1. Primary vertex reconstruction with the SPD
E. Crescio, M. Masera, F. Prino
INFN and Università di Torino
A. Dainese
INFN LNL
ALICE PHYSICS WEEK – Münster – February 12th 2007

2. Outline Summarize status of SPD vertexing algorithms
AliITSVertexerZ
AliITSVertexer3D
Show recent modifications/upgrades
AliITSVertexer3D is relatively new (first version commited to CVS on October 30th 2006) and is still under development/optimization
Answer to specific issues arisen in First Physics meeting of January 19th 2007
Vertexeing with tracks not discussed
see talk by A. Dainese on Friday

3. AliITSVertexerZ – the method
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
HijingParam1500
 New version, committed to CVS on Feb. 9th

4. Z distributions for single events: examples
p-p event
Z distributions for single events: examples
signal
Combinatorial background
Higher multiplicity event
(Hijing parametrized
with 1500 charged
primary particles
in 2 units of rapidity)

5. Vertexer3D – the method
Build tracklets (= straight lines) from pairs of Clusters on layers 1, 2
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 ESD 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
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

6. Events used in this study
Event generation
AliRoot v4-04-Release of June 2007
pp collisions (kPyMb) at s=14 TeV
Vertex smearing on x,y (50 mm) and z (5.3 cm)
NO TRIGGER information
4 sets of pp events:
9800 events with beam centered in (0, 0)
9400 events with beam centered in (500 mm, 0)
9200 events with beam centered in (5 mm, 0)
10000 events with beam centered in (1 cm, 0)
VertexerZ and Vertexer3D performance studied in bins of Ntracklets in SPD (AliMultiplicity::GetNumberOfTracklets)
Last version of AliMultiplicity on CVS gives the number of associated Clusters as discussed on January 19th meeting

7. Overall efficiency
Efficiency = Number of event with vertex / Total number of events
Vertexer3D efficiency increased by ≈4% with DCA cut optimization
Vertexer3D requires at least 2 selected tracklets
Events with just one tracklet are ≈10%
VertZ
Vert3D
kPyMb
84%
72%
Non diffr.
98%
90%
Single Diffr.
53%
33%
Double diffr.
57%
32%

8. More on efficiency
GetNContributors() for VertexerZ information on where inefficiencies come from:
>0  Vertex OK (≈84% of events)
0  error in the vertex finding procedure (0%)
-1  no tracklets (≈0.25%)
-2  no Clusters in 1 SPD layer (≈16%)

9. Residuals
Zfound – Ztrue distribution for VertexerZ in the 6 multiplicity bins
Good agreement between gaussian sigma and histo RMS

10. 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 of AliITSVertexerZ.cxx

11. Vertexer3D: average of residuals
No apparent bias

12. Resolutions Resolution improved for both vertexers in last weeks
New procedure for VertexerZ
Cut optimization for Vertexer3D

13. Pulls Distributions of (Zfound-Ztrue)/Zerr
Only for VertexerZ, error calculation for Vertexer3D still under development

14. VertexerZ: pulls
Pull distribution has RMS ≈ 1.5 independent of multiplicity
Checked also with higher multiplicity events
Errors slightly underestimated
Other sources of error:
Multiple scattering (Beam Pipe +Layer 1)  found to be negligible
Beam smearing effect (see next slide)
Error on radius (under study)

15. Beam smearing contribution
HijingParam generation with 500 particles in |y|<2
Beam X,Y smearing = 50 mm
Pull = 1.5
Beam X,Y smearing = 0
Pull = 1.36

16. VertexerZ vs. vertex position (I)
Efficiency and RMS worsen for large | zvertex|
Due to smaller number of tracklets in acceptance
Average of residual stable against zvertex
No bias as a function of zvertex

17. Ntracklets vs. Ztrue SPD extends from z=-14 cm to z= 14 cm
For |Zvertex| >≈ 14 cm the number of SPD tracklets (and ESD tracks with SPD points) starts to decrease

18. VertexerZ vs. vertex position (II)
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

19. Beam offset in X and 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

20. Summary Vertexer Z (items under study are in red color)
Very high overall efficiency (98% for non diffractive events)
Inefficiencies essentially due to events with no Clusters on SPD layer 2
Suitable for all multiplicities --> p-p ; p-A; Pb-Pb interactions
No bias
Error calculated for all events but slightly underestimated (pulls≈1.5)
Performance dramatically worsens with increasing X,Y beam offset
If the X,Y position of the beam is known the performance is completely recovered
Under study the application for pile-up detection
Vertexer3D (items under study are in red color)
Efficiency limited by the need of at least 2 tracklets
Performance remains practically the same also for large beam offsets
CPU time and memory consumption presently under test
Important for application to Pb-Pb interactions
Error calculation under study (VertexFitter of AliVertexerTracks should be used)
Under study the application for measuring average x,y beam position in the LHC fill

21. Backup slides

22. VertexerZ and beam offset (I)
Beam offset up to 1 cm assumed unknown
Efficiency not affected
Resolution dramatically worsens with increasing beam offset

23. 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

24. Vertexer3D and beam offset
Beam offset up to 1 cm assumed unknown
Performance maintained

25. Efficiency – detail (Z)

26. Residuals 3D (X)

27. Residuals 3D (Y)

28. Residuals 3D (Z)

29. VertexerZ at higher multiplicities

30. VertexerZ at higher multiplicities

31. VertexerZ at higher multiplicities

32. 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