From a8b711edf00104cf0025da5d2cf7ce320498bef5 Mon Sep 17 00:00:00 2001
From: Will Leight <wleight@cern.ch>
Date: Mon, 12 Mar 2018 13:03:46 +0100
Subject: [PATCH] Fix for ATR-16605, round two
As discussed in !2959, a bug in the extrapolator was in rare cases causing thousands of scatterers to be added to a track.
Thanks to Peter Kluit, we believe that we have now tracked this down to a bug in the STEP_Propagator.
The propagator was getting stuck in a volume and iterating forward in steps of 1-10 um, adding a scatterer each time.
At the time, we implemented a "fix" that simply aborted the track-finding when more than 1000 scatterers were found on a track.
Now, the underlying issue should finally be resolved.
It appears that the problem was occurring when the propagator found two solutions from the starting surface, the first of which was trivial and the second of which gave a negative distance estimate.
In this case, the propagator would automatically flip direction, bringing it back almost to the starting point, where it would add a scatterer.
Then the process would repeat thousands of times, until it got far enough away from the start position that there was no second solution, or one with a postive distance estimate.
By identifying these cases and rejecting their negative-distance second solutions, this problem is avoided.
Given the rarity of this problem, FT0 tests naturally show no violation, results are available at /afs/cern.ch/work/w/wleight/public/MuonSW/tooManyScatterers/RunTier0Tests.log
Former-commit-id: 0c1c0d3b0c6bd977208a58a2fc9c9062d4516c7c
---
.../TrkExSTEP_Propagator/src/STEP_Propagator.cxx | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)
diff --git a/Tracking/TrkExtrapolation/TrkExSTEP_Propagator/src/STEP_Propagator.cxx b/Tracking/TrkExtrapolation/TrkExSTEP_Propagator/src/STEP_Propagator.cxx
index baa278ef735..ce79ab4bdaa 100755
--- a/Tracking/TrkExtrapolation/TrkExSTEP_Propagator/src/STEP_Propagator.cxx
+++ b/Tracking/TrkExtrapolation/TrkExSTEP_Propagator/src/STEP_Propagator.cxx
@@ -1360,12 +1360,14 @@ bool
std::vector<DestSurf >::iterator sBeg = sfs.begin();
unsigned int numSf=0;
unsigned int iCurr=0; // index for m_currentDist
-
+ int startSf = -99;
for (; sIter!=sfs.end(); sIter++) {
Trk::DistanceSolution distSol = (*sIter).first->straightLineDistanceEstimate(position,direction0);
double distEst = -propDir*maxPath;
+ double dist1Est = -propDir*maxPath;
if (distSol.numberOfSolutions()>0 ) {
distEst = distSol.first();
+ dist1Est = distSol.first();
if ( distSol.numberOfSolutions()>1 && ( fabs(distEst) < tol || (propDir*distEst<-tol && propDir*distSol.second()>tol)) ) distEst = distSol.second();
}
// select input surfaces;
@@ -1384,6 +1386,7 @@ bool
// save the nearest distance to surface
m_currentDist[iCurr]=std::pair<int,std::pair<double,double> >(-1,std::pair<double,double>(distSol.currentDistance(),distSol.currentDistance(true)));
}
+ if(fabs(dist1Est)<tol) startSf = (int) iCurr;
iCurr++;
}
@@ -1672,7 +1675,15 @@ bool
fabs(distSol.second()*propDir+distanceStepped-previousDistance) ){
distanceEst = distSol.second();
}
+// Peter Kluit: avoid jumping into other (distSol.first->second) distance solution for start surface with negative distance solution
+// negative distanceEst will trigger flipDirection = true and will iterate to the start surface
+// this will lead to very similar positions for multiple propagator calls and many tiny X0 scatterers
+ if(ic==startSf&&distanceEst<0&&distSol.first()>0) distanceEst = distSol.first();
}
+ //if(ic==startSf) std::cout << " start Surface " << ic << std::endl;
+ //std::cout << " surface nr ic " << ic << " distEst " << distanceEst << " previousDistance " << previousDistance << " distanceStepped " << distanceStepped << " tol " << tol << " path " << path << std::endl;
+ //if(distSol.numberOfSolutions()>1) std::cout << " surface nr ic " << ic << " sol first " << distSol.first() << " second " << distSol.second() << " current " << distSol.currentDistance(true) << std::endl;
+
// eliminate close surface if path too small
if (ic==nextSf && fabs(distanceEst)<tol && fabs(path)<tol) {
(*vsIter).first=-1; vsIter=vsBeg; restart=true; distanceToTarget=maxPath; nextSf=-1;
--
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