Performance of ATLAS

1
Performance of ATLAS CMS Silicon Tracker
International Europhysics Conference
on High Energy Physics EPS 2003, July 17th-23rd
2003, Aachen, Germany

• Alessia Tricomi
• University and INFN Catania

2
What LHC means

• p-p collision _at_ vs 14 TeV
• bunch spacing of 25 ns
• Luminosity
• low-luminosity 21033cm-2s-1 (first years)
• high-luminosity 1034cm-2s-1
• 20 minimum bias events per bunch crossing
• 1000 charged tracks per event
• Radius 2cm 10cm 25cm
  60cm
• NTracks/(cm225ns) 10.0 1.0 0.10 0.01
• Severe radiation damage to detectors

Plus 22 minimum bias events
Challenging requirements for the Tracking system
3
Tracker Requirements

• Efficient robust Pattern Recognition algorithm
• Fine granularity to resolve nearby tracks
• Fast response time to resolve bunch crossings
• Ability to reconstruct narrow heavy object
• 12 pt resolution at 100 GeV
• Ability to operate in a crowded environment
• Nch/(cm225ns) 1.0 at 10 cm from PV
• Ability to tag b/t through secondary vertex
• Good impact parameter resolution
• Reconstruction efficiency
• 95 for hadronic isolated high pt tracks
• 90 for high pt tracks inside jets
• Ability to operate in a very high radiation
  environment
• Silicon detectors will operate at -7C ? -10C to
  contain reverse annealing and limit leakage
  current

4
Two different strategies

• ATLAS Inner Detector
• ID inside 2T solenoid field
• Tracking based on many points
• Precision Tracking
• Pixel detector (2-3 points)
• Semiconductor Tracker
• SCT (4 points)
• Continuous Tracking
• (for pattern recognition e id)
• Transition Radiation Tracker
• TRT (36 points)

5
Two different strategies
CMS has chosen an all-silicon configuration

• CMS Tracker
• Inside 4T solenoid field
• Tracking rely on few measurement layers, each
  able
• to provide robust (clean) and precise coordinate
  determination
• Precision Tracking
• Pixel detector (2-3 points)
• Silicon Strip Tracker (220 m2) SST (10
  14 points)

22m Long, 15m Diameter, 14000 Ton Detector
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The ATLAS Pixel Detector

• 3 barrel layers
• r 5.05 cm (B-layer), 9.85 cm, 12.25 cm
• 3 pairs of Forward/Backward disks
• r 49.5 cm, 6.0 cm, 65.0 cm
• 2 of tracks with less than 3 hits
• Fully insertable detector
• Pixel size
• 50 mm x 300 mm (B layer) 50 mm x 400 mm
• 2.0 m2 of sensitive area with 8 x 107 ch
• Modules are the basic building elements
• 1456 in the barrel 288 in the endcaps
• Active area 16.4 mm x 60.8 mm
• Sensitive area read out by 16 FE chips each
  serving a 18 columns x 160 row pixel matrix
• Several changes from TDR

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The ATLAS SCT Detector

• Endcap 9 wheel pairs
• pitch 70 - 80 mm
• 3 types of modules
• Inner (400)
• Middle (640 incl. 80 shorter)
• Outer (936)

• Barrel 4 layers
• pitch 80 mm
• radii 284 335 427 498 mm
• 2112 modules, with 2 detectors per side,
• read out in the middle

• All detectors are double-sided
• (40 mrad stereo angle)
• 4088 modules
• 61 m2 of silicon
• 6.3 x 106 channels

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The CMS Pixel Detector

• 3 barrel layers
• r 4.1 4.6 cm, 7.0 7.6 cm, 9.9 10.4 cm
• 32 x 106 pixels
• 2 pairs of Forward/Backward disks
• Radial coverage 6 lt r lt 15 cm
• Average z position 34.5 cm, 46.5 cm
• Later update to 3 pairs possible
• (ltzgt 58.2 cm)
• Per Disk 3 x 106 pixels
• ? 3 high resolution space points for h lt 2.2
• Pixel size 150 mm x 150 mm driven by FE chip
• ? Hit resolution
• r-f s 10 mm
• (Lorentz angle 28 in 4 T field)
• r-z s 17 mm
• Modules are the basic building elements
• 800 in the barrel 315 in the endcaps

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The CMS Silicon Strip Tracker
9648128 strips ? channels 75376 APV
chips 6136 Thin sensors 18192 Thick
sensors 440 m2 of silicon wafers 210 m2 of
silicon sensors 3112 21512 Thin
modules 5496 21800 Thick modules ss
dsb-to-b (100mrad) 17000 modules 25000000
Bonds p strips on n-type bulk lt100gt crystal
lattice orientation Polysilicon resistors to
bias the strips Strip width over pitch
w/p0.25 Metal overhang and multiguard structure
to enhance breakdown performance

• Outer Barrel (TOB) 6 layers
• Thick sensors (500 mm)
• Long strips

• Endcap (TEC) 9 Disk pairs
• r lt 60 cm thin sensors
• r gt 60 cm thick sensors

Silicon sensors
CF frame
Strip length ranges from 10 cm in the inner
layers to 20 cm in the outer layers. Pitch ranges
from 80mm in the inner layers to near 200mm in
the outer layers

• Inner Disks (TIB) 3 Disk pairs
• Thin sensors

• Inner Barrel (TIB) 4 layers
• Thin sensors (320 mm)
• Short strips

Black total number of hits Green double-sided
hits Red ds hits - thin detectors Blue ds hits
- thick detectors
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Track reconstruction efficiency
Efficiency for p is lower compared to m due to
secondary interactions in the Tracker Efficiency
can be increased by relaxing track selection
CMS
Global efficiency selected Rec.Tracks / all
Sim.Tracks Algorithmic efficiency selected
Rec.Tracks / selected Sim.Tracks (Sim.Track
selection at least 8 hits, at least 2 in
pixel) Global efficiency limited by pixel
geometrical acceptance
Efficiency for particles in a 0.4cone around jet
axis No significant degradation compared to
single pions Loss of efficiency is dominated by
hadronic interactions in Tracker material
11
Track resolutions
ATLAS CMS have similar performance
Good track parameter resolution already with 4 or
more hits
s(d0) mm
s(pT)/pT
For lower pt tracks multiple scattering becomes
significant and the h dependence reflects the
amount of material traversed by tracks
12
ATLAS CMS performances

• ATLAS and CMS have thick trackers
• each pixel layer contributes gt2 X0
• plus global support and cooling structures and
  thermal/EMI screens
• The momentum impact parameter resolution
  depends strongly on
• radius of innermost pixel layer
• thickness of pixel layers
• radius and thickness of beam pipe
• Example
• effect of the new ATLAS layout now (TDR)

s(d0) mm
s(1/pT) TeV-1
13
The dark side material budget in the Tracker
Degrades tracking performance, due to multiple
scattering, Bremsstrahlung and nuclear
interactions (see pt resolution and
reconstruction efficiency)
Dominates energy resolution for electrons
14
ATLAS CMS Silicon Tracker vertexing
At LHC design luminosity 20 interaction per
beam crossing spread out by s(z)5.6 cm
Identification of primary and secondary vertices
fundamental
Pixel detectors allow fast vertex reconstruction
with s(z)lt50mm
Slower but better resolution (15 mm) achievable
using the full Tracker
s 40mm
easy channel difficult channel
s 16mm
uu 100 GeV hlt1.4
Pixel
Full Tracker
Several algorithms available
15
ATLAS CMS Silicon Tracker vertexing
Secondary Vertex Exclusive Vertices
The basic tool for the vertexing classes is a
general purpose fitter. Test on B0s ? J/? f,
with J/? ? mm and f ? KK
Secondary Vertex Inclusive Vertices

• Useful for b and t tagging
• Two methods available and tested (Combinatorial
  method, d0/F method)

The typical resolution using RecTracks is 55 mm
in the transverse plane and 75 mm in z
Typical efficiency ranges from 35 to 25 for
Track Puritygt50
Difference between the simulated Bs decay vertex
and the fitted one in transverse and
longitudinal directions
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ATLAS CMS Silicon Tracker btagging

• Typical performance for both experiments
• average e(u) 1 for e(b) 60 for
  interesting jet pT range (50 lt pT lt 130 GeV)
  and all h
• best e(u) 0.2 for e(b) 50 for pT 100
  GeV and central rapidity

• Several algorithms tried by CMS and ATLAS, based
  on
• impact parameter (track counting and jet
  probability
• secondary vertex reconstruction
• decay length

CMS 2-D 3-D I.P. prob.e(b) vs
e(u) ATLAS 2-D I.P. prob. e(u) vs pT (all h)
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Conclusions

• Tracking at LHC is a very challenging task
• Very high rates
• Very harsh radiation environment
• High accuracy needed
• Extensive RD programs carried on to design
  detectors which fulfil these requirements
• Design of ATLAS CMS Trackers almost complete
• Production and construction of various
  components/detectors already started
• Both ATLAS CMS have robust performances
• Pixel detectors allow for fast and efficient
  track seed generation as well as vertex
  reconstruction
• pt resolution of 1 for 100 GeV muons over
  about 1.7 units of rapidity
• Robust efficient track reconstruction
  algorithms available (see D.Rousseau Talk)
• Jet flavour tagging under study to improve and
  extend the Physics reach
• Extensive use of track information _at_ HLT (see G.
  Bagliesis Talk)