1 / 27

The LHCb VELO and its use in the trigger

The LHCb VELO and its use in the trigger. Thomas Ruf. Vertex 2001 23-28 September 2001. Introduction Silicon R&D Second Level trigger Pile up VETO trigger. Introduction. VE rtex LO cator of the LHCb experiment. LHCb requires:.

oistin
Télécharger la présentation

The LHCb VELO and its use in the trigger

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The LHCb VELO and its use in the trigger Thomas Ruf Vertex 200123-28 September 2001 Introduction Silicon R&D Second Level trigger Pile up VETO trigger

  2. Introduction VErtex LOcator of the LHCb experiment LHCb requires: LHCb: Systematic studies of CP in the beauty sector by measuring particle - antiparticletime dependent decay rate asymmetries. • Reconstruction of pp-interaction point • Reconstruction of decay vertex of beauty and charm hadrons • Standalone and fast track reconstruction in second Level trigger (L1)

  3. Introduction Vacuum Vessel  silicon sensors are placed in vacuum ~1m Number of silicon sensors: 104 Area of silicon: 0.32m2Number of channels: 204,800 VELO Overview Precise vertexing requires, to be: • as close as possible to the decay vertices • with a minimum amount of material between the first measured point and the vertex

  4. Introduction VELO Setup Positioning and number of stations is defined by the LHCb forward angular coverage of 15mrad <  <390mrad together with the inner/outer sensor dimensions Also need to account for • spread of interaction region: s =5.3 cm • partial backward coverage for improved primary vertex measurement One detector half: Interaction region p p forward Final configuration: • 25 stations • 1 station = 2 modules (left and right) • 1 module = 2 sensors

  5. Introduction Radiation • Sensors have to work in a harsh radiation environment: • max. fluences: 0.5 x 1014 - 1.3 x 1014neq / cm2 / year N*r-a a = 1.6  2.1 neq = damage in silicon equivalent to neutrons of 1 MeV kinetic energy

  6. Introduction Sensor Design • Azimuthal symmetry of the events suggests sensors which measure f and R coordinates. • Advantages of Rf geometry: • Resolution: Smallest strip pitch where it is needed, optimizing costs/resolution. • Radiation: Short strips, low noise, (strixels, 40mm x 6283mm) close to beam • L1: Segmented R-sensor (45o) information is enough for primary vertex reconstruction and impact parameter measurement. • Inner radius is defined by the closest possible approach of any material to the beam:8mm (sensitive area) LHC machine • Outer radius is constrained by the practical wafer size: 42mm

  7. Introduction • 2048 strips / sensor  16 FE chips Sensor Design • Strips are readout by using a double metal layer • Analog readout for better hit resolution and monitoring

  8. Tested Prototypes Double sided DELPHI sensor thickness: 310 m Silicon R&D VELO Technology Choices • Thickness • Oxygenation • Cryogenic Operation • Segmentation p or n strips ? VELO Design Challenges • Varying strip lengths • Double metal layer • Regions of fine pitch • Large and non-uniform irradiation

  9. Silicon R&D VELO Technology Choices • Thickness • Oxygenation • Cryogenic Operation • Segmentation p or n strips ? VELO Design Challenges • Varying strip lengths • Double metal layer • Regions of fine pitch • Large and non-uniform irradiation Tested Prototypes Hamamatsu n-on-n thickness: 300 m

  10. Silicon R&D VELO Technology Options • Thickness • Oxygenation • Cryogenic Operation • Segmentation p or n strips ? VELO Design Challenges • Varying strip lengths • Double metal layer • Regions of fine pitch • Large and non-uniform irradiation Tested Prototypes MICRON p-on-n thickness: 200/300 m

  11. Silicon R&D R&D Results First confrontation with alignment issues ~5% of VELO sensors tested with beam 40MHz readout chip SCT128A Resolution vs angle and pitch See talk of M. Charles Best resolution: 3.6 m Trigger performance simulation http://lhcb-vd.web.cern.ch/lhcb-vd/TDR/TDR_link.htm May 2001 Results about irradiated sensors

  12. Silicon R&D n-on-n Prototypes Variable irradiation with 24 GeV protons at the CERN-PS Sensor readout with 25ns electronics (SCT128A) Repeater card S/N = 21.5 3-chip hybrids Most irradiated region corresponds to 2 years of LHCb operation for the innermost sensor part. Temperature probes

  13. Silicon R&D Resolution [mm] Charge Collection Efficiency p-on-n Prototypes After irradiation, silicon needs to be fully depleted, otherwise: • Resolution degrades

  14. Silicon R&D p-on-n Prototypes After irradiation, silicon needs to be fully depleted, otherwise: • Charge is lost to double metal layer 2nd metal layer Size of boxes proportional to CCE

  15. Silicon R&D VELO Technology Choices • n-strips, safest solution, sensors can be operated underdepleted • Thickness = 300 m, OK for radiation hardness and material budget • Oxygenation: nice to have, but not mandatory, less of interest for p-on-n • Fully depleted for >2 years, Ubias<400VPrototype n-on-n sensors even for 4 years • With n-on-n, can accept 40% under-depletion  extending the lifetime even further • Operation model: • 100 days constant fluence, T=-50C • 14 days at +22oC Prototype n-on-n sensors behaved much better than expected for “standard” silicon

  16. p p Data is kept in analog/digital pipelines L0: look for high pt particleshadron (HCAL), muon (MUON) or electron (ECAL) pile up VETO 40 MHz  1 MHzFixed latency: 4.0 ms L0 YES: Data is digitized, but stays in memory of individual detector electronics L1: vertex trigger 1 MHz  40 kHz L1 buffer = 1927 events L1 YES: Digitized data is read-out, event building  CPU farm L2: refined vertex trigger 40 kHz  5 kHz L3: online event selection 5 kHz  200 Hz LHCb Trigger System

  17. LHCb Trigger Occupancy < 1% per channel ~ 1.5 tracks in the seeding region(innermost 450 sectors of R-sensors) track reconstruction is straightforward L1 (Vertex) Trigger Input rate: 1 MHz Output rate: 40 kHz Maximum latency: ~2 ms Purpose: Select events with detached secondary vertices Needs: Standalone tracking and vertex reconstruction

  18. L1 Vertex Trigger r 2d track reconstruction using only R-hits triplets Efficiency: 98% z Primary vertex: Histogram of 2-track crossings  zF-segmentation  x,y sz,y 70 mm sx,y 20 mm Reject events with multiple interactions Impact Parameter Test beam 3d reconstruction for tracks with large impact parameter Efficiency: 95% Search for 2 tracks close in space Calculate probability to be a non b-event based on impact parameter and geometry Algorithm

  19. L1 Vertex Trigger 150-250 processors Basic Idea: x24 x100 1 MHz CPU farm SWITCH Readout Unit Digitizer Board:zero-suppression,common mode correction, cluster finding 2d torus topology based on SCI shared memory 680 Mb/s, limited by number of senders Implementation Challenge: 4 Gb/s and small event fragments of ~170bytes

  20. L1 Vertex Trigger Bp+p- Minimum bias L1 latency Execution Time of Algorithm 450 MHz Pentium III running Windows NT Expect CPU’s to be 10 times faster in 2005

  21. L1 Vertex Trigger Physics Performance Technical Proposal With new Event Generator, performance degraded by a factor of 2 ! Minimum bias retension • Main limitations: • No momentum information significance of impact parameter • No particle ID Working point40kHz Signal efficiency Link L0 objects: large pt(e, m, h) with VELO tracks. Investigate effect of possible momentum information. Recent developments:

  22. L1 Trigger New ideas Super Level 1 matching efficiency Bp+p- : 78% for one p BJ/y(mm)Ks: 96% for one m HCAL MUON ECAL Example: Matching with HCAL clusters Bdl=4Tm, pt kick=1.2GeV/c New Bp+p- BJ/y(mm)Ks Bp+p- TP BJ/y(mm)Ks L1 rate In general, gain back factor 2, in some channels even more.

  23. L1 Trigger New ideas Mini Level 1 Silicon tracking station 30% additional data to be send to L1 farm momentum resolution:s(pt)/pt 20% B0s Ds-(K+K--)K+B0d +- For final answer: See Trigger TDR in 2002

  24. LHCb Trigger Luminosity Bunch crossing frequency  30 MHz LHCb chooses luminosity for maximum number of single interactions with Cross-section L0 Pile Up VETO Purpose: Remove events with multiple interactions. Why ? Multiple interactions are more difficult to reconstruct (specially for L1) and fill bandwidth of L0 (~ 2x probability to pass L0). Input rate: 40 MHz Output rate: 1 MHz Latency: 4.0 ms b-event rate: 25kHz Number of inelastic interactions/bunch crossing as a function of luminosity At L=2x1032cm-2s-1: P>1/ P124% b-events: P>1/ P141%

  25. L0 Pile Up VETO True combinations All combinations 2 silicon R-stations upstream of VELO B A RB [cm] RB RA RA [cm] RA [cm] ZA ZPV ZB ZPV [cm] Concept If hits are from the same track:  ZPV • mask hits belonging to P1 • search next highest peak P2 (peak size S2) • classify: if S2<Slimit then single, else multiple • build a ZPV histogram • search highest peak P1

  26. L0 Pile Up VETO Implementation • 2 R-stations upstream of the VELO • OR of 4 channels, binary readout, 80Mbit LVDS, 512 lines • Frontend chip: BEETLE running in comparator mode • Large FPGA: >350k gates and >600 I/O pins. Candidates: Altera EP20K400, XILINX XC4000, … Performance: Gain of 30-40% of single bb-events at optimal luminosity

  27. Summary • Technical Design Report of the LHCb VErtex LOcator is completed: • n-on-n silicon strip sensors are the baseline. • Prototyping with different companies continues. • The VELO plays an important role in the second level trigger: • Standalone track finding, • primary vertex reconstruction, • impact parameter determination • Silicon sensors are also used in the first level trigger for a fast determination of the number of interactions.

More Related