20 May 2009 Offline/Prod. Rev. Adam Jacholkowski 1

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

20 May 2009 Offline/Prod. Rev. Adam Jacholkowski 2

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)

Prolegomena

20 May 2009 Offline/Prod. Rev. 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

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Generated & Reconstructed (1)

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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

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Vertex Residuals (abs and norm)

20 May 2009 Offline/Prod. Rev. 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

20 May 2009 Offline/Prod. Rev. Adam Jacholkowski 9

Normalized (pseudo)residuals

Notn-gaussiandistributions:~exp(-b*sqrt(abs(x))

Explanation in the correlations

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Residuals vs Sigma (Tracks)

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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

20 May 2009 Offline/Prod. Rev. Adam Jacholkowski 12

σ(Xvertex) vs NContributors (SPD)

IN case VertexerZ is usedXRes set to 50 micWhile σ(x-diamond) = 60 mic

[cm]

(From CAF )

NContribBAq

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Sigmas(SPD-3D) extrapolation: Xvert

Practical highmultiplicitylimit with SPD

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Sigma(Tracks) extrapolation: Xv

Limit for VertexerTracks

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Sigma(SPD-3D) extrapolation: Zvert

Practical highmultiplicitylimit with SPD

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Sigma(Tracks) extrapolation: Zv

Limit for VertexerTracks

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Average&Spread(SPD-3D) : Xvert

Asymptotic spread/RMSshould give beam diamondSize estimate

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Average&Spread(Tracks) : Xvert

?

Counter-intuitive due to the average vertexConstraint, but the limit RMS ~OK

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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 and alignment.

MC data without the Mean Vertex Constraint would be useful to test the beam reconstruction procedure

20 May 2009 Offline/Prod. Rev. Adam Jacholkowski 20

ALICE coordinates

BACKUP

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Vertex reconstruction logics When Ntracklets > 1, Vertexer3D called, then eventually (if it

failes) VertexerZ When Ntracklets= 1 VertexerZ called directly vtxGetStatus() 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