G. Cerati (UCSD) TRK POG meeting /09/09 2 Introduction
G. Cerati (UCSD) TRK POG meeting /09/09 3 Vertex Acceptance Sample of 20 muons per event, with pT=10 GeV and random |eta| 5%.Note that this is very optimistic scenario with only 20 tracks per event; with large PU performance should dramatically degrade outside the pixel acceptance. First question: what is the maximum beam spot width we can tolerate?
G. Cerati (UCSD) TRK POG meeting /09/09 4 Vertex Density RelVal samples ( =20) 2012 geometry Phase1 geom 1.28 vtx/mm 0.65 vtx/mm MC association: sim vtx matches reco vtx if within 3σ z plot by M.Geisler to 0th order this does not depend on PU, need to check higher order effects (worse tracking performance in high density region) Second question: what is the maximum vertex density we can tolerate? Efficiency plateau down to ~1 vtx/mm. Done on low PU samples, does the picture change ay high PU? (see later in this talk)
G. Cerati (UCSD) TRK POG meeting /09/09 5 Private samples Release used: CMSSW_6_1_2_SLHC8_patch1 MinBias sample for PU: 10k events TTbar sample: 500 events Geometry: Phase1 PU conditions: AVE_140_BX_25ns Reconstruction: PU140 tracking The produced samples do not contain TrackingParticles and TrackingVertices; comparison with MC truth using GenParticles and PileupSummaryInfo
G. Cerati (UCSD) TRK POG meeting /09/09 8 Simulated Beam Spot Verified that the two samples have ~same number of simulated vertices, distributed along z with a gaussian (σ=5.3 cm) and a flat distribution (z=[-11,11] cm) respectively. z [cm]
G. Cerati (UCSD) TRK POG meeting /09/09 9 Reco Results BS shapes roughly as expected. Flat BS sample has more vertices and more HP tracks per event. caveat: HP selection depends on BS and could be not properly defined for flat BS
G. Cerati (UCSD) TRK POG meeting /09/09 10 Vertex Σp T 2 Additional vertices in flat BS sample mostly at low Σp T 2. Distribution of Max(Σp T 2 ) compatible with current statistics.
G. Cerati (UCSD) TRK POG meeting /09/09 11 Vertex merging MC association: sim vtx matches reco vtx if within 3σ z P(Merged): number of reco vertex associated to >1 sim vtx / number of sim vertices. Flat BS has flat merged rate vs z, gaussian BS has more merged vertices for |z|<5 cm.
G. Cerati (UCSD) TRK POG meeting /09/09 12 Vertex efficiency 1.28 vtx/mm0.65 vtx/mm MC association: sim vtx matches reco vtx if within 3σ z Efficiency: number of associated reco vertices / number of simulated vertices. No selection on simulated vertices (plateau at 80% efficiency). Efficiency losses can be due to: vertex not reconstructed at all, vertex reconstructed out of 3σ z, vertex merging. Efficiency vs density does not depend on PU (as expected). Flat BS has flat efficiency vs z, gaussian BS loses up to 10% efficiency at |z|<5 cm.
G. Cerati (UCSD) TRK POG meeting /09/09 13 Tracking Efficiency MC: GenParticles (from ttbar event only), association by pull (chi2<50) Efficiency: number of associated tracks / number of simulated tracks. Selection: loose track quality, p T (sim)>0.9 GeV Similar tracking efficiency for signal event for the two beam spot configurations.
G. Cerati (UCSD) TRK POG meeting /09/09 14 Conclusions Acceptance is maximum for |z|<15 cm With Phase1 detector (4-pixel layers), vertex reconstruction efficiency drops below density of 1 vtx/mm algorithms not optimized for high PU, can possibly be improved With 140PU, a flat beam spot with z=[-11,11] cm has a lower density than a gaussian with σ=5.3 cm lower merged vertices rate for |z|<5 cm higher vertex efficiency, mostly for |z|<5 cm and low Σp T 2 Signal vertex well reconstructed in both cases similar Max(Σp T 2 ) distribution similar tracking efficiency a bigger sample could help finding small effects; other variables may show bigger differences ‣ other POG/PAG are welcome to peform other tests too