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Primary Vertex Overview (LHC09a4 MC: pp MB 10TeV B=0.5T)

Primary Vertex Overview (LHC09a4 MC: pp MB 10TeV B=0.5T). 2009. Prolegomena. Primary Vertex with Tracks Average X & Y vertex used as a constraint (for real data foreseen only in the second pass !) Better resolution in X/Y (factor ~5-6) than SPD vertex

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Primary Vertex Overview (LHC09a4 MC: pp MB 10TeV B=0.5T)

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  1. Primary Vertex Overview (LHC09a4 MC: pp MB 10TeV B=0.5T) 2009 Adam Jacholkowski

  2. Prolegomena • Primary Vertex with Tracks • Average X & Y vertex used as a constraint (for real data foreseen only in the second pass !) • Better resolution in X/Y (factor ~5-6) than SPD vertex • Z vertex quality comparable to the SPD vertexer performance • Under condition of perfect TPC – ITS alignment • Questions: • is the vertex constraint working ok ? • how to reconstruct beam parameters (position & diamond) Adam Jacholkowski

  3. Ref: Reconstruction of the interaction vertex in pp collisions at LHC with ALICE, ALICE-INT-2003-27, A. Dainese and M. Masera Adam Jacholkowski

  4. Adam Jacholkowski

  5. Generated & Reconstructed (1) Adam Jacholkowski

  6. Generated & Reconstructed (2) • Effect of the constraint  reconstructed distributions narrower than the true (MC) ones • Is strength of the constraint correct ? • The answer can come only from the inspection of the residuals/pulls Adam Jacholkowski

  7. Vertex Residuals (abs and norm) Adam Jacholkowski

  8. Vertex Residuals (abs and norm) • Normalized residuals look correct  sigmas take correctly into account effects of the constraint and of the vertex resolution • A test - taking (normalized by sigmas) pseudo residuals defined as (VTXreco - VTXaverage)/sigma  non-gaussian with RMS ≥1.0 in spite of the narrowness of the X/Y vertex distributions Adam Jacholkowski

  9. Normalized (pseudo)residuals Notn-gaussian distributions: ~exp(-b*sqrt(abs(x)) Explanation in the correlations Adam Jacholkowski

  10. Residuals vs Sigma (Tracks) Adam Jacholkowski

  11. Beam Diamond size estimate • Arbitrary good statistical precision of the average beam position possible (systematics dominating !) under condition of beam stability • Size of the diamond measurement – needs high multiplicity event selection (beware of pile-up!) and a careful subtraction of the vertex resolution Adam Jacholkowski

  12. σ(Xvertex) vs NContributors (SPD) (From CAF ) IN case VertexerZ is used XRes set to 50 mic While σ(x-diamond) = 60 mic [cm] Adam Jacholkowski

  13. Sigmas(SPD-3D) extrapolation: Xvert Practical high multiplicity limit with SPD Adam Jacholkowski

  14. Sigma(Tracks) extrapolation: Xv Limit for VertexerTracks Adam Jacholkowski

  15. Sigma(SPD-3D) extrapolation: Zvert Practical high multiplicity limit with SPD Adam Jacholkowski

  16. Sigma(Tracks) extrapolation: Zv Limit for VertexerTracks Adam Jacholkowski

  17. Average&Spread(SPD-3D) : Xvert Asymptotic spread/RMS should give beam diamond Size estimate Adam Jacholkowski

  18. Average&Spread(Tracks) : Xvert ? Counter-intuitive due to the average vertex Constraint, but the limit RMS ~OK Adam Jacholkowski

  19. Conclusion(s) • X/Y vertex components much better determined by Tracks (in case of perfect alignment !) than by SPD tracklets (B field ON) while Z vertex only marginally better with tracks • Mean Vertex constraint in the Vertex Fit with tracks distorting X/Y of the reconstructed vertex but the (standard) residuals are OK • Extraction of σ(X/Y) beam interaction diamond not trivial – needs subtraction (deconvolution) of the vertex resolution sigma, which depends on multiplicity, tracking precision andalignment. • MC data without the Mean Vertex Constraint would be useful to test the beam reconstruction procedure Adam Jacholkowski

  20. ALICE coordinates BACKUP Adam Jacholkowski

  21. Vertex reconstruction logics • When Ntracklets > 1, Vertexer3D called, then eventually (if it failes) VertexerZ • When Ntracklets= 1 VertexerZ called directly • vtxGetStatus() returns: • kTRUE if vertex found, NContributors = N of used tracklets (not quite clear) • kFALSE if VertexerZ failed, then Vertex set to (0,0,0) • NContributors =-2 if no SPD clusters • NContributors = -1 if no valid tracklet found • If Vertexer3D failes (but not VertexerZ), then only X,Yvert set to 0: Vertex=(0,0,Zvert) • VertexerTracks called always if Ntracks ≥ 2 Adam Jacholkowski

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