The LHCb VErtex LOcator

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The LHCb VErtex LOcator
Tracking, Vertexing and Triggering in a harsh
radiation environment
Doris Eckstein, CERN
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The LHCb Experiment

• Dedicated to the study of CP violation in the B
  system

• LHC -pp collisions _at_ 14TeV
• -full spectrum of B hadrons (
  )
• -high intensity

• LHCb -single arm spectrometer
• -15-300mrad angular acceptance
• -recently optimised to minimise
• material

Vertex Locator
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The requirements for the VELO

• Reconstruction of pp interaction vertex
• wide spread of interaction region in z
  (sz5.3cm)
• many stations around z0
• Reconstruction of b-hadron decay vertex
• short track extrapolation distances
• measure at smallest radii
• minimal multiple scattering
• minimise material between interaction and
• first measured point
• VELO is the tracker before the LHCb magnet
• Angular coverage of full downstream detector
• angular range
• 21 Silicon stations allowing to measure at
  least 3 hits/track
• 2 R- and 2 F-measuring sensors per station
• overlap (acceptance, alignment)

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The VELO Design
Mechanical design as consequence of these criteria

• VELO sensors as close as possible to beam
• no beam pipe, sensors 7mm away from beam

• Injection larger aperture required retraction by
  30mm
• Protect sensors against RF pickup from the LHC
  beam
• Protect the LHC Vacuum from possible outgasing of
  detector modules

• Place sensors in a secondary vacuum Roman pots

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Secondary Vacuum RF Foil

• Made from 250mm thick Al
• Inner corrugations
• Minimal material before the first
• sensor is hit
• Outer corrugations
• allow for overlap of detector halves
• for full azimuthal coverage and for
• alignment
• Prototyping at NIKHEF
• method Hotgas Forming
• full size foil
• vacuum tight and stiff

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More requirements for the VELO

• Rejection of multiple interactions in L0 Trigger
• additional VETO stations upstream of
• interaction point
• Fast stand-alone tracking and vertexing for L1
  Trigger
• motivates R-and F-measuring sensors
• Design allows to optimise resolution vs.
• number of channels

• Baseline design of sensors
• Active area 8mm to 42 mm
• R measuring sensors
• division into 45o sectors
• F measuring sensors
• inner/outer region
• increasing pitch from inner towards outer radii
• 2nd metal layer to route signal to chips

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Second Level Vertex Trigger

• Forward flight direction of B Rz impact
  parameter
• First step 2D
• -build Triplets of clusters in R sensors
• -form tracks in Rz
• -fill z-vertex histogram
• -preselect large impact parameter
• tracks in Rz
• -match to m

Second step 3D -preselected (5-10) 2D tracks
-add information from F sensors for 3D
reconstruction -match to L0 and TT
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Optimisation of VELO sensor design

• L1 Trigger speed, number of ghost tracks
• sector division
• Clustering/tracking efficiency Signal to Noise
• 
  strip length

• Options of design studied (keeping constant
  number of strips)
• Different strip pitches
• Does impact parameter
• resolution suffer?
• max. 5
• design chosen with gradual
• increase of pitch (40mm
• to 103mm)

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Even more requirements for the VELO
This detector has to operate in an extreme
radiation environment

• Maximum irradiation per station
• 5x1012 to 1.3x1014 neq/cm2/year
• Strongly non-uniform dependence on R and station
  (z)
• Maintain a good S/N performance for at
  least 2 years (replacement)
• Extensive RD program to select
• Sensor and Front-End chip

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Prototype testing in Lab and Test beam
Test beam CERN SPS (120 GeV p and m)
Irradiated Not irradiated
Irradiated Not irradiated
DELPHI-ds sensor Irradiated/Not irradiated
PR03 sensors
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Tests of the p-on-n prototype

• Efficiency of cluster reconstruction close to
  track
• Box size efficiency

• PR02 F-sensor routing lines in outer region
• none in
  inner region

• More efficient in inner region
• Less efficient in irradiated region

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P-on-n vs n-on-n

• P-on-n
• -Fraction of charge in routing line
• reaches 20 in outer region
• -5 in inner region
• -Detector has undepleted and insulating
• layer after irradiation
• -Expected to be less for n-on-n

• Compare efficiency for p-on-n and
• n-on-n for different depletion depth
• P-on-n efficiency degrades fast
• N-on-n efficiency 100 for only 60 depletion
  depth

Chose n-on-n for VELO
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Front-End chip decision

• Two parallel developments SCTA_VELO (DMILL) and
  Beetle (0.25mm CMOS)
• Features 128 input channels
• 40MHz sampling (LHC clock)
• Hybrids equipped with 16 chips tested in test
  beam
• Decision taken at beginning of this year to use
  Beetle
• Performance equally good
• Availability, radiation hardness and usage in
  LHCb

SCTA_VELO
Beetle1.1
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Beetle chip tests

• Currently Beetle1.2 under study
• Test set-up in Lab with one chip reading out a
  n-on-n 200mm thick
• prototype sensor
• Sensor close to final design
• Measure S/N with Sr source
• Prepare for test beam
• Hybrid with 16 Beetle1.2
• chips reading out a full
• sensor
• MPW submission of improved
• Beetle1.3 soon

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Summary Outlook

• VELO design is close to completion
• Important decisions finalised sensor design
• choice of Front-End chip
• Successfully tested module prototypes consisting
• of Sensor, Hybrid and 16 chips
• Plan to have first module end of 2003
• Complete VELO in 2006