Primary vertex reconstruction with the SPD

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 ?f lt0.01 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
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 ?f lt 0.5 rad, DCA to beam lt 2.5
  cm, zINTlt5.3 cm
• Combine tracklet pairs and select them according
  to
• small DCA (lt 1 mm) between the two tracklets
• Tracklet intersection close to beam axis (rINT lt
  2.5 cm )
• Tracklet intersection in the diamond region
  (zINTlt 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 ?f lt 0.01 rad, DCA to beam lt
  0.5 cm, zINT - zVERTlt0.5 cm
• Combine tracklet pairs and select them according
  to
• small DCA (lt1 mm) between the two tracklets
• Tracklet intersection close to the estimated
  vertex (Dr lt 0.5 cm)
• Tracklet intersection in the diamond region (Dz lt
  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 ?s14 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 (AliMultiplicityGetNum
  berOfTracklets)
• 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

8
More on efficiency

• GetNContributors() for VertexerZ ?information on
  where inefficiencies come from
• gt0 ? 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. 1.17 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 ylt2

• 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 gt 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

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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

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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 --gt p-p p-A
  Pb-Pb interactions
• No bias
• Error calculated for all events but slightly
  underestimated (pulls1.5)
• 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
• No bias
• 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
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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)
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Residuals 3D (X)
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Residuals 3D (Y)
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Residuals 3D (Z)
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VertexerZ at higher multiplicities
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VertexerZ at higher multiplicities
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VertexerZ at higher multiplicities
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Pile-up

• Expected interaction rate 2105 Hz at a
  luminosity of 51030 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 gt600 µm the vertex of
  the event with higher multiplicity is found
• Vertices with distances lt600 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