Performance of ATLAS & CMS Silicon Tracker

Performance of Performance of ATLAS & CMS Silicon TrackerATLAS & CMS Silicon Tracker

Alessia TricomiUniversity and INFN Catania

International Europhysics Conference on High Energy PhysicsEPS 2003, July 17th-23rd 2003, Aachen, Germany

Alessia Tricomi - University & INFN Catania EPS 2003 17-23 July, Aachen

What LHC means…

• 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 eventRadius: 2cm 10cm 25cm 60cmNTracks/(cm2*25ns) 10.0 1.0 0.10 0.01

• Severe radiation damage to detectors

H bb event

Plus 22 minimum

bias events

H bb event @ high luminosity

Challenging requirements for the Tracking system


Tracker Requirements• Efficient & robust Pattern Recognition algorithmEfficient & robust Pattern Recognition algorithm

– Fine granularity to resolve nearby tracksFine granularity to resolve nearby tracks– Fast response time to resolve bunch crossingsFast response time to resolve bunch crossings

• Ability to reconstruct narrow heavy objectAbility to reconstruct narrow heavy object– 1~2% p1~2% ptt resolution at ~ 100 GeV resolution at ~ 100 GeV

• Ability to operate in a crowded environmentAbility to operate in a crowded environment– Nch/(cm2*25ns) = 1.0 at 10 cm from PV

• Ability to tag b/Ability to tag b/ through secondary vertex through secondary vertex– Good impact parameter resolutionGood impact parameter resolution

• Reconstruction efficiencyReconstruction efficiency– 95% for hadronic isolated high p95% for hadronic isolated high ptt tracks tracks– 90% for high p90% for high ptt tracks inside jets tracks inside jets

• Ability to operate in a very high radiation Ability to operate in a very high radiation environmentenvironment– Silicon detectors will operate at -7°C Silicon detectors will operate at -7°C -10°C to contain -10°C to contain

reverse annealing and limit leakage current reverse annealing and limit leakage current


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

ATLASATLAS Inner DetectorID inside 2T solenoid fieldTracking based on many pointsPrecision Tracking:• Pixel detector (2-3 points)• Semiconductor Tracker – SCT (4 points)Continuous Tracking:(for pattern recognition & e id)• Transition Radiation Tracker – TRT (36 points)


CMS

5.4

m

Outer Barrel –TOB-

Inner Barrel –TIB-

End cap –TEC-Pixel

2.4

m

volume 24.4 m3

running temperature – 10 0Cdry atmosphere for YEARS!

Inner Disks –TID-

Two different strategies…

22m Long, 15m Diameter, 14’000 Ton Detector

CMS Tracker Inside 4T solenoid fieldTracking rely on “few” measurement layers, each ableto provide robust (clean) and precise coordinate determinationPrecision Tracking:• Pixel detector (2-3 points)• Silicon Strip Tracker (220 m2) – SST (10 – 14 points)

13m x 6m Solenoid: 4 Tesla Field Tracking up to ~ 2.4

ECAL & HCALInside solenoid

Muon system in return yoke

First muon chamber just after solenoid Extended lever arm for pt measurement

CMS has chosen an all-silicon configuration


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 m x 300 m (B layer) & 50 m x 400

m • ~ 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


The ATLAS SCT Detector

5.6 m

1.53 m

1.0

4 m

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

Endcap: 9 wheel pairs• pitch 70 - 80 m• 3 types of modules

Inner (400) Middle (640 incl. 80

shorter) Outer (936)All detectors are double-sided

(40 mrad stereo angle)• 4088 modules• 61 m2 of silicon• 6.3 x 106 channels


• 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 < 2.2• Pixel size: 150 m x 150 m driven by FE

chip Hit resolution:

– r-~ 10 m(Lorentz angle 28° in 4 T field)

– r-z~ 17 m• Modules are the basic building elements

– 800 in the barrel + 315 in the endcaps

The CMS Pixel Detector

Occupancy is ~ 10-4

Pixel seeding fastest starting point for track reconstruction despite the extremely high track density


The CMS Silicon Strip TrackerOuter Barrel (TOB): 6 layers• Thick sensors (500 m)• Long strips

Endcap (TEC): 9 Disk pairs• r < 60 cm thin sensors• r > 60 cm thick sensors

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

6 layers6 layersTOBTOB

4 layers4 layersTIBTIB

3 disks TID3 disks TID

Radius ~ 110cm, Length ~ 270cmRadius ~ 110cm, Length ~ 270cm ~1.7~1.7

~2.4~2.4

9 disks TEC9 disks TECInner Disks (TIB): 3 Disk pairs• Thin sensors

9’648’128 strips channels

75’376 APV chips

6’136 Thin sensors18’192 Thick sensors

440 m2 of silicon wafers 210 m2 of silicon sensors

3’112 + 2*1’512 Thin modules5’496 + 2*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 stripsStrip width over pitch w/p=0.25

Metal overhang and multiguard structure to enhance breakdown performance

FE hybrid FE hybrid with FE with FE ASICSASICS

Pitch Pitch adapteradapter

Silicon Silicon sensorssensors

CF frameCF frame

12 layers with (pitch/12 layers with (pitch/12) spatial 12) spatial resolution and 110 cm radius resolution and 110 cm radius give a momentum resolution ofgive a momentum resolution of

Tevp

BT

Lm

mpitch

pp T

141.1

10012.0

121

A typical pitch of order A typical pitch of order mmis required in the is required in the coordinate coordinate

To achieve the required resolutionTo achieve the required resolution

Black: total number of hitsGreen: double-sided hitsRed: ds hits - thin detectorsBlue: ds hits - thick detectors

Strip length ranges fromStrip length ranges from 10 cm10 cm in the in the inner layers toinner layers to 20 cm 20 cm in the outer in the outer layers.layers.

Pitch ranges fromPitch ranges from 80 80mm in the inner in the inner layers to nearlayers to near 200200mm in the outer in the outer layerslayers


99%99% 99%99%

Single

Track reconstruction efficiency

Global efficiency: selected Rec.Tracks / all Sim.TracksGlobal efficiency: selected Rec.Tracks / all Sim.Tracks

Algorithmic efficiency: selected Rec.Tracks / selected Sim.TracksAlgorithmic efficiency: selected Rec.Tracks / selected Sim.Tracks

(Sim.Track selection: at least 8 hits, at least 2 in pixel)(Sim.Track selection: at least 8 hits, at least 2 in pixel)

Global efficiency limited by pixel geometrical acceptanceGlobal efficiency limited by pixel geometrical acceptance

Efficiency for particles in a Efficiency for particles in a 0.4cone around jet axis0.4cone around jet axis

No significant degradation No significant degradation compared to single pionscompared to single pions

Loss of efficiency is dominated by Loss of efficiency is dominated by hadronic interactions in Tracker hadronic interactions in Tracker materialmaterial

Efficiency for is lower compared to due to secondary interactions in the Tracker

Efficiency can be increased by relaxing track selection

ET = 200 GeV Fake Rate < 8 *10-3

ET = 50 GeV Fake Rate < 10-3

<10-

5

Dijet events

CMS


Track resolutionsGood track parameter resolution

already with 4 or more hits

CMS CMS

ATLAS & CMS have similar performance

For lower pt tracks multiple scattering becomes significant and the dependence reflects the amount of material traversed by tracks

CMS CMS

(p

T)/

pT

(d

0) m


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)m

t0 p

)106.8(67.611.7(10.5))σ(d

1

TT

TeVp

18.0(14.7)0.6(0.4)

p1

σ

(1/p

T)

TeV

-1(

d0) m


Degrades tracking performance, due to Degrades tracking performance, due to multiple scattering, Bremsstrahlung and multiple scattering, Bremsstrahlung and

nuclear interactionsnuclear interactions(see p(see ptt resolution and reconstruction efficiency) resolution and reconstruction efficiency)

The dark side: material budget in the Tracker

-4 -2 0 2 4

2

1.5

1

0.5

0

X/X

0

ATLAS

Reduces (somewhat) efficiency forReduces (somewhat) efficiency forusefully reconstructing usefully reconstructing H H

Dominates energy resolutionDominates energy resolutionfor electronsfor electrons

CMS CMS


ATLAS

Primary vertex in A

ATLAS & CMS Silicon Tracker: vertexing

At LHC design luminosity ~ 20 interaction per beam crossingspread out by (z)=5.6 cm

Identification of primary and secondary vertices fundamental

CMS

H 4

“easy” channel

“difficult” channel

Pixel detectors allow fast vertex reconstruction with (z)<50m

Slower but better resolution (15 m) achievable using the full Tracker

Pixel

Several algorithms available

~m

~m

Full Tracker

uu 100 GeV<1.4


ATLAS & CMS Silicon Tracker: vertexing

Secondary Vertex: Exclusive Vertices

The basic tool for the vertexing classes is a general purpose fitter. Test on B0

s J/, with J/and

Difference between the simulated Bs decay vertex and the fitted one in transverse and longitudinal directions

Secondary Vertex: Inclusive Vertices Useful for b and tagging

Two methods available and tested (Combinatorial method, d0/ method)

Typical efficiency ranges from ~35% to ~25% for Track Purity>50%

The typical resolution using RecTracks is ~55 m in the transverse plane and ~75 m in z


ATLAS & CMS Silicon Tracker: btaggingSeveral 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: (u) ~ 1% for (b) = 60% for “interesting” jet pT range (50 < pT < 130 GeV) and all • best: (u) ~ 0.2% for (b) = 50% for pT ~ 100 GeV and central rapidity

CMS:

2-D & 3-D I.P. prob.:(b) vs (u)

ATLAS:

2-D I.P. prob.: (u) vs pT (all )


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 information @ HLT (see G. Bagliesi’s Talk)

Performance of ATLAS & CMS Silicon Tracker

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