ITS tracking: Introduce configurable minimum pt per track length

The changes introduce a configurable minimum PT value per track length in the ITS
tracking parameters. This allows for more fine-grained control over the minimum
PT requirement, which can be useful for different tracking scenarios.

The main changes are:

- Modify the `TrackingParameters` struct to include an array of 4 minimum PT
  values, one for each track length (7, 6, 5, 4).
- Update the track fitting logic in `TrackerTraits::findRoads()` to use the
  appropriate minimum PT value based on the track length.
- Update the default minimum PT value in `TrackingInterface::initialise()` to
  use the new array-based approach.

These changes provide more flexibility in tuning the ITS tracking parameters.

Detectors/ITSMFT/ITS/macros/test/CheckTracksCA.C

@@ -87,6 +87,7 @@ void CheckTracksCA(bool doFakeClStud = false,
   TTree* mcTree = (TTree*)gFile->Get("o2sim");
   mcTree->SetBranchStatus("*", 0); // disable all branches
   mcTree->SetBranchStatus("MCTrack*", 1);
+  mcTree->SetBranchStatus("MCEventHeader*", 1);
 
   std::vector<o2::MCTrack>* mcArr = nullptr;
   mcTree->SetBranchAddress("MCTrack", &mcArr);
@@ -115,10 +116,13 @@ void CheckTracksCA(bool doFakeClStud = false,
   std::cout << "** Filling particle table ... " << std::flush;
   int lastEventIDcl = -1, cf = 0;
   int nev = mcTree->GetEntriesFast();
-  std::vector<std::vector<ParticleInfo>> info(nev);
+  std::vector<std::vector<ParticleInfo>> info;
+  info.resize(nev);
+  TH1D* hZvertex = new TH1D("hZvertex", "Z vertex", 100, -20, 20);
   for (int n = 0; n < nev; n++) { // loop over MC events
     mcTree->GetEvent(n);
     info[n].resize(mcArr->size());
+    hZvertex->Fill(mcEvent->GetZ());
     for (unsigned int mcI{0}; mcI < mcArr->size(); ++mcI) {
       auto part = mcArr->at(mcI);
       info[n][mcI].event = n;
@@ -196,7 +200,6 @@ void CheckTracksCA(bool doFakeClStud = false,
         info[evID][trackID].track.getImpactParams(info[evID][trackID].pvx, info[evID][trackID].pvy, info[evID][trackID].pvz, bz, ip);
         info[evID][trackID].dcaxy = ip[0];
         info[evID][trackID].dcaz = ip[1];
-        Info("", "dcaxy=%f dcaz=%f bz=%f", ip[0], ip[1], bz);
       }
 
       fakes += fake;
@@ -286,6 +289,10 @@ void CheckTracksCA(bool doFakeClStud = false,
   clone->Divide(clone, den, 1, 1, "b");
   clone->SetLineColor(3);
   clone->Draw("histesame");
+  TCanvas *c2 = new TCanvas;
+  c2->SetGridx();
+  c2->SetGridy();
+  hZvertex->DrawClone();
 
   std::cout << "** Streaming output TTree to file ... " << std::flush;
   TFile file("CheckTracksCA.root", "recreate");

Detectors/ITSMFT/ITS/tracking/include/ITStracking/Configuration.h

@@ -94,7 +94,7 @@ struct TrackingParameters {
   unsigned long MaxMemory = 12000000000UL;
   float MaxChi2ClusterAttachment = 60.f;
   float MaxChi2NDF = 30.f;
-  float MinPt = 0.f;
+  float MinPt[4] = {0.f};
   unsigned char StartLayerMask = 0x7F;
   bool FindShortTracks = false;
   bool PerPrimaryVertexProcessing = false;

Detectors/ITSMFT/ITS/tracking/src/TrackerTraits.cxx

@@ -618,7 +618,7 @@ void TrackerTraits::findRoads(const int iteration)
       temporaryTrack.resetCovariance();
       temporaryTrack.setChi2(0);
       fitSuccess = fitTrack(temporaryTrack, mTrkParams[0].NLayers - 1, -1, -1, mTrkParams[0].MaxChi2ClusterAttachment, mTrkParams[0].MaxChi2NDF, 50.f);
-      if (!fitSuccess || temporaryTrack.getPt() < mTrkParams[iteration].MinPt) {
+      if (!fitSuccess || temporaryTrack.getPt() < mTrkParams[iteration].MinPt[7 - temporaryTrack.getNClusters()]) {
         continue;
       }
       tracks[trackIndex++] = temporaryTrack;

Detectors/ITSMFT/ITS/tracking/src/TrackingInterface.cxx

@@ -47,7 +47,7 @@ void ITSTrackingInterface::initialise()
     trackParams[2].TrackletMinPt = 0.1f;
     trackParams[2].CellDeltaTanLambdaSigma *= 4.;
     trackParams[2].MinTrackLength = 4;
-    trackParams[2].MinPt = 0.2f;
+    trackParams[2].MinPt[3] = 0.2f;
     trackParams[2].StartLayerMask = (1 << 6) + (1 << 3);
     if (o2::its::TrackerParamConfig::Instance().doUPCIteration) {
       trackParams[3].TrackletMinPt = 0.1f;