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[ALICE3] Protect LUT smearing against negative eigenvalues - #18726
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mario-krueger merged 1 commit intoNov 13, 2026
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A LUT covariance matrix close to singular (strongly correlated track parameters at low pT) can carry a tiny negative eigenvalue (~ -1e-7) from numerical precision. std::sqrt of it gives NaN, the smeared track gets NaN parameters and is dropped by the on-the-fly tracker (hNaNBookkeeping). This shows up as holes in the tracking efficiency in single pT bins and eta halves, e.g. for e, pi, K, p between 0.09 and 0.25 GeV/c with 0.5 T LUTs. Clamp the eigenvalue at zero before taking the square root: such a mode has practically zero width, so it is simply not smeared.
mario-krueger
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njacazio
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November 13, 2026 10:10
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O2 linter results: ❌ 0 errors, |
mario-krueger
enabled auto-merge (squash)
November 13, 2026 10:20
njacazio
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Nov 13, 2026
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A LUT covariance matrix close to singular (strongly correlated track parameters at low pT) can carry a tiny negative eigenvalue (~ -1e-7) from numerical precision. std::sqrt of it gives NaN, the smeared track gets NaN parameters and is dropped by the on-the-fly tracker (hNaNBookkeeping). This shows up as holes in the tracking efficiency in single pT bins and eta halves, e.g. for e, pi, K, p between 0.09 and 0.25 GeV/c with 0.5 T LUTs.
Clamp the eigenvalue at zero before taking the square root: such a mode has practically zero width, so it is simply not smeared.