Primary vertex reconstruction with the SPD

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

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

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

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)

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 • Combine tracklet pairs and select them according to: • 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

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

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%

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%)

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

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

Vertexer3D: average of residuals • No apparent bias

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

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

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)

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

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

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

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 VertexerZ vs. vertex position (II)

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

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 • 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

Backup slides

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

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

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

Efficiency – detail (Z)

Residuals 3D (X)

Residuals 3D (Y)

Residuals 3D (Z)

VertexerZ at higher multiplicities

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

Primary vertex reconstruction with the SPD