Fxt offset vtx (#720)

This PR adjusts the selection of candidate tracks for use in Minuit vertex-finding to be offset vertically by -2 cm (downward in y) from the z-axis when processing with the `FXT` chain option. The z-axis is otherwise the default for the Minuit vertex-finding `RImpact` cut.
star-bnl · Dec 10, 2024 · 640288b · 640288b
1 parent 252a596
commit 640288b
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Showing 4 changed files with 17 additions and 2 deletions.
diff --git a/StRoot/StBFChain/StBFChain.cxx b/StRoot/StBFChain/StBFChain.cxx
@@ -580,6 +580,7 @@ Int_t StBFChain::Instantiate()
       if (GetOption("usePct4Vtx" ) )      mk->SetAttr("PCT"           	, kTRUE);
       if (GetOption("useBTOF4Vtx") )      mk->SetAttr("BTOF"          	, kTRUE);
       if (GetOption("useBTOFmatchOnly") ) mk->SetAttr("useBTOFmatchOnly", kTRUE);
+      if (GetOption("FXT"        ) )      mk->SetAttr("FXT"             , kTRUE);
 
       // X-tended works only for VFPPV, VFPPVnoCTB, VFPPVev for now but could be re-used
       // However, we will change this to a more flexible arbitrarry setting later

diff --git a/StRoot/StGenericVertexMaker/Minuit/StMinuitVertexFinder.cxx b/StRoot/StGenericVertexMaker/Minuit/StMinuitVertexFinder.cxx
@@ -55,6 +55,7 @@ StMinuitVertexFinder::StMinuitVertexFinder(VertexFit_t fitMode) :
   mExternalSeedPresent = kFALSE;
   mRequireCTB        = kFALSE;
   requireCTB         = kFALSE;
+  mFXT               = kFALSE;
   mUseITTF           = kFALSE;
   mUseOldBEMCRank    = kFALSE;
   mLowerSplitVtxRank = kFALSE;
@@ -407,19 +408,29 @@ int StMinuitVertexFinder::fit(StEvent* event)
     Int_t n_bemc_match_tot = 0;
     Int_t n_cross_tot = 0;
 
+    // In FXT mode, we want to have a bias = -2 cm below the z-axis
+    //   for the approximate physical target location. This could
+    //   potentially be a database table (PrimaryVertexCuts) parameter,
+    //   but the bias is approximate and highly unlikely to change/finetune.
+    StThreeVectorD beamAxis(0.0, mFXT ? -2.0 : 0.0, 0.0);
+    double RImpactMax2 = mRImpactMax*mRImpactMax;
+
     for (const StTrackNode* stTrack : event->trackNodes())
     {
       StGlobalTrack* g = ( StGlobalTrack*) stTrack->track(global);
       if (!accept(g)) continue;
       StDcaGeometry* gDCA = g->dcaGeometry();
       if (! gDCA) continue;
-      if (TMath::Abs(gDCA->impact()) >  mRImpactMax) continue;
+      StPhysicalHelixD gHelix = gDCA->helix();
+      StThreeVectorD DCAPosition = gHelix.at(gHelix.pathLength(beamAxis.x(),beamAxis.y())) - beamAxis;
+      double RImpact2 = DCAPosition.perp2();
+      if (RImpact2 > RImpactMax2) continue; // calculate square once instead of sqrt N times
       mDCAs.push_back(gDCA);
       // 	  StPhysicalHelixD helix = gDCA->helix(); 
       // 	  mHelices.push_back(helix);
       mHelices.push_back(g->geometry()->helix());
       mHelixFlags.push_back(1);
-      Double_t z_lin = gDCA->z();
+      Double_t z_lin = DCAPosition.z();
       mZImpact.push_back(z_lin);
 
       Bool_t shouldHitCTB = kFALSE;

diff --git a/StRoot/StGenericVertexMaker/Minuit/StMinuitVertexFinder.h b/StRoot/StGenericVertexMaker/Minuit/StMinuitVertexFinder.h
@@ -130,6 +130,7 @@ class StMinuitVertexFinder: public StGenericVertexFinder
     void            setFlagBase();
     void            SetFitPointsCut(Int_t fitpoints) {mMinNumberOfFitPointsOnTrack = fitpoints;}
     void            SetMinimumTracks(Int_t n) {mMinTrack = n;}
+    void            SetFXT(bool fxt=kTRUE) { mFXT = fxt; }
 
 private:
 
@@ -151,6 +152,7 @@ class StMinuitVertexFinder: public StGenericVertexFinder
     bool                   mLowerSplitVtxRank;// Use lower rank for split vertices
     UInt_t                 mFlagBase;         // ITTF track flag
     bool                   mRequireCTB;       // Set maker to use CTB
+    bool                   mFXT;              // Switch on FXT mode
     UInt_t                 mMinNumberOfFitPointsOnTrack;
     Float_t                mDcaZMax;
     Double_t               mRImpactMax;       // Max distance between helix and nominal beamline (0,0,z)

diff --git a/StRoot/StGenericVertexMaker/StGenericVertexMaker.cxx b/StRoot/StGenericVertexMaker/StGenericVertexMaker.cxx
@@ -112,6 +112,7 @@ Int_t StGenericVertexMaker::Init()
 
     if (IAttr("VFMinuit") ) ((StMinuitVertexFinder*) theFinder)->useOldBEMCRank();
     if (IAttr("VFMinuit3") ) ((StMinuitVertexFinder*) theFinder)->lowerSplitVtxRank();
+    if (IAttr("FXT") ) ((StMinuitVertexFinder*) theFinder)->SetFXT();
     if (minTracks > 0) ((StMinuitVertexFinder*) theFinder)->SetMinimumTracks(minTracks);
 
   } else if ( IAttr("VFppLMV")){