};
-    o2::track::TrackParCov trdTrackParCov; // TRD track state for propagation to the common local X = 275 cm plane; filled only with --store-full-trackpar
+    o2::track::TrackParCov trdTrackParCov; // TRD track state for propagation to the common local X = 275 cm plane
     bool hasTRDTrackPar = false;
   };
 
@@ -398,10 +399,8 @@ class TPCTimeSeries : public Task
             trdData.trackletIndices[iLay] = trkltId;
           }
         }
-        if (mStoreFullTrackPar) {
-          trdData.trdTrackParCov = o2::track::TrackParCov(trdTrack); // TrackTRD base state, not validated as the outer/entrance state (open Phase 0.4 finding)
-          trdData.hasTRDTrackPar = true;
-        }
+        trdData.trdTrackParCov = o2::track::TrackParCov(trdTrack); // TrackTRD base state, not validated as the outer/entrance state (open Phase 0.4 finding)
+        trdData.hasTRDTrackPar = true;
         tpcToTRDMap[refTPC] = trdData;
       }
     }
@@ -1217,7 +1216,7 @@ class TPCTimeSeries : public Task
   float mMaxdEdxRegionRatio{0.5};                                          ///< maximum abs dedx region ratio: log(dedx_region/dedx)
   float mSamplingFactor{0.1};                                              ///< sampling factor in case sampling is used for unbinned data
   bool mSampleTsallis{false};                                              ///< perform sampling of unbinned data
-  bool mStoreFullTrackPar{false};                                          ///< experimental: compute and store tpcOut/trdIn/tpcAtVtx TrackPar and TPC-TRD chi2 (constant per job)
+  bool mStoreFullTrackPar{false};                                          ///< experimental: store tpcOut/trdIn/tpcAtVtx as TrackPar (constant per job)
   uint8_t mTRDInReadout{255};                                              ///< TRD in detector readout (GRPECS): 1 yes, 0 no, 255 unknown
   std::vector<std::mt19937> mGenerator;                                    ///< random generator for debug tree sampling
   std::vector<std::unique_ptr<o2::utils::TreeStreamRedirector>> mStreamer; ///< streamer for unbinned data
@@ -1485,46 +1484,55 @@ class TPCTimeSeries : public Task
           }
         }
 
-        // Optional TPC-out / TRD-in states on a common local X plane (experimental, --store-full-trackpar, default off).
+        // TPC-out / TRD-in states on a common local X plane, computed for every written entry (Architect decision 2026-10-28:
+        // residuals always stored; only the full TrackPar objects depend on --store-full-trackpar).
         // Open Phase 0.4 findings, deferred: the target is local X = kTPCTRDMiddle in the frame rotated to the track
         // position angle (lab radius = sqrt(X^2 + Y^2), not exactly 275 cm), and the TRD starting state is the TrackTRD
-        // base state, not validated as the outer/entrance state. With the switch off nothing below is computed.
-        static constexpr float kTPCTRDMiddle = 275.f; // cm, common local X plane between TPC outer wall and TRD layer 0
-        o2::track::TrackParCov tpcOutCov;             // local temporary with covariance (needed for chi2); persisted as TrackPar
+        // base state (TrackTRD carries a single parameter set), not validated as the outer/entrance state.
+        static constexpr float kTPCTRDMiddle = 275.f;            // cm, common local X plane between TPC outer wall and TRD layer 0
+        static constexpr uint32_t kDeltaTRDTPCMask = 0xFFFFE000; // keep 10 mantissa bits: float16-like relative precision ~1e-3
+        o2::track::TrackParCov tpcOutCov;                        // local temporary with covariance (needed for chi2); persisted as TrackPar
         o2::track::TrackParCov trdInCov;
-        uint8_t tpcOutOK = 255;     // 0 propagated, 2 propagation failed, 255 switch off (not computed)
-        uint8_t trdInOK = 255;      // 0 propagated, 1 not attempted (no TRD match or tpcOutOK != 0), 2 propagation failed, 255 switch off; count TRD matches with trdPattern > 0
-        float chi2PhiTPCTRD = -1.f; // -1: not computed (switch off, trdInOK != 0) or singular covariance
+        uint8_t tpcOutOK = 2;       // 0 propagated, 2 propagation failed
+        uint8_t trdInOK = 1;        // 0 propagated, 1 not attempted (no TRD match or tpcOutOK != 0), 2 propagation failed; count TRD matches with trdPattern > 0
+        float chi2PhiTPCTRD = -1.f; // -1: not computed (trdInOK != 0) or singular covariance
         float chi2ZTPCTRD = -1.f;
-        if (mStoreFullTrackPar) {
-          // D1: TPC outer → kTPCTRDMiddle
-          tpcOutCov = o2::track::TrackParCov(tracksTPC[iTrk].getParamOut());
-          tpcOutOK = 2; // default: propagation failed
-          {
-            // rotate to sector angle at current position
-            float sn = std::sin(tpcOutCov.getAlpha());
-            float cs = std::cos(tpcOutCov.getAlpha());
-            float gy = tpcOutCov.getX() * sn + tpcOutCov.getY() * cs;
-            float gx = tpcOutCov.getX() * cs - tpcOutCov.getY() * sn;
-            if (tpcOutCov.rotate(std::atan2(gy, gx)) &&
-                propagator->propagateTo(tpcOutCov, kTPCTRDMiddle, false, mMaxSnp, mCoarseStep, mMatType)) {
-              tpcOutOK = 0;
-            }
+        // TRD - TPC residuals at the common plane (same alpha, same X), parameters 0..4 = Y, Z, Snp, Tgl, Q2Pt; -999 if trdInOK != 0
+        float deltaP0TRDTPC = -999.f;
+        float deltaP1TRDTPC = -999.f;
+        float deltaP2TRDTPC = -999.f;
+        float deltaP3TRDTPC = -999.f;
+        float deltaP4TRDTPC = -999.f;
+        // D1: TPC outer → kTPCTRDMiddle
+        tpcOutCov = o2::track::TrackParCov(tracksTPC[iTrk].getParamOut());
+        {
+          // rotate to sector angle at current position
+          float sn = std::sin(tpcOutCov.getAlpha());
+          float cs = std::cos(tpcOutCov.getAlpha());
+          float gy = tpcOutCov.getX() * sn + tpcOutCov.getY() * cs;
+          float gx = tpcOutCov.getX() * cs - tpcOutCov.getY() * sn;
+          if (tpcOutCov.rotate(std::atan2(gy, gx)) &&
+              propagator->propagateTo(tpcOutCov, kTPCTRDMiddle, false, mMaxSnp, mCoarseStep, mMatType)) {
+            tpcOutOK = 0;
           }
+        }
 
-          // D2: TRD matched → kTPCTRDMiddle, rotated to SAME alpha as TPC out
-          trdInOK = 1; // default: not attempted
-          if (itTRD != tpcToTRDMap.end() && itTRD->second.hasTRDTrackPar && tpcOutOK == 0) {
-            trdInCov = o2::track::TrackParCov(itTRD->second.trdTrackParCov);
-            // rotate to TPC alpha FIRST, then propagate — guarantees same frame + same X
-            if (trdInCov.rotate(tpcOutCov.getAlpha()) &&
-                propagator->propagateTo(trdInCov, kTPCTRDMiddle, false, mMaxSnp, mCoarseStep, mMatType)) {
-              trdInOK = 0;
-              chi2PhiTPCTRD = chi2Match_Phi(tpcOutCov, trdInCov);
-              chi2ZTPCTRD = chi2Match_Z(tpcOutCov, trdInCov);
-            } else {
-              trdInOK = 2; // propagation failed
-            }
+        // D2: TRD matched → kTPCTRDMiddle, rotated to SAME alpha as TPC out
+        if (itTRD != tpcToTRDMap.end() && itTRD->second.hasTRDTrackPar && tpcOutOK == 0) {
+          trdInCov = o2::track::TrackParCov(itTRD->second.trdTrackParCov);
+          // rotate to TPC alpha FIRST, then propagate — guarantees same frame + same X
+          if (trdInCov.rotate(tpcOutCov.getAlpha()) &&
+              propagator->propagateTo(trdInCov, kTPCTRDMiddle, false, mMaxSnp, mCoarseStep, mMatType)) {
+            trdInOK = 0;
+            chi2PhiTPCTRD = chi2Match_Phi(tpcOutCov, trdInCov);
+            chi2ZTPCTRD = chi2Match_Z(tpcOutCov, trdInCov);
+            deltaP0TRDTPC = o2::math_utils::truncateFloatFraction(trdInCov.getParam(0) - tpcOutCov.getParam(0), kDeltaTRDTPCMask);
+            deltaP1TRDTPC = o2::math_utils::truncateFloatFraction(trdInCov.getParam(1) - tpcOutCov.getParam(1), kDeltaTRDTPCMask);
+            deltaP2TRDTPC = o2::math_utils::truncateFloatFraction(trdInCov.getParam(2) - tpcOutCov.getParam(2), kDeltaTRDTPCMask);
+            deltaP3TRDTPC = o2::math_utils::truncateFloatFraction(trdInCov.getParam(3) - tpcOutCov.getParam(3), kDeltaTRDTPCMask);
+            deltaP4TRDTPC = o2::math_utils::truncateFloatFraction(trdInCov.getParam(4) - tpcOutCov.getParam(4), kDeltaTRDTPCMask);
+          } else {
+            trdInOK = 2; // propagation failed
           }
         }
 
@@ -1693,12 +1701,17 @@ class TPCTimeSeries : public Task
                             << "nTRDTracklets=" << nTRDTracklets
                             << "trdTracklets=" << trdTrackletVec
                             << "trdCalibTracklets=" << trdCalibVec
-                            // TPC out / TRD in on the common local X = 275 cm plane: status and matching chi2
-                            // (255 / -1 when --store-full-trackpar is off; the TrackPar objects are written above)
+                            // TPC out / TRD in on the common local X = 275 cm plane: status, matching chi2 and TRD - TPC residuals
+                            // (the full TrackPar objects are written above, only with --store-full-trackpar)
                             << "tpcOutOK=" << tpcOutOK
                             << "trdInOK=" << trdInOK
                             << "chi2PhiTPCTRD=" << chi2PhiTPCTRD
                             << "chi2ZTPCTRD=" << chi2ZTPCTRD
+                            << "deltaP0TRDTPC=" << deltaP0TRDTPC
+                            << "deltaP1TRDTPC=" << deltaP1TRDTPC
+                            << "deltaP2TRDTPC=" << deltaP2TRDTPC
+                            << "deltaP3TRDTPC=" << deltaP3TRDTPC
+                            << "deltaP4TRDTPC=" << deltaP4TRDTPC
                             // TRD in detector readout for this run (GRPECS): 1 yes, 0 no, 255 unknown
                             << "trdInReadout=" << mTRDInReadout
                             << "chi2match_ITSTPC=" << chi2match_ITSTPC
@@ -2149,7 +2162,7 @@ o2::framework::DataProcessorSpec getTPCTimeSeriesSpec(const bool disableWriter,
       {"sample-unbinned-tsallis", VariantType::Bool, false, {"Perform sampling of unbinned data based on Tsallis function"}},
       {"sampling-factor", VariantType::Float, 0.001f, {"Sampling factor in case sample-unbinned-tsallis is used"}},
       {"disable-min-bias-trigger", VariantType::Bool, false, {"Disable the minimum bias trigger for skimmed data"}},
-      {"store-full-trackpar", VariantType::Bool, false, {"Experimental: compute TPC-out/TRD-in states on the common local X = 275 cm plane and the TPC-TRD matching chi2, and store tpcOut, trdIn, tpcAtVtx as TrackPar in the unbinned tree"}},
+      {"store-full-trackpar", VariantType::Bool, false, {"Experimental: store tpcOut, trdIn (common local X = 275 cm plane) and tpcAtVtx as TrackPar in the unbinned tree"}},
       {"out-file-unbinned", VariantType::String, "time_series_tracks.root", {"name of the output file for the unbinned data"}},
       {"max-occupancy-bins", VariantType::Int, 912, {"Maximum number of occupancy bins"}}}};
 }