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Primary vertex with TPC-only tracks

Primary vertex with TPC-only tracks. Motivation: TPC stand-alone analyses (multiplicity, spectra). Andrea Dainese INFN Legnaro. (twice: ITS+TPC, TPC only; both stored in ESD). (twice: ITS+TPC, TPC only). (twice: ITS+TPC, TPC only; both stored in ESD). Barrel Tracking & Vertex Recons.

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Primary vertex with TPC-only tracks

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  1. Primary vertexwith TPC-only tracks Motivation: TPC stand-alone analyses (multiplicity, spectra) Andrea Dainese INFN Legnaro ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  2. (twice: ITS+TPC, TPC only; both stored in ESD) (twice: ITS+TPC, TPC only) (twice: ITS+TPC, TPC only; both stored in ESD) Barrel Tracking & Vertex Recons. • ITS local reconstruction (➔ RecPoints) • Vertex (x,y,z) from SPD tracklets • Barrel Tracking • Vertex with tracks • Compute diamond transverse profile, as mean and RMS of vertices in high-multiplicity events of each “LHC fill” • Repeat zvtx with SPD starting from mean (x,y) • important, to have zvtx for all events (even with one tracklet), w/o bias • Barrel Tracking • Vertex with tracks, using info on diamond profile • final (x,y) resolution better than diamond RMS for ALL events 1st pass on subsample 2nd pass full statistics ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  3. Vertex reconstruction with tracksAliVertexerTracks • Three main “tasks” implemented: • TRACKS SELECTION • try reject secondary tracks • VERTEX FINDING • give first estimate of vtx position • VERTEX FITTING: • give optimal estimate of the position of the vertex • give vertex covariance matrix • give a c2 (vertex quality) ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  4. Primary vertex reco with tracksAliVertexerTracks::FindPrimaryVertex() • Primary vertex reconstruction: iterative procedure • Select tracks • track points to fiducual cylinder (R<3 cm, |z|<30 cm) • for ITS+TPC: kITSrefit and >4 ITS clusters • for TPC: kTPCin, >50 cls and |tgl|<1 • 0th iteration: • VERTEX FINDING, using track pairs  vtx0 • 1st iteration: • TRACKS SELECTION: |d03D(vtx0)| < 0.5 cm for ITS+TPC < 2.5 cm for TPC • VERTEX FINDING • VERTEX FITTING  vtx1 • 2nd iteration: • TRACKS SELECTION: |d0rf(vtx1)| < 3  sd0 • VERTEX FINDING • VERTEX FITTING  vtx2 (final estimate) ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  5. Impact parameter distr. of TPC tracks z rf reasonable pulls ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  6. Diamond constraint • Why diamond constraint? • vertex also for events with 1 “primary” track • better rejection of secondaries • (x,y) resolution always better than diamond size • Exercise on CAF (as it could be done on the data) • use 10k triggered events, pp 900 GeV • select vertices with > 10 tracks • weighted average of (x,y,z) • diamond: • centre: weighted average (tried also with plain average: compatible results) • sigma: error of weighted average (20-30 mm)  diamond rms from machine (200 mm @ 900 GeV) • use this diamond as constraint and re-evaluate vertices (2nd reconstruction pass)

  7. TPC Vertex efficiency • Efficiency: events with vertex / triggered (MB1=SPD||V0A||V0C) • Different normalization: events with vertex / events triggered and with >0 TPC tracks pointing to the beam pipe

  8. TPC Vertex efficiency PHOJET 900 0T PHOJET 900 0T PHOJET 900 0.5T PHOJET 900 0.5T

  9. Resolutions and pulls (PHOJET 900 0T) closed markers: RMS; open markers: s fit no constraint with constraint Ideal simulation and reconstruction First Physics test production (TPC misaligned, miscalibrated)‏ with constraint no constraint

  10. Code • AliVertexerTracks can take an array (TObjArray) of AliExternalTrackParam tracks • Separate “MeanVertex” objects in OCDB for ITS+TPC and TPC-only • Selection criteria for AliVertexerTracks kept in new class AliGRPRecoParam (from rec.C or OCDB) • separate criteria (incl. constraint flag) for ITS and TPC “mode” • performance optimisation without touching compiled code • In AliReconstruction: AliVertexerTracks tVertexer; // TPC + ITS primary vertex tVertexer.SetVtxStart(fDiamondProfile); // from OCDB tVertexer.SetITSMode(grpRecoParam->GetITSCuts()); ... find vertex  ESD // TPC-only primary vertex tVertexer.SetVtxStart(fDiamondProfileTPC); // from the OCDB tVertexer.SetTPCMode(grpRecoParam->GetTPCCuts()); ... find vertex  ESD ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  11. Next • The extraction of the mean vertex and storage in the OCDB is run manually on CAF • OK for first data at 900 GeV • check stability of diamond position vs time, run#, ... • Later the procedure will have to become automatic and run on ESDs accessed on grid • how to identify the run/event/time at which the diamond moves?

  12. Comparison with MC vertex: shift Ideal simulation and reconstruction First Physics test production (TPC misaligned, miscalibrated)‏

  13. y track x vertex need an estimate of the vertex and its cov. matrix Tracks Selection • Idea: use “primary”tracks for vertex reconstruction • Only tracks with 5 or 6 points in ITS • Select primary tracks on the basis of their d0 • Cut |d0| < nsd0(pt) sd0(pt) = svtxstrack(pt) from track’s covariance matrix ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

  14. 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 Offline Week, CERN, 22.10.08 Andrea Dainese

  15. Easy to add “beam constraint” • Same procedure as before + constraint: vertex vertex covariance matrix where: ALICE Offline Week, CERN, 22.10.08 Andrea Dainese

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