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[ALICE3] Protect LUT smearing against negative eigenvalues #18726

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Merged
mario-krueger merged 1 commit into from
Nov 13, 2026
Merged

[ALICE3] Protect LUT smearing against negative eigenvalues #18726

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6 changes: 5 additions & 1 deletion ALICE3/Core/FlatTrackSmearer.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -21,6 +21,7 @@

#include <TRandom.h>

#include <algorithm>
#include <cmath>
#include <cstddef>
#include <cstdint>
Expand Down Expand Up @@ -323,7 +324,10 @@ bool TrackSmearer::smearTrack(O2Track& o2track, const lutEntry_t* lutEntry, cons
for (int j = 0; j < kNumEigenModes; ++j) {
val += lutEntry->eigvec[j][i] * o2track.getParam(j);
}
params[i] = gRandom->Gaus(val, std::sqrt(lutEntry->eigval[i]));
// A LUT covariance close to singular (strongly correlated parameters at low pT) can carry a tiny negative
// eigenvalue from numerical precision. std::sqrt of it gives NaN and the track is dropped later on, which
// shows up as holes in the tracking efficiency. Such a mode has practically zero width: do not smear along it.
params[i] = gRandom->Gaus(val, std::sqrt(std::max(0.f, lutEntry->eigval[i])));
}

// Transform back params vector
Expand Down
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