MatchCosmics: compare same-side TPC-only legs refitted at a common time

A TPC-only track has no time measurement: its time0 is the tracker's guess
(GPUTPCGMTrackParam::ShiftZ: z = 0 at the beam line if the helix can come
from it, otherwise the latest cluster at z = 0.521 * x, then clamped to the
drift volume). For a cosmic, which does not come from the beam line, the
two legs get independent guesses, typically tens of mus apart, so their
clusters were distortion-corrected at different z and the legs disagree
(y, snp, q/pt) although they are one track. For legs on opposite sides the
time comes from z continuity, but legs on the same side were compared as
tracked.

New MatchCosmicsParams refitSameSideAtCommonTime (default false, not in
the preset physics-v1, see below): after the cheap cuts (tgl, q/pt, same half), same-side
TPC-only legs are refitted at the centre of their brackets' overlap (the
time the refit of the winners uses) and compared there
(MatchCosmics::refitSeedAtTime: TPC refit from the outer parameters, then the
propagation to the DCA as for the seeds; energy loss along the muon's flight
as in refitWinners). The TPC refitter is created once per TF in process() and
shared with refitWinners. The matcher debug tree writes the legs compared
and a commonTime flag.

Cosmic MC with PbPb-like distortions (190 TFs, preset physics-v1, off -> on):
correctly matched muons 6848 -> 7324 of 8639 (79.3 -> 84.8 %); same side
77.0 -> 84.2 %, its loss to the crude chi2 cut 6.9 -> 0.8 %; A×C unchanged;
0 wrong-muon pairs in both; matcher +12 ms per TF (~100 refits). With the
true MC time instead of the window centre the same-side pass rate of the
chi2 cut would be 99.2 % (centre 98.0 %, legs at their own time0s 84.7 %).
PbPb 2025 (4 TFs): ~530 refits per TF, +15 ms per TF.

Not in the preset: in PbPb (568041, 310 CTFs, 16541 TFs) it adds mostly
pairs of collision tracks, whose time0 was about right and which the
refit at the window centre mis-corrects: +352 same-side cosmics but only
+5 TOF-tagged (2.6 % of the new ones vs 15.3 % of the common ones), and
~35 A×C cosmics lose a leg to a refitted same-side pair.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: matthias-kleiner

Detectors/GlobalTracking/include/GlobalTracking/MatchCosmics.h

@@ -41,7 +41,8 @@ class VDriftCorrFact;
 namespace gpu
 {
 class TPCFastTransformPOD;
-}
+class GPUO2InterfaceRefit;
+} // namespace gpu
 namespace globaltracking
 {
 
@@ -137,6 +138,7 @@ class MatchCosmics
  private:
   void updateTimeDependentParams();
   RejFlag checkPair(int i, int j);
+  bool refitSeedAtTime(const TrackSeed& seed, float timeMUS, TrackSeed& out);
   void registerMatch(int i, int j, float chi2, float tCommon = 0.f, float tCommonErr = -1.f);
   void suppressMatch(int partner0, int partner1);
   void createSeeds(const o2::globaltracking::RecoContainer& data);
@@ -164,6 +166,9 @@ class MatchCosmics
   float mQ2PtCutoff = 1e9;
   float mQ2PtCutoffOppositeSides = 1e9;
   const MatchCosmicsParams* mMatchParams = nullptr;
+  const o2::globaltracking::RecoContainer* mRecoData = nullptr; ///< inputs of the TF being processed
+  o2::gpu::GPUO2InterfaceRefit* mTPCRefitter = nullptr;         ///< TPC refitter of the TF being processed (owned by process())
+  size_t mNRefitsCommonTime = 0;                                ///< seeds refitted at the common time of a same-side pair in this TF
 
   std::vector<o2d::TrackCosmics> mCosmicTracks;
   std::vector<o2::MCCompLabel> mCosmicTracksLbl;

Detectors/GlobalTracking/include/GlobalTracking/MatchCosmicsParams.h

@@ -38,9 +38,10 @@ struct MatchCosmicsParams : public o2::conf::ConfigurableParamHelper<MatchCosmic
   float minSeedPt = 0.10f;     // use only tracks above this pT (scaled with field)
   int minSeedNClTPC = 0;       // use only TPC-only seeds with at least this number of clusters (0: no cut)
   float minSeedDCAxy = 0.f;    // use only tracks with |DCA_xy| to the beam line >= this [cm] (0: no cut; rejects collision tracks in physics data)
-  float minSeedDCAxyNSigma = 0.f; // use only tracks with |DCA_xy| >= this * sigma(DCA_xy) (0: no cut; poorly measured collision tracks)
-  bool constrainTPCOnlyZ = false; // TPC-only legs: test z at a common time (same side, or a leg with known time), else require the time implied by z continuity in both brackets
-  bool vetoSameHalf = false;      // reject pairs whose two legs lie on the same side of the closest approach (two pieces of one leg)
+  float minSeedDCAxyNSigma = 0.f;         // use only tracks with |DCA_xy| >= this * sigma(DCA_xy) (0: no cut; poorly measured collision tracks)
+  bool constrainTPCOnlyZ = false;         // TPC-only legs: test z at a common time (same side, or a leg with known time), else require the time implied by z continuity in both brackets
+  bool vetoSameHalf = false;              // reject pairs whose two legs lie on the same side of the closest approach (two pieces of one leg)
+  bool refitSameSideAtCommonTime = false; // TPC-only legs on the same side: compare them refitted at the centre of their brackets' overlap instead of at their own time0s
   float nSigmaTError = 4.f;    // number of sigmas on track time error for matching (except for TPC which provides an interval)
   float tpcExtraZError2 = 1.f; // extra error^2 on the TPC-only track Z coordinate
   float fiducialRIP = 1.0f;    // consider track having |Y@x=0|< this as passing DCA cut (if requested)

Detectors/GlobalTracking/src/MatchCosmics.cxx

@@ -111,6 +111,17 @@ void MatchCosmics::process(const o2::globaltracking::RecoContainer& data)
       }
     }
   }
+  // TPC refitter of this TF: same-side TPC-only legs compared at a common time (checkPair) and the refit of the winners
+  std::unique_ptr<o2::gpu::GPUO2InterfaceRefit> tpcRefitter;
+  if (data.inputsTPCclusters) {
+    tpcRefitter = std::make_unique<o2::gpu::GPUO2InterfaceRefit>(&data.inputsTPCclusters->clusterIndex, mTPCCorrMaps, mBz, data.getTPCTracksClusterRefs().data(), 0,
+                                                                 data.clusterShMapTPC.data(), data.occupancyMapTPC.data(), data.occupancyMapTPC.size(), nullptr,
+                                                                 o2::base::Propagator::Instance());
+  }
+  mTPCRefitter = tpcRefitter.get();
+  mRecoData = &data;
+  mNRefitsCommonTime = 0;
+
   // sort in time bracket lower edge, putting rejected tracks in the end
   std::vector<int> sortID(ntr);
   std::iota(sortID.begin(), sortID.end(), 0);
@@ -134,6 +145,11 @@ void MatchCosmics::process(const o2::globaltracking::RecoContainer& data)
 
   selectWinners();
   refitWinners(data);
+  if (mNRefitsCommonTime) {
+    LOGP(info, "{} seeds refitted at the common time of same-side pairs", mNRefitsCommonTime);
+  }
+  mTPCRefitter = nullptr;
+  mRecoData = nullptr;
 
   mTFCount++;
 }
@@ -144,16 +160,7 @@ void MatchCosmics::refitWinners(const o2::globaltracking::RecoContainer& data)
   LOG(info) << "Refitting " << mWinners.size() << " winner matches";
   int count = 0;
   auto tpcTBinMUSInv = 1. / mTPCTBinMUS;
-  const auto& tpcClusRefs = data.getTPCTracksClusterRefs();
-  const auto& tpcClusShMap = data.clusterShMapTPC;
-  const auto& tpcClusOccMap = data.occupancyMapTPC;
-  std::unique_ptr<o2::gpu::GPUO2InterfaceRefit> tpcRefitter;
-  if (data.inputsTPCclusters) {
-    tpcRefitter = std::make_unique<o2::gpu::GPUO2InterfaceRefit>(&data.inputsTPCclusters->clusterIndex,
-                                                                 mTPCCorrMaps, mBz,
-                                                                 tpcClusRefs.data(), 0, tpcClusShMap.data(),
-                                                                 tpcClusOccMap.data(), tpcClusOccMap.size(), nullptr, o2::base::Propagator::Instance());
-  }
+  auto* tpcRefitter = mTPCRefitter; // created in process()
 
   const auto& itsClusters = prepareITSClusters(data);
   // RS FIXME: this is probably a temporary solution, since ITS tracking over boundaries will likely change the TrackITS format
@@ -461,6 +468,9 @@ MatchCosmics::RejFlag MatchCosmics::checkPair(int i, int j)
   float chi2 = 1.e9f;
   float tCommon = 0.f;     // time fixed by z continuity of TPC-only legs on opposite sides, used by the refit
   float tCommonErr = -1.f; // its error (< 0: not fixed)
+  TrackSeed seed0Common;   // same-side TPC-only legs refitted at a common time (refitSameSideAtCommonTime)
+  TrackSeed seed1Common;
+  bool commonTime = false;
 
   // check
   // 1) crude check on tgl and q/pt (if B!=0). Note: back-to-back tracks will have mutually params (see TrackPar::invertParam)
@@ -504,49 +514,58 @@ MatchCosmics::RejFlag MatchCosmics::checkPair(int i, int j)
         break;
       }
     }
-    o2::track::TrackParCov seed1Inv = seed1;
+    // TPC-only legs on the same side: the tracker transformed each leg's clusters with its own time0, a guess (the two legs of a cosmic
+    // get guesses tens of mus apart), so the legs were distortion-corrected at different z and disagree although they are one track;
+    // compare them refitted at one common time, the centre of their brackets' overlap (the time the refit of the winners uses)
+    if (mMatchParams->refitSameSideAtCommonTime && seed0.tpcSide != 0 && seed0.tpcSide == seed1.tpcSide) { // tpcSide != 0: TPC-only one-side legs
+      const float tPair = seed0.tBracket.getOverlap(seed1.tBracket).mean();
+      commonTime = refitSeedAtTime(seed0, tPair, seed0Common) && refitSeedAtTime(seed1, tPair, seed1Common);
+    }
+    const TrackSeed& leg0 = commonTime ? seed0Common : seed0;
+    const TrackSeed& leg1 = commonTime ? seed1Common : seed1;
+    o2::track::TrackParCov seed1Inv = leg1;
     seed1Inv.invert();
     for (int i = 0; i < o2::track::kNParams; i++) { // add systematic error
       seed1Inv.updateCov(mMatchParams->systSigma2[i], o2::track::DiagMap[i]);
     }
 
-    if (!seed1Inv.rotate(seed0.getAlpha()) ||
-        !o2::base::Propagator::Instance()->PropagateToXBxByBz(seed1Inv, seed0.getX(), mMatchParams->maxSnp, mMatchParams->maxStep, mMatchParams->matCorr)) {
+    if (!seed1Inv.rotate(leg0.getAlpha()) ||
+        !o2::base::Propagator::Instance()->PropagateToXBxByBz(seed1Inv, leg0.getX(), mMatchParams->maxSnp, mMatchParams->maxStep, mMatchParams->matCorr)) {
       rej = RejProp;
       break;
     }
-    auto dSnp = seed0.getSnp() - seed1Inv.getSnp();
-    if (dSnp * dSnp > (seed0.getSigmaSnp2() + seed1Inv.getSigmaSnp2()) * mMatchParams->crudeNSigma2Cut[o2::track::kSnp]) {
+    auto dSnp = leg0.getSnp() - seed1Inv.getSnp();
+    if (dSnp * dSnp > (leg0.getSigmaSnp2() + seed1Inv.getSigmaSnp2()) * mMatchParams->crudeNSigma2Cut[o2::track::kSnp]) {
       rej = RejSnp;
       break;
     }
-    auto dY = seed0.getY() - seed1Inv.getY();
-    if (dY * dY > (seed0.getSigmaY2() + seed1Inv.getSigmaY2()) * mMatchParams->crudeNSigma2Cut[o2::track::kY]) {
+    auto dY = leg0.getY() - seed1Inv.getY();
+    if (dY * dY > (leg0.getSigmaY2() + seed1Inv.getSigmaY2()) * mMatchParams->crudeNSigma2Cut[o2::track::kY]) {
       rej = RejY;
       break;
     }
-    bool ignoreZ = seed0.origID.getSource() == o2d::GlobalTrackID::TPC || seed1.origID.getSource() == o2d::GlobalTrackID::TPC;
+    bool ignoreZ = leg0.origID.getSource() == o2d::GlobalTrackID::TPC || leg1.origID.getSource() == o2d::GlobalTrackID::TPC;
     if (ignoreZ && mMatchParams->constrainTPCOnlyZ) {
       // a TPC-only track with clusters on one side has z relative to its time0: z(t) = z + side * vD * (t - tRef); a CE-crossing or
       // non-TPC-only track has an absolute z (side 0). Bring both legs to a common time where possible and test z; for legs on opposite
       // sides, z continuity fixes the common time, which must lie in both time brackets.
-      const int side0 = seed0.tpcSide;
-      const int side1 = seed1.tpcSide;
-      const float sigZ2 = (seed0.getSigmaZ2() + seed1Inv.getSigmaZ2()) * mMatchParams->crudeNSigma2Cut[o2::track::kZ];
+      const int side0 = leg0.tpcSide;
+      const int side1 = leg1.tpcSide;
+      const float sigZ2 = (leg0.getSigmaZ2() + seed1Inv.getSigmaZ2()) * mMatchParams->crudeNSigma2Cut[o2::track::kZ];
       if (side0 == 0 || side1 == 0 || side0 == side1) {
-        float dZ = seed0.getZ() - seed1Inv.getZ();
+        float dZ = leg0.getZ() - seed1Inv.getZ();
         float dZTimeTol = 0.f;          // the time of a non-TPC absolute leg is only known within its bracket (tRef is the bracket centre)
         if (side0 != 0 && side1 != 0) { // same side: the z offset is fixed by the difference of the reference times
-          dZ -= side0 * mTPCVDrift * (seed0.tRef - seed1.tRef);
-        } else if (side1 != 0) { // seed0 absolute: move seed1 to the time of seed0
-          dZ -= side1 * mTPCVDrift * (seed0.tRef - seed1.tRef);
-          if (seed0.origID.getSource() != o2d::GlobalTrackID::TPC) {
-            dZTimeTol = 0.5f * mTPCVDrift * seed0.tBracket.delta();
+          dZ -= side0 * mTPCVDrift * (leg0.tRef - leg1.tRef);
+        } else if (side1 != 0) { // leg0 absolute: move leg1 to the time of leg0
+          dZ -= side1 * mTPCVDrift * (leg0.tRef - leg1.tRef);
+          if (leg0.origID.getSource() != o2d::GlobalTrackID::TPC) {
+            dZTimeTol = 0.5f * mTPCVDrift * leg0.tBracket.delta();
           }
-        } else if (side0 != 0) { // seed1 absolute: move seed0 to the time of seed1
-          dZ += side0 * mTPCVDrift * (seed1.tRef - seed0.tRef);
-          if (seed1.origID.getSource() != o2d::GlobalTrackID::TPC) {
-            dZTimeTol = 0.5f * mTPCVDrift * seed1.tBracket.delta();
+        } else if (side0 != 0) { // leg1 absolute: move leg0 to the time of leg1
+          dZ += side0 * mTPCVDrift * (leg1.tRef - leg0.tRef);
+          if (leg1.origID.getSource() != o2d::GlobalTrackID::TPC) {
+            dZTimeTol = 0.5f * mTPCVDrift * leg1.tBracket.delta();
           }
         }
         const float dZTol = std::sqrt(sigZ2) + dZTimeTol;
@@ -555,20 +574,20 @@ MatchCosmics::RejFlag MatchCosmics::checkPair(int i, int j)
           break;
         }
       } else { // opposite sides
-        const float t = 0.5f * (side0 * (seed1Inv.getZ() - seed0.getZ()) / mTPCVDrift + seed0.tRef + seed1.tRef);
+        const float t = 0.5f * (side0 * (seed1Inv.getZ() - leg0.getZ()) / mTPCVDrift + leg0.tRef + leg1.tRef);
         const float tTol = std::sqrt(sigZ2) / (2.f * mTPCVDrift);
-        if (t < std::max(seed0.tBracket.getMin(), seed1.tBracket.getMin()) - tTol || t > std::min(seed0.tBracket.getMax(), seed1.tBracket.getMax()) + tTol) {
+        if (t < std::max(leg0.tBracket.getMin(), leg1.tBracket.getMin()) - tTol || t > std::min(leg0.tBracket.getMax(), leg1.tBracket.getMax()) + tTol) {
           rej = RejZ;
           break;
         }
         tCommon = t;
-        tCommonErr = std::sqrt(seed0.getSigmaZ2() + seed1Inv.getSigmaZ2()) / (2.f * mTPCVDrift);
+        tCommonErr = std::sqrt(leg0.getSigmaZ2() + seed1Inv.getSigmaZ2()) / (2.f * mTPCVDrift);
       }
     }
     // the z of a TPC-only leg refers to its own time0: it is tested above at a common time with constrainTPCOnlyZ, otherwise ignored
     if (!ignoreZ) { // both legs have an absolute z
-      auto dZ = seed0.getZ() - seed1Inv.getZ();
-      if (dZ * dZ > (seed0.getSigmaZ2() + seed1Inv.getSigmaZ2()) * mMatchParams->crudeNSigma2Cut[o2::track::kZ]) {
+      auto dZ = leg0.getZ() - seed1Inv.getZ();
+      if (dZ * dZ > (leg0.getSigmaZ2() + seed1Inv.getSigmaZ2()) * mMatchParams->crudeNSigma2Cut[o2::track::kZ]) {
         rej = RejZ;
         break;
       }
@@ -580,7 +599,7 @@ MatchCosmics::RejFlag MatchCosmics::checkPair(int i, int j)
       seed1Inv.setCov(0., o2::track::CovarMap[o2::track::kZ][o2::track::kQ2Pt]);
     }
     // calculate chi2 (expensive)
-    chi2 = seed0.getPredictedChi2(seed1Inv);
+    chi2 = leg0.getPredictedChi2(seed1Inv);
     if (chi2 > mMatchParams->crudeChi2Cut) {
       rej = RejChi2;
       break;
@@ -594,12 +613,14 @@ MatchCosmics::RejFlag MatchCosmics::checkPair(int i, int j)
 
 #ifdef _ALLOW_DEBUG_TREES_
   if (mDBGOut && ((rej == Accept && isDebugFlag(MatchTreeAccOnly)) || isDebugFlag(MatchTreeAll))) {
-    auto seed1I = seed1;
+    const TrackSeed& dbgLeg0 = commonTime ? seed0Common : seed0; // the legs compared (refitted at the common time if they were)
+    auto seed1I = commonTime ? seed1Common : seed1;
     seed1I.invert();
-    if (seed1I.rotate(seed0.getAlpha()) && o2::base::Propagator::Instance()->PropagateToXBxByBz(seed1I, seed0.getX(), mMatchParams->maxSnp, mMatchParams->maxStep, mMatchParams->matCorr)) {
+    if (seed1I.rotate(dbgLeg0.getAlpha()) && o2::base::Propagator::Instance()->PropagateToXBxByBz(seed1I, dbgLeg0.getX(), mMatchParams->maxSnp, mMatchParams->maxStep, mMatchParams->matCorr)) {
       int rejI = int(rej);
+      int commonTimeI = commonTime;
       (*mDBGOut) << "match"
-                 << "tf=" << mTFCount << "seed0=" << seed0 << "seed1=" << seed1I << "chi2Match=" << chi2 << "rej=" << rejI
+                 << "tf=" << mTFCount << "seed0=" << dbgLeg0 << "seed1=" << seed1I << "chi2Match=" << chi2 << "rej=" << rejI << "commonTime=" << commonTimeI
                  << "side0=" << int(seed0.tpcSide) << "side1=" << int(seed1.tpcSide) << "tCommon=" << tCommon << "tCommonErr=" << tCommonErr << "\n";
     }
   }
@@ -608,6 +629,38 @@ MatchCosmics::RejFlag MatchCosmics::checkPair(int i, int j)
   return rej;
 }
 
+//________________________________________________________
+bool MatchCosmics::refitSeedAtTime(const TrackSeed& seed, float timeMUS, TrackSeed& out)
+{
+  // TPC-only seed refitted with its clusters transformed at the time timeMUS, then treated as the seeds in createSeeds and process() (muon,
+  // extra z error, propagation to the DCA to the beam line); energy loss along the muon's flight in the refit and the propagation, as in
+  // refitWinners. Its z then refers to timeMUS
+  if (!mTPCRefitter || !mRecoData) {
+    return false;
+  }
+  const auto& tpcTrack = mRecoData->getTPCTrack(seed.origID);
+  o2::track::TrackParCov trk = tpcTrack.getParamOut();
+  trk.setPID(o2::track::PID::Muon, true);
+  trk.resetCovariance();
+  // the muon flies downward: inward along a leg pointing up (top leg) is along its flight (loss), along the bottom leg against it (gain)
+  std::array<float, 3> momentum{};
+  trk.getPxPyPzGlo(momentum);
+  const int eLossSign = momentum[1] > 0.f ? ELossLoss : ELossGain;
+  if (mTPCRefitter->RefitTrackAsTrackParCov(trk, tpcTrack.getClusterRef(), timeMUS / mTPCTBinMUS, nullptr, false, true, eLossSign) < 0) {
+    return false;
+  }
+  trk.setCov(mMatchParams->tpcExtraZError2 + trk.getSigmaZ2(), o2::track::kSigZ2);
+  const o2::dataformats::VertexBase v;
+  if (!o2::base::Propagator::Instance()->propagateToDCABxByBz(v, trk, mMatchParams->maxStep, mMatchParams->matCorr, nullptr, nullptr, eLossSign)) {
+    return false;
+  }
+  out = seed;
+  static_cast<o2::track::TrackParCov&>(out) = trk;
+  out.tRef = timeMUS;
+  mNRefitsCommonTime++;
+  return true;
+}
+
 //________________________________________________________
 void MatchCosmics::registerMatch(int i, int j, float chi2, float tCommon, float tCommonErr)
 {