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A vertex trigger for LHCb

This presentation discusses the use of the Si vertex detector at the first and second trigger levels in the LHCb experiment. It covers the L0 pile-up veto, L1 high PT and impact parameter triggers, as well as the L2+L3 triggers for high PT, displaced vertex, B-mass, and PID. The performance and efficiency of the pile-up veto algorithm and the L1 vertex trigger implementation are also discussed.

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A vertex trigger for LHCb

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  1. The trigger for LHCb … .. and the use of the Si vertex detector at the first and second trigger levels Vertex2002 Hawai’i, 7 Nov 2002 Niels Tuning (CERN) (on behalf of LHCb) A vertex trigger for LHCb Niels Tuning - Vertex 2002

  2. LHCbA Large Hadron Collider Beauty Experiment for Precision Measurements of CP-Violation and Rare Decays • Colliding beams: • 25 ns • 7 TeV x 7 TeV pp • L = 2.1032 cm-2 s-1 • (visible) = 68 mb • (ppbbX) = 0.5 mb •  ~1012 bb / year • BR(interesting channels) ~10-2 – 10-9 • Finding B-mesons: • High PT decay products • Large lifetime  sec.vertex • Invariant mass A low multiplicity B+- event LHCb trigger = looking for a needle in a haystack… …every 25 ns! Niels Tuning - Vertex 2002

  3. LHCb Trigger – Overview • L0: high PT • Pile-up Veto, using VETO detector • High ET calorimeter objects • High PT muons • L1: high PT + impact parameter • High impact parameter tracks, using VELO detector • High PT tracks, using TT detector and L0 info • L2+L3: high PT + displaced vertex + B-mass + PID • Use tracking stations and RICH Niels Tuning - Vertex 2002

  4. LHCb detector TT T1 T2 T3 L0 veto L1 L0 trigger Niels Tuning - Vertex 2002

  5. 320 strips 16 mm 84 mm L0 – Pile-up VETO(L0 = first trigger level) • Purpose: remove multiple interactions • Nominal luminosity: L = 2.1032 cm-2 s-1 • Single : Double : Triple  16 : 4 : 1  75% : 20% : 5% • Why? • More difficult to find high IP tracks at L1 • Reduce bandwidth for L0 • Detector: • 2 Si disks (4 sensors) • Same sensors as VErtex LOcator (see talk J.Palacios) • Only R information • Use Beetle chip • OR of 4 strips: comparator output of 4 channels •  1280 channels for 2 disks Niels Tuning - Vertex 2002

  6. Rb (cm) true all Ra (cm) combinations Z vtx (cm) L0 – Pile-up VETO algorithm • Calculate vertex for all combinations of 2 points a and b. • Find highest peak (PV) • Remove the hits and find 2nd peak • Veto if peak>threshold • (Zvtx)  2.8 mm, (beam)  53 mm Niels Tuning - Vertex 2002

  7. Same L0 output rate! L0 – Pile-up VETO performance • B+-L0 efficiency increase • from 50% to 60% •  the L0 PT,HADR threshold can be lowered from 4 GeV to 3.6 GeV • Reduce bandwidth and enhance purity: • Pileup VETO vetoes ~15% of all events • Vetoed events are more likely to trigger • Only small inefficiency for single interactions: ~5% • Reject ~30% of multiple interactions (NB: multiple interactions include inelastic+elastic !) Niels Tuning - Vertex 2002

  8. CPU SCI RU Scheduling network L1 trigger: vertex trigger(L1 = second trigger level) • Implementation: • Clustering in FPGA on front-end • Send data to RU (3-4 GB/s) • CPU-farm: • 300 – 400 CPUs • 2D torus • Use scheduler • Prototype with 32 CPUs running at 1.24 MHz • Buffer depth: 1820 events  Latency = 1.65 ms • Strategy: • Find 2d-tracks with R-sensors and reconstruct vertex • Reconstruct high-impact parameter tracks in 3d • Extrapolate to TT through small magnetic field  PT • Match track to L0 Muon objects  PT and PID • Select B–events using impact parameter and PT information Niels Tuning - Vertex 2002

  9.  sensor 2048 strips R sensor 2048 strips  sensor: stereo angle +10o,-20o R sensor: 4 inner and 2 outer  sectors L1: VErtex LOcator(see talk by Juan Palacios) • Si: 220 m thick, n-on-n, • Pitch:  37–98 m, R 40–92 m • Sens. area: 0.8 cm < R < 4.2 cm • 21 stations (84 sensors) • -17.5 cm < Z < 75 cm • 170,000 channels RF foil: Very thin “beampipe” to separate prim. and sec. vacua Niels Tuning - Vertex 2002

  10. L1: input data Cluster resolution: (testbeam) =14 m • Velo clusters: • Clusters are found in FPGAs per groups of 32 strips. • Digital (offline is analog) • ~1000 clusters • ~0.1% noise clusters (200) • TT clusters • ~300 clusters • L0 objects •  3 muons • Some calorimeter data Resolution (m) Pitch (m) 1000 clusters (simulation) Niels Tuning - Vertex 2002

  11. L1: track reconstruction ~60 • Look only for R-clusters: 2d RZ-tracks • Fast! • Find “triplets” of clusters • Combine triplets • ~98% efficiency for B-tracks multiplicity 2d tracks in a 90o sector: • VELO is being redesigned to 45o sectors: • faster L1 tracking • lower noise • less 2d tracks Z vtx histogram X,Y vtx Niels Tuning - Vertex 2002

  12. Impact parameter: Vertex resolution: 2d 3d X Z Lifetime L1: primary vertex • Primary vertex reconstructed with 2d tracks • XY information from  segmentation • Flight direction of B is forward  RZ projection of impact parameter contains most information Niels Tuning - Vertex 2002

  13. L1: PT information – L0 CAL • Match VELO track to Muon from L0: •  PID •  Momentum • Efficient selection of • BsJ/()  • BdJ/() Ks VELO TT MUON MAGNET dp/p=4.8% • Enhance -tagged sample Niels Tuning - Vertex 2002

  14. L1: algorithm (1)Preliminary J/ mass • Good momentum resolution, cut on J/ mass: • ~60% of events contain both muons • <1% min.bias retention • OR require 1 muon with high PT, high IP • ~75 % efficiency • ~3% retention • Work ongoing: • achieved 90% eff. using neural net Performance with all event info(except TT) Niels Tuning - Vertex 2002

  15. RICH-1 MAGNET VELO TT 141 cm 116 cm L1: Trigger Tracker • TT = Tracking station before the magnet • Design still under study • Si: 400 - 500 m thick • Wide pitch: 200 m • Sensor dimensions:7.8 x 11.0 cm2 • 4 layers (x,u [30 cm gap] v,x) • Stereo angle: 00, -50, +50, 00 • To be optimized • 836 sensors (~7 m2) Niels Tuning - Vertex 2002

  16.  dp/p 30 GeV --- 30% ---  p (GeV) L1: PT information – TT • High impact parameter 2d tracks are reconstructed in 3d and extrapolated to TT1 • Magnetic field between VELO and TT: •  B dl  0.1 Tm • Ensures momentum information • dp/p ~ 30% Niels Tuning - Vertex 2002

  17. Bd+- BsDs-K+ Signal Minimum bias L1: algorithm (2)Preliminary • Get two highest PT tracks, using TT • Consider impact parameter and PT of these tracks • Look in plane IP/(IP) vs PT Niels Tuning - Vertex 2002

  18. Events Tracking+Vertexing ---- 17 ms time (ms) L1: performance - timing • Remember: latency ~1.7 ms • Possibly x32 more. • A more flexible system is under study were CPUs from DAQ can be used for the TRIGGER and vice versa • Tracking+Vertexing: < 20 ms • 2007 CPUs: x8 faster • Optimize algorithm+code •  in the right ballpark! • 2d tracking: ~70% • Vertex: ~15% • 3d tracking (a few) ~15% Niels Tuning - Vertex 2002

  19. L0L1 L1L2 L1: performance • Efficiency vs retention: (Example B+- ) Bd+- • Expected overall trigger performance: (cumulative) L1+L0 info L1 output rate (MHz) 40 kHz L1+PT info Bd+- efficiency Niels Tuning - Vertex 2002

  20. Conclusions • The pileup-VETO detector efficiently rejects multiple primary vertices @ 40 MHz at L0 • The VELO detector reconstructs primary vertices at L1 with excellent resolution • (Zvtx)  60 m •  high impact parameter tracks can be identified • The TT detector - or L0 information - enables measuring the momentum of tracks •  Efficient L1 selection algorithms under study • Efficiency of 70% (90%) for B+- ( BJ/() Ks) reachable at 4% minimum bias retention Niels Tuning - Vertex 2002

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