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

2. ALICE Offline Week, CERN, 22.10.08 Andrea Dainese 2 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 z vtx with SPD starting from mean (x,y) important, to have z vtx 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 1 st pass on subsample 2 nd pass full statistics (twice: ITS+TPC, TPC only; both stored in ESD) (twice: ITS+TPC, TPC only)

3. ALICE Offline Week, CERN, 22.10.08 Andrea Dainese 3 Vertex reconstruction with tracks AliVertexerTracks 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   (vertex quality)

4. ALICE Offline Week, CERN, 22.10.08 Andrea Dainese 4 Primary vertex reco with tracks AliVertexerTracks::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 |tg |<1 0 th iteration:  VERTEX FINDING, using track pairs  vtx0 1 st iteration:  TRACKS SELECTION: |d 0 3D (vtx0)| < 0.5 cm for ITS+TPC < 2.5 cm for TPC  VERTEX FINDING  VERTEX FITTING  vtx1 2 nd iteration:  TRACKS SELECTION: |d 0 r  (vtx1)| < 3   d0  VERTEX FINDING  VERTEX FITTING  vtx2 (final estimate)

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

6. 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  m)  diamond rms from machine (200  m @ 900 GeV) use this diamond as constraint and re-evaluate vertices (2 nd reconstruction pass)

7. 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. 8 TPC Vertex efficiency PHOJET 900 0T PHOJET 900 0.5T

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

10. ALICE Offline Week, CERN, 22.10.08 Andrea Dainese 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

11. 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. 12 Comparison with MC vertex: shift First Physics test production (TPC misaligned, miscalibrated)‏ Ideal simulation and reconstruction

13. ALICE Offline Week, CERN, 22.10.08 Andrea Dainese 13 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 d 0 Cut |d 0 | < n  d0 (p t )  d0 (p t ) =  vtx   track (p t ) track vertex x y from track’s covariance matrix need an estimate of the vertex and its cov. matrix

14. 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  2 is written as the sum of the single track  2 s w.r.t. a generic vertex position r vtx : where r i is the (x,y,z) position of the i th track and V i = W i -1 is the cov matrix of r i, extracted from the track’s cov matrix The solution that minimizes this  2 is analytic: vertex covariance matrix

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