Primary vertex reconstruction with the SPD

1. Primary vertex reconstruction with the SPD E. Crescio, M. Masera, F. Prino INFN e Università di Torino Offline week, reconstruction meeting – October 2nd 2006

2. Vertexers • AliITSVertexerZ: uses information from SPD • Goals: high efficiency, constrain the tracking • Before tracking • AliITSVertexer3D: extension of AliITSVertexerZ to x and y • New class presently under test • Before tracking (needs just SPD tracklets) • AliITSVertexerIons: uses information from SPD • measurement of Xv,Yv,Zv through fit procedures • Not working for low multiplicities • AliVertexerTracks: uses tracks • Goals: accuracy and good error determination • Measurement of Xv,Yv,Zv • After recostruction in the barrel

3. Events used in this study • Event generation • AliRoot v4-04-Release • pp collisions (kPyMb) • Vertex smearing on x,y (50 mm) and z (5.3 cm) • FIRST SET: • 9800 events with beam centered in 0,0 • SECOND SET: • 9400 events with beam centered in (500 mm, 0) • ONGOING GENERATION: • events with 5 mm and 1 cm of beam offset • VertexerZ and Vertexer3D performance studied in 6 bins of Ntracklets in SPD (AliMultiplicity::GetNumberOfTracklets) • Efficiency = ratio between events with reconstructed vertex and total number of events • Residual distributions = distribution of zmeasured-ztrue • Estract resolution, pulls and bias

4. AliITSVertexerZ – the method Layer 2 • Build “tracklets” from SPD RecPoints • associate each point of the two central SPD modules of layer1 to all the points of layer2 within a window Δφ <0.01 rad • Calculate Zi = Z of closest approach of tracklet and nominal beam axis. • Fill histograms of Zi with 3 different bin sizes • 5 m (fine: for the actual measure) • 100 m (coarse: to single out the peak corresponding to the actual vertex) • 300 m (very coarse: used when the number of tracklets is very low) • Define a z window (200 or 600 m wide)around the “peak” of the coarse histogram. • Window adjusted to be symmetric around the centroid of the peak • Calculate on the “fine” histogram: • Zv = average of the Zi of the tracklets in the window • Dispersion parameter (= sqrt of the sample variance) Layer 1 Beam axis

5. Number of contributors • GetNContributors() to check the “quality” of the vertex • >0  Vertex OK • 0  error in the vertex finding procedure • -1  no tracklets • recoverable with iterative procedure • -2  no recpoints in SPD

6. Performance: efficiency • Black = beam in nominal position • Red = beam offset 500 mm – assumed unknown • Blue = beam offset 500 mm – assuming to know the beam position • Optimized with an iterative procedure to reduce the fraction of events with no tracklets (NContributors=-1) • Committed on June 1st (AliITSVertexerZ Rev 1.11)

7. Performance: resolution and bias • Black = beam in nominal position • Red = beam offset 500 mm – assumed unknown • Blue = beam offset 500 mm – assuming to know the beam position • Bias reduced (from ≈20 to ≈5 mm) since Rev. 1.14 • Committed in HEAD, but not in v4-04-Release • Good performance for beam offset of 500 mm if the offset is known

8. Peformance: pulls • From distributions of Δz/Error • error on the zmeasured as given by AliITSVertexerZ • RMS of pull distributions should be 1 for gaussian estimators • to be checked at higher multiplicities

9. Vertexer3D – the method • Pairs of RecPoints on layer 1 and layer 2 taken as candidate tracklets • selection cut: Δφ <0.01 rad (i.e. straight lines) • Tracklet pairs are combined and selected according to: • small DCA • Intersection close to beam axis • Intersection in the diamond region • Use tracklets as “straight-line-tracks” and apply the same vertex finder algorithms used with ESD tracks (AliVertexerTracks) • Background tracklets must be removed as much as possible before applying the algorithm tuning of these cuts presently under study Layer 2 Layer 1 Beam axis

10. Vertexer3D - efficiency • Overall efficiency ≈ 65% • Efficiency improvements under study

11. Vertexer3D – bias and resolution • Bias not present • Hint that the problem with the VertexerZ is in the vertexing algorithm and not in SPD geometry • Good resolution!

12. Vertexer 3D and beam offset • Black = beam in nominal position • Red = beam offset 500 mm – assumed unknown • Blue = beam offset 500 mm – assuming to know the beam position • Large bias on X (the coordinate where the beam has the offset) • ≈ 1/2 of the beam offset • Due to Δφ< 0.01 rad selection cut (select straight lines pointing to the beam axis given as input)

13. Correcting the 3D bias (I) • First (very naïve) idea: iterative procedure • For each event use x, y vertex positions found in iteration i as nominal beam positions for iteration i+1 • Works sufficiently well for high multiplicity (i.e. good resolution) • Price to pay: loss of resolution • due to events with worse vertex determination at the 1st iteration? • May be recovered using nominal positions averaged over several high multiplicity events

14. Correcting the 3D bias (II) • Second (very naïve) idea: enlarge Δφ cut • From trigonometry: 500 mm of offset give a max. Δφ of 0.011 rad. • Does not allow to completely cancel the bias • no improvement when enlarging Δφ from 0.03 to 0.05 • Price to pay: loss of resolution • Main drawback: requires huge enlargement in case of larger offsets • Third idea (presently under development): change the tracklet selection • use a DCA cut between pairs of tracklets crossing the beam pipe instead of the Δφ cut

15. Pile-up • Expected interaction rate = 2×105 Hz at a luminosity of 5×1030 cm-2s-2 • 1 interaction every 200 bunch crossings • Foreseen SPD strobe duration is 200 ns • 8 bunch crossings (0.04 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: found the vertex of the event with higher multiplicity • Vertices with distances <600 μm: found an intermediate value of z • 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