1. Performance of ATLAS & CMS Silicon Tracker
International Europhysics Conference on High Energy Physics
EPS 2003, July 17th-23rd 2003, Aachen, Germany
Performance of ATLAS & CMS Silicon Tracker
Alessia Tricomi
University and INFN Catania

2. What LHC means… H  bb event p-p collision @ √s = 14 TeV
bunch spacing of 25 ns
Luminosity
low-luminosity: 2*1033cm-2s-1 (first years)
high-luminosity: 1034cm-2s-1
~20 minimum bias events per bunch crossing
~1000 charged tracks per event
Radius: cm 10cm 25cm 60cm
NTracks/(cm2*25ns)
Severe radiation damage to detectors
H  bb event
@ high luminosity
Plus 22
minimum
bias events
Challenging requirements for the Tracking system
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

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
1~2% pt resolution at ~ 100 GeV
Ability to operate in a crowded environment
Nch/(cm2*25ns) = 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 -7°C  -10°C to contain reverse annealing and limit leakage current
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

4. Two different strategies…
46m Long, 22m Diameter, 7’000 Ton Detector
2.3 m x 5.3 m Solenoid ~ 2 Tesla Field
~ 4 Tesla Toroid Field
ATLAS
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)
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

5. Two different strategies…
CMS has chosen an all-silicon configuration
13m x 6m Solenoid: 4 Tesla Field
 Tracking up to h ~ 2.4
ECAL & HCAL
Inside solenoid
Muon system in return yoke
First muon chamber just after solenoid
 Extended lever arm for pt measurement
5.4 m
Outer Barrel –TOB-
Inner Barrel –TIB-
End cap –TEC-
Pixel
2.4 m
volume 24.4 m3
running temperature – 10 0C
dry atmosphere for YEARS!
Inner Disks –TID-
CMS
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, 14’000 Ton Detector
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

6. The ATLAS Pixel Detector
3 barrel layers*
r = 5.05 cm (B-layer), 9.85 cm, 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 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
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

7. The ATLAS SCT Detector 1.53 m 1.04 m 5.6 m Endcap: 9 wheel pairs
pitch mm
3 types of modules
Inner (400)
Middle (640 incl 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
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

8. 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 < r < 15 cm
Average z position: 34.5 cm, 46.5 cm
Later update to 3 pairs possible
(<z> ~ 58.2 cm)
Per Disk: ~3 x 106 pixels
 3 high resolution space points for h < 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 in the endcaps
Occupancy is ~ 10-4
Pixel seeding fastest starting point for track reconstruction despite the extremely high track density
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

9. The CMS Silicon Strip Tracker
12 layers with (pitch/12) spatial resolution and 110 cm radius give a momentum resolution of
A typical pitch of order 100mm
is required in the f coordinate
To achieve the required resolution
Outer Barrel (TOB): 6 layers
Thick sensors (500 mm)
Long strips
9’648’128 strips  channels
75’376 APV chips
6’136 Thin sensors
18’192 Thick sensors
440 m2 of silicon wafers
210 m2 of silicon sensors
3’ *1’512 Thin modules
5’ *1’800 Thick modules
ss ds=b-to-b (100mrad)
~17’000 modules
~25’000’000 Bonds
p+ strips on n-type bulk
<100> crystal lattice orientation
Polysilicon resistors to bias the strips
Strip width over pitch w/p=0.25
Metal overhang and multiguard structure to enhance breakdown performance
Endcap (TEC): 9 Disk pairs
r < 60 cm thin sensors
r > 60 cm thick sensors
6 layers
TOB
4 layers
TIB
3 disks TID
Radius ~ 110cm, Length ~ 270cm
h~1.7
h~2.4
9 disks TEC
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
FE hybrid with FE ASICS
Pitch adapter
Black: total number of hits
Green: double-sided hits
Red: ds hits - thin detectors
Blue: ds hits - thick detectors
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

10. Track reconstruction efficiency
ET = 200 GeV Fake Rate < 8 *10-3
ET = 50 GeV Fake Rate < 10-3
<10-5
99%
Single m
Efficiency for p is lower compared to m due to secondary interactions in the Tracker
Efficiency can be increased by relaxing track selection
Dijet events
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
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

11. Track resolutions ATLAS & CMS Good track parameter resolution
have similar performance
Good track parameter resolution
already with 4 or more hits
CMS
CMS
s(pT)/pT
s(d0) mm
For lower pt tracks multiple scattering becomes significant and the h dependence reflects the amount of material traversed by tracks
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

12. ATLAS & CMS performances
ATLAS and CMS have thick trackers:
each pixel layer contributes >2% 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
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

13. The dark side: material budget in the Tracker2
1.5 1
0.5
X/X0
ATLAS
Degrades tracking performance, due to multiple scattering, Bremsstrahlung and nuclear interactions
(see pt resolution and reconstruction efficiency)
CMS
Dominates energy resolution
for electrons
Reduces (somewhat) efficiency for
usefully reconstructing H gg
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

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)<50mm
Slower but better resolution (15 mm) achievable using the full Tracker
CMS
H  4 m
ATLAS
Primary vertex in A  tt
s ~ 40mm
“easy” channel
“difficult” channel
s ~ 16mm
uu 100 GeV
h<1.4
Pixel
Full Tracker
Several algorithms available
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

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 Purity>50%
Difference between the simulated Bs decay vertex and
the fitted one in transverse and longitudinal directions
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

16. ATLAS & CMS Silicon Tracker: btagging
Several algorithms tried by CMS and ATLAS, based on:
impact parameter (track counting and jet probability
secondary vertex reconstruction
decay length
Typical performance for both experiments:
average: e(u) ~ 1% for e(b) = 60% for “interesting” jet pT range (50 < pT < 130 GeV) and all h
best: e(u) ~ 0.2% for e(b) = 50% for pT ~ 100 GeV and central rapidity
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)
Alessia Tricomi - University & INFN Catania
EPS July, Aachen

17. Conclusions Tracking at LHC is a very challenging task:
Very high rates
Very harsh radiation environment
High accuracy needed
Extensive R&D 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 HLT (see G. Bagliesi’s Talk)
Alessia Tricomi - University & INFN Catania
EPS July, Aachen