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Vertex reconstruction in ALICE

Vertex reconstruction in ALICE. Andrea Dainese in collaboration with E.Bruna, E.Crescio, S.Gorbunov, I.Kisel, S.Masciocchi, M.Masera, F.Prino. alieve, M.Tadel. Outline. Interaction diamond at LHC (pp) Primary vertex reconstruction s along z with SPD tracklets in 3D with SPD tracklets

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Vertex reconstruction in ALICE

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  1. Vertex reconstructionin ALICE Andrea Dainese in collaboration with E.Bruna, E.Crescio, S.Gorbunov, I.Kisel, S.Masciocchi, M.Masera, F.Prino alieve, M.Tadel ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  2. Outline • Interaction diamond at LHC (pp) • Primary vertex reconstructions • along z with SPD tracklets • in 3D with SPD tracklets • in 3D with tracks • Secondary vertex reconstruction (heavy-flavour) • present algorithm • developments in progress ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  3. Primary vertex in pp collisions • Primary vertex resolution affects impact param. resolution: • E-by-E vertex position reconstructed in 3D using tracks (or SPD tracklets, see talk by F.Prino) • Then, interaction diamond profile from many events in fill • Nominal spread of the diamond at LHC: ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  4. Algorithms for vertex reco • AliITSVertexerZ: z from SPD tracklets • PROS: almost 100% efficient in triggered events; • CONS: only z; with large beam offsets has problems (loss of precision and accuracy); • AliITSVertexer3D: xyz from SPD tracklets • PROS: 3D; no problem with beam offset; • CONS: not 100% efficient; cannot account for curvature; • AliVertexerTracks: xyz from tracks • PROS: most precise (curvature known); gives cov matrix and c2; • CONS: can be done only after tracking; • Optimal performance by exploiting the pros of the different algorithms during event reconstruction (next slide) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  5. Barrel Tracking & Vertex Recons. • Vertex (x,y,z) from SPD tracklets (AliITSVertexer3D) • only z with AliITSVertexerZ when AliITSVertexer3D fails • Barrel Tracking • Compute diamond transverse profile, as mean and RMS of vertices in high-multiplicity events of each “LHC fill” ➔ stored in OCDB • Repeat step 1) • important, to have zvtx for all events (even with one tracklet) • Barrel Tracking • Vertex with tracks (AliVertexerTracks), using info on diamond profile, taken from OCDB (in progress; not in PDC06) final (x,y) resolution better than diamond RMS for ALL events 1st pass on subsample 2nd pass full statistics ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  6. AliITSVertexerZ – the method F.Prino, Münster • Build “tracklets” from SPD Clusters • associate each Cluster on layer1 to all the Clusters on layer2 within a window Δφ <0.01 rad • Calculate Zi = Z of closest approach of tracklet and nominal beam axis. • Fill a histogram of Zi with 100 mm (200 in pp) bin size • define a z window (2 mm wide)around the “peak” • Calculate vertex position and error • Zv = weighted average of the Zi of the tracklets in the window • after symmetrization around the peak • Error = propagation of errors on Clusters • also in case of just 1 tracklet Layer 2 Layer 1 Beam axis ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  7. AliITSVertexer3D – the method F.Prino, Münster • Build tracklets (= straight lines) from pairs of Clusters • First selection done w.r.t. nominal beam axis • Loose selection: Δφ < 0.5 rad, DCA to beam < 2.5 cm, |zINT|<5.3 cm • Combine tracklet pairs and select them according to: • small DCA (< 1 mm) between the two tracklets • Tracklet intersection close to beam axis (rINT < 2.5 cm ) • Tracklet intersection in the diamond region (zINT< 5.3 cm) • Get a first estimate of the vertex from selected tracklets • same vertex finder algorithm used with tracks (AliVertexerTracks) • Re-build tracklets • Selection done w.r.t. beam position from vertex estimate in previuos step • Tight selection: Δφ < 0.01 rad, DCA to beam < 0.5 cm, |zINT - zVERT|<0.5 cm • Combine tracklet pairs and select them according to: • small DCA (<1 mm) between the two tracklets • Tracklet intersection close to the estimated vertex (Dr < 0.5 cm) • Tracklet intersection in the diamond region (Dz < 0.5 cm) • Calculate the vertex using the selected tracklets ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  8. Vertexing with SPD: Efficiency • pp 14 TeV events (PYTHIA minimum bias) • MC vertex: (0,0,0) with ss = (50 mm, 50 mm, 5.3 cm) [b*=10m] • No trigger selection • Efficiency = events with vertex / all events • Vertexer3D requires at least 2 selected tracklets • Events with just one tracklet are ≈10% SPD pp14TeV B = 0 SPD pp14TeV B = 0.5 T ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  9. Vertexing with SPD: Resolutions • Resolution improved for both vertexers in last weeks • New procedure for VertexerZ • Cut optimization and improved track selection for Vertexer3D SPD pp14TeV B = 0 SPD pp14TeV B = 0.5 T • Everything working with B=0 • better resolution in (x,y) because tracks are straight ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  10. Vertexing with SPD:Beam offset in (x,y) • VertexerZ resolution dramatically worsens in case of large (and unknown) beam offset • The performance is completely recovered if the X,Y position of the beam is known • Vertexer3D performance not affected also in case of large and unknown beam offset better resolution with large offset due to geom. effect ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  11. Barrel Tracking & Vertex Recons. • Vertex (x,y,z) from SPD tracklets (AliITSVertexer3D) • only z with AliITSVertexerZ when AliITSVertexer3D fails • Barrel Tracking • Compute diamond transverse profile, as mean and RMS of vertices in high-multiplicity events of each “LHC fill” ➔ stored in OCDB • Repeat step 1) • important, to have zvtx for all events (even with one tracklet) • Barrel Tracking • Vertex with tracks (AliVertexerTracks), using info on diamond profile, taken from OCDB final (x,y) resolution better than diamond RMS for ALL events 1st pass on subsample 2nd pass full statistics ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  12. AliVertexerTracks • Three main “tasks” implemented: • TRACKS SELECTION • reject secondary tracks • VERTEX FINDING • give first estimate of vertex position • method: approximate tracks as straight lines and calculate the minimum-distance point among all the tracks at once • VERTEX FITTING: • give optimal estimate of the position of the vertex • give vertex covariance matrix • give a c2 (vertex quality) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  13. VertexerTracks::Vertex Fitter • “Fast Vertex Fitting”, CMS Note 1997/051 • Tracks are propagated to the point given by the vtx finder • Each track gives an independent measurement of the vertex position: a c2 is written as the sum of the single track c2s w.r.t. a generic vertex position rvtx: where ri is the (x,y,z) position of the ith track and Vi = Wi-1 is the cov matrix of ri, extracted from the track’s cov matrix • The solution that minimizes this c2 is analytic: vertex covariance matrix vertex ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  14. Easy to add “beam constraint” • Same procedure as before + constraint: vertex vertex covariance matrix where: ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  15. VertexerTracks – the method • Primary vertex reconstruction: iterative procedure • Input: interaction diamond (vtx0) • Select tracks with kITSrefit and 5 or 6 ITS clusters • 1st iteration: • TRACK SELECTION: |d0rf(vtx0)| < 15  sd0 • VERTEX FINDING • VERTEX FITTING  vtx1 • 2nd iteration: • TRACK SELECTION: |d0rf(vtx1)| < 3  sd0 • VERTEX FINDING • VERTEX FITTING  vtx2 (final estimate) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  16. VertexerTracks: Efficiency • pp 14 TeV events (PYTHIA minimum bias) • MC vertex: (0,0,0) with ss = (50 mm, 50 mm, 5.3 cm) [b*=10m] • No trigger selection • Vertex found if distance to MC vertex in (x,y) is < 1 mm • Efficiency: events with vertex found / all events (with beam constraint; not yet in PDC, but soon) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  17. VertexerTracks:Resolutions & Pulls res: from gaussian fit of residuals distribution pulls: from gaussian fit of residual/error distribution 3s track-cut 3s track-cut (with beam constraint; not yet in PDC, but soon) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  18. VertexerTracks:c2/ndf ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  19. New development in VertexerTracks: possibility to remove tracks • Why remove tracks? • systematic underestimate of d0 for tracks used for vertex fit • primary vertex has to be reconstructed for each D/B candidate excluding its daughters: time-consuming! • Shortcut: • our fit is a “weighted mean over tracks” • ➔ can calculate ri and Wi and subtract them from rvtx and V • without recalculating the vertex from scratch • implemented in AliVertexerTracks::RemoveTrackFromVertex() ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  20. VertexerTracks w/o beam constraint(importance of event display) problem spotted with event display scanning (Y.Foka, M.Tadel) tracks used for vertex “bad” vertices vertex not found tracks in TPC&ITS(≥5 pts) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  21. Ladies and Gentlemen, (in)famous event #24 ! after before VertexerTracks w/o beam constraint(importance of event display) vertex not found ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  22. SPD barrel VertexerTracks- pp 0.9 TeV B = 0 s = (198mm,198mm,7.9cm) - pp14TeV B = 0.5 T pp0.9TeV B = 0 ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  23. Münster x x pull resolution [mm] z z pull resolution [mm] Secondary vertex reconstructionAliVertexerTracks::VertexForSelectedTracks() • Finder+Fitter ➔ errors ➔ heavy flvr cuts on normalized vars • Test on D0Kp vertex: same resolution as V0 vertexer • Test on D+Kpp in progress (E.Bruna, F.Prino) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  24. Implementation of a new vertexing package (Gorbunov, Kisel, Masciocchi) • Vertexing package based on the Kalman filter • S. Gorbunov and I. Kisel, CBM-SOFT-note-2006-001, CBM-SOFT-note-2006-002 • Used by HERA-B, CBM • Advanced fitting functionality included: • Use of full track kinematics • Invariant mass constraint • Topological constraint (pointing to the primary) • Adapted to ALICE track format (AliESDtrack) • Tests in progress, using D0Kp as a benchmark ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  25. New vertexing package- resolutions and pulls on D0 vertex - pull x x resolution [mm] Preliminary, Work in progress z z pull resolution [mm] ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  26. Summary • 3 complementary (not alternative) vertex reconstruction methods: all used during event reconstruction • AliITSVertexerZ: z from SPD tracklets • PROS: almost 100% efficient in triggered events; error provided; • AliITSVertexer3D: xyz from SPD tracklets • PROS: 3D; no problem with beam offset; • AliVertexerTracks: xyz from tracks • PROS: most precise (curvature known); gives cov matrix and c2; allows fast removal of tracks from fit; • being tested for heavy-flavour vertexing (D0, D+,...): good peformance • Kalman-filter vertex fitter on the way ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  27. EXTRA SLIDES ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  28. VertexerZ: average of residuals • Unbiased! • The small bias (≈5 mm) which was present in the previous releases of the code was due to a systematic error in finding the maximum of the histograms for few low multiplicity events • Corrected since Rev. 1.17 of AliITSVertexerZ.cxx ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  29. Vertexer3D: average of residuals • No apparent bias ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  30. In events generated with AliRoot rev. after June 12th 2006 a bias (slope vs. ztrue) is observed see Jan Fiete talk at first physics meeting on Jan 19th 2007 Major modifications in ITS geometry ITS geometry changed to TGeo SPD chip thickness reduced to 150 mm The bias is introduced by the SPD chip thickness Bug found and fixed by L. Gaudichet on February 5th 2007 VertexerZ vs. vertex position (II) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  31. VertexerZ and beam offset (I) • Beam offset up to 1 cm assumed unknown • Efficiency not affected • Resolution dramatically worsens with increasing beam offset ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  32. VertexerZ and beam offset (II) • Beam offset up to 1 cm assuming to know the beam position • Good performance for offsets up to 1 cm if the beam position is known ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  33. Vertexer3D and beam offset • Beam offset up to 1 cm assumed unknown • Performance maintained ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  34. Pile-up • Expected interaction rate = 2×105 Hz at a luminosity of 5×1030 cm-2s-2 • 1 interaction every 200 bunch crossings • In case of SPD strobe duration of 100 ns • 4 bunch crossings (0.02 interactions) • All events in the strobe are overlapped even if not belonging to the same bunch-cross • Caveat: high- multiplicity triggers will select piled-up events • First check on AliITSVertexerZ in the case of pile-up • “Manual merging” of recpoints with an “ad hoc” macro • Results: • Vertices with distances >600 μm: the vertex of the event with higher multiplicity is found • Vertices with distances <600 μm: an intermediate value of z is found • Under study: check if the vertexer can be used to “detect” the pile-up, searching for two peaks (possible in the case of well separated peaks) • Study to be performed also on the Vertexer3D ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  35. VertexerTracks: Efficiency for triggered events • pp 14 TeV events (PYTHIA minimum bias) • MC vertex: (0,0,0) with ss = (50 mm, 50 mm, 5.3 cm) [b*=10m] • Select triggered events: [ (V0A or V0C) and SPDFO ] • Vertex found if distance to MC vertex in (x,y) is < 1 mm • Efficiency: events with vertex found / triggered events triggered minimum bias (with beam constraint) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  36. Performance w/o beam constraint(as in PDC events) without constraint with constraint ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  37. AliVertexerTracks Residuals vs zMC • Small bias in zrec (“pulled towards z=0”) • Larger bias observed in vertex with SPD (see talk by F.Prino) • It was due to bug in the SPD geometry, introduced in September, now corrected • However, for large z there can be acceptance-induced bias ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  38. New development: possibility to remove (add) tracks • One-shot fit procedure • Select tracks with kITSrefit and 5 or 6 ITS clusters • Prpogate them to diamond centre (vtxmean) • TRACKS SELECTION: |d0rf(vtxmean)| < 3 sd0 • VERTEX FITTING standard procedure one-shot fit OK! ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

  39. Efficiencies vs zMC events with vertex / triggered events events with vertex / all events VertexerTracks (triggered events) VertexerZ (all events) ALICE Physics Forum, CERN - 07.03.07 Andrea Dainese

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