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The LHC TrackersThe LHC Trackers

ATLAS

27 km LHC tunnel

Mohsen Khakzad (St FX University)December 16, 2008

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LiteratureLiterature1) Gluckstern R.L.

Uncertainties in track momentum and direction, due to multiple scattering and measurement errors NIM 24 p. 381 (1963)

2) Bock R. K., Grote H., Notz D., Regler M.

Data Analysis technique for high energy physics experiments; Cambridge University Press 1990

3) Blum W., RolandiL.

Particle detection with drift chambers; Springer-Verlag1993

4) Avery P.

Applied fitting theory I: General Least Squares Theory, CLEO note CBX 91-72 (1991):

see: http//www.phys.ufl.edu/~avery/fitting.html

5) Fru�hwirthR.

Application of Kalman Filtering to Track and Vertex Fitting; NIM A262 p. 444 (1987)

6) BilloirP.

Track Element Merging Strategy and Vertex Fitting in Complex Modular Detectors; NIM A241 p. 115 (1985)

7) Francesco Ragusa,

An introduction to Charged Particles Tracking, Itao-Hellenic School of Physics (2006)

See: http://www.le.infn.it/lhcschool/talks/Ragusa.pdf

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• Introduction and basic concepts

• The challenge of tracking at LHC.

• ATLAS and CMS.

OutlineOutline

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CERN Bubble ChamberCERN Bubble Chamber

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CMS and ATLAS Tracking ChambersCMS and ATLAS Tracking Chambers

CMS ATLAS

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When we talk about “tracking,” we want to do the following:

• Measure the true path of the charged particle, which let’s us know...

• The momentum (3-momentum) if we know the magnetic field

• The sign of the charge of the particle

• With other constraints or assumptions, the “origin” in space of theparticle

Basic IdeaBasic Idea

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Basic Concepts of TrackingBasic Concepts of Tracking

Tracking means reconstruction ofcharged particles trajectory to performseveral measurements

momentum (magnetic field)

p = 0.3·B·R

Vertexing:Lifetime Tag

the sign of the charge

particle ID (mass), notnecessarily with the samedetector

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Basic Concepts: Motion inBasic Concepts: Motion in Magnetic FieldMagnetic Field

In a magnetic field the motion of a char-ged particle is determined bythe Lorentz force. For homogenous B (solenoidal field) the trajectory isgiven by an helix

Where λ the dip angle and h=±1 is thesense of rotation.

The projection of the helixIn the transverse plane (x,y) is a circle

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Basic ConceptsBasic Concepts: Radius of Curvature: Radius of Curvature

Important to dimension the tracking system and to calculate the number ofmeasuring points for a given transverse momentum (cut-off in pt).

Important also to calculate the average radius of the “loopers”. Lowmomentum particles carry no basic information on the physics of the hardprocesses while they might jeopardize pattern recognition by increasing theoccupancy in the innermost layers.

For pt<300MeV<25cm in CMS (4T) pixel only<50cm in ATLAS (2T) pixel and Si-microstrips

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Basic ConceptsBasic Concepts:: Momentum MeasurementMomentum Measurement

In hadronic colliders we want to measure mainly the transverse momentum sinceelementary processes happens among partons that are not at rest in thelaboratory (momentum conservation only in the transverse plane)

The momentum of the particle is projectedalong two directions

in the R − φ plane we measure the transversemomentum

in the R − z plane we measure the dip angleλ

R

R

11For small momenta is y a periodic function of z

For large momenta we have a straight line as a function of z

Aleph:Aleph: Track ReconstructionTrack Reconstruction

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Basic Concepts: Basic Concepts: Sagitta Sagitta MeasurementMeasurement

A few examples assuming a track length of 1m and magnetic fields of 2 and 4T

Pt=1 GeV R(2T)=1,67m R(4T)= 0,83m s(2T)=75mm s(4T)= 150mmPt=10 GeV R(2T)=16,7m R(4T)= 8,3m s(2T)=7,5mm s(4T)= 15mmPt=100 GeV R(2T)=167m R(4T)= 83m s(2T)=0,75mm s(4T)= 1,5mmPt=1 TeV R(2T)=1670m R(4T)= 830m s(2T)=0,075mm s(4T)= 0,15mm

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Measuring the Momentum ofMeasuring the Momentum of a Tracka Track

Since the transverse momentum is proportional to the bending radius, themomentum resolution depend on the accuracy in measuring R

The error in measuring momenta is proportional to momentum, decreases linearlywith the accuracy of the measurements and is inversely proportional to the bendingpower BL2. A big lever arm is the most effective tools. Beam spot and last layersare crucial.

Sagitta

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Momentum Momentum ResulationResulation

Useful formulas for practical purposes

The dependance on the number of measurements is weak. Still BL2 dominatesUnfortunately solenoidal magnets with large B and large L are very expensive.The cost scales with the stored energy. And also the tracking systems are notcheap.

Cost of a solenoidal magnet (M$)=0.523[(E/1 MJ)]0.662

E= B2V/2μ0 (V=π L2l ) where L is the radius and l is the length of the solenoid

The CMS solenoid stores 2.6 x 109 Joule and costs about 100M$

When N measurement points are distributed along the trajectory.

For N>10 ; CN= 180/N+4.

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Impact Parameter Resolution: Sign of ChargeImpact Parameter Resolution: Sign of Charge

Simple considerations lead to an error on the impact parameterdominated by the precision of the first measuring layers, theirdistance from the interaction point and the precision on themeaurement of the slope given by the entire tracker

Using typical resolution of about 10μm at a few cm distance from the beamline 10-15 μm ip resolution are easilyachievable

The maximum momentum which allows theidentification of the charge depends again on BL2.

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Basic Parameter for the LHC TrackersBasic Parameter for the LHC Trackers

• Optimal momentum resolution (Higgs-> 4μ; better cuts on the Zmass and use of invariant mass in general to reduce the irreduciblebackgrounds).

• High efficiency in reconstruction of tracks bothisolated (muons and electrons) and within hightransverse energy jets. (muon trigger validation,isolation cuts for single photons (H -> γγ and tracksin general).

Del pt/pt=0.2-0.4 pt (TeV)

• ξ > 95% for pt>2GeV

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Basic Parameter for the LHC Trackers Basic Parameter for the LHC Trackers ……

• Good impact parameter resolution (10-20μm)Reconstruction of different primary vertices within the same highluminosity event. Tagging capability for b and tau jets

• Radiation resistance: 10 years of running (1.5-2.4x1014n/cm2)

• Amount of material kept under control: to minimize photonconversion and secondary interactions within the tracker itself.

• Costs within the maximum available budget: 70-80MCHF.

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Energy LossEnergy Loss

• If a charged particle passes through material, it can loseenergy and slow down and change direction somewhat

• As a particle bends in the magnetic field, it can emitbremsstrahlung and slow down

Our model of the trajectory of the charged particle has to takethese effects into account if they are important

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Tracking Detector

• Should have the least amount of material as possible

• Should have as many measurements of the trajectory as Possible Tomeasure pT well, the longer the lever arm the better

• Measurement points should be as precise as possible Some of the maintechnologies in use right now covert the energy lost by a chargedparticle with

• Gas and wire: ions in gas drift to wire under influence of electric field.Drift time and position must be precisely known

• Scintillating fibers

• Semiconductor: usually silicon, fully depleted. Charge is collected inprecisely laid down strips or pixels

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Silicon microstrip detectors allow a very good point resolution (10-30 micron) that coupled to lever arms around 1m in solenoidal fieldof 2-4T would allow an adequate momentum resolution, good impactparameter resolution for b-tagging and excellent measurement ofthe charge up to 1TeV and beyond.

Single bunch crossing resolution is feasible in silicon(collection time < 10 ns) with fast read-out electronics.

The real challenge is pattern recognition for track reconstruction:the high density of tracks typical of the inner regions of highluminosity hadronic colliders can be tackled with extremesegmentations both in r-phi and r-z : pixel detectors and siliconmicrostrip modules.

The approachThe approach

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Let’sconsider a large drift chamber 1m radius and 6m length. Let’sput a wire every 3mm (impossible to do in real life, wire tension,sagitta etc); 300k electronics channels and 300 detection planes;assume also that we have found a gas so fast to collect all chargein 25ns.

With this chamber we can reasonably afford pattern recognitionproblems for events producing 30 charged tracks every 25ns(Average occupancy 300 layersx30 tracks/300K channels= 3%).

If now every 25ns you produce 1000 charged tracks you canmaintain the same reconstruction efficiency only bySEGMENTING in Z the CHAMBER (1 chamber 6m long -> 30chambers 20cm each one).

Is the basic idea we had for the CMS Tracker. The segmentationincreases in the most congested regions: 20cm-10cm-1cm

Pattern Recognition: High GranularityPattern Recognition: High Granularity

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This approach has been followed very aggressively by CMS andthe collaboration agreed to build the first full silicon tracker inHEP

• more challenge in terms of technology and costs• higher performance particularly in pattern recognition

Atlas has adopted a more traditional approach based on ahybrid tracker: pixel and silicon microstrip detectors in theinnermost part and a large gaseous detectors in the outer part(straw tubes).

•development of new technologies limited to pixels•higher risks in terms of performance (high occupancyforeseen in the TRT for the high luminosity run of LHC).

Two Different StrategiesTwo Different Strategies

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ATLASATLAS

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Two different strategiesTwo 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)

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The ATLAS TrackerThe ATLAS Tracker

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Transition Radiation TrackerTransition Radiation Tracker

4mm straws Small drift tubes

Combined tracking and sensitivity totransition radiation X-rays for e-hadron identification

Radiator

Straws

Radiator

Straws

End-cap

End-cap

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Charged Particle TrackingCharged Particle Tracking

TRT

Silicon strip detectors

Siliconpixel

detectors

Computer reconstruction Pattern recognition This particular event shows the

characteristics of “jets” Jets are created from the quarks and

gluons formed in the collisions. The quarks and gluons combine to

form hadrons(pions, kaons, protons…)that are detected(if charged) by thetracking detectors or by thecalorimetry(charged or neutral).

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The ATLAS Tracker The ATLAS Tracker ……

Pixel Detector3 barrels, 3+3 disks: 80 X106 pixelsbarrel radii: 4.7, 10.5, 13.5 cm pixelsize 50 x 400 μ mσrφ= 6-10 μ m σz = 66 μ m

SCT4 barrels, disks: 6.3 X 106 stripsbarrel radii:30, 37, 44 ,51 cm strippitch 80 μ m stereo angle ~40 mrσrφ= 16 μ m σz = 580μ m

TRTbarrel: 55 cm < R < 105 cm 36 layersof straw tubesσrφ= 170 μ m 400.000 channels

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We can now give a rough estimate ofthe momentum resolution of theATLAS tracking systems

ATLAS Momentum ResolutionATLAS Momentum Resolution

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ATLAS Impact Parameter ResolutionATLAS Impact Parameter Resolution

For the ATLAS detector Monte Carlo studies have shownthat the resolutions on impact parameter and momentum

can be parametrized as

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The CMSThe CMS DetectorDetector

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13 precision measuring points per track + 4T solenoidal field

The CMSThe CMS TrackerTracker

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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 strategiesTwo different strategies……

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

CMS TrackerInside 4T solenoid fieldTracking rely on “few”measurement layers, each ableto provide robust (clean) andprecise 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 η ~ 2.4

ECAL & HCALInside solenoid

Muon system in return yoke

First muon chamber justafter solenoid→ Extended lever armfor pt measurement

CMS has chosen an all-silicon configuration

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The CMSThe CMS Full Silicon TrackerFull Silicon Tracker

Pixel Detector2 barrels, 2 disks: 40 x 106 pixelsbarrel radii: 4.1, ~10. cmpixel size 100 x 150 μ mσrφ = 10 μ m σz = 10 μ m

Internal Silicon Strip Tracker4 barrels, many disks: 2 x106 stripsbarrel radii:strip pitch 80,120 μ mσrφ = 20μm σz = 20 μ m

External Silicon Strip Tracker6 barrels, many disks: 8 x 106 stripsbarrel radii: max 110 cmstrip pitch 80, 120 μ mσrφ= 30 μ m σz = 30 μ m

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The CMSThe CMS Full Silicon Tracker Full Silicon Tracker ……

• 207m2 of microstrip silicon detectors 15.232 modules

• 6136 thin sensors, 320μm (HPK) and 19292, thicksensors 500μm (HPK + STM) all produced on 6” wafers

• 60M channels pixel detector

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•Track reconstruction is a vital ingredient inthe analysis chain of a high-energy physicsexperiment.

•This task is often divided into two differentsubtasks:

•Track finding/pattern recognition•Track fitting/parameter estimation

•Track finding: starts out with a set ofposition measurements (provided by a trackingdetector).

•The aim is to group these measurementstogether in subsets, each subset containingmeasurementsorginating from one charged particle.

Track ReconstructionTrack Reconstruction

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Track Reconstruction Track Reconstruction ……

Measurements from threetracks coming from origin

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Track Reconstruction Track Reconstruction ……

Track Finding

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•Track fitting: for each of the subsets providedby the track finder, find the optimal estimate of aset of parameters uniquely describing the state ofthe particle somewhere in the detector.

• Example: momentum (absolute value), directionand position at the surface of the detector unitclosest to the beam.

•The parameters of the tracks are used in higherlevel analyses, for instance in vertexreconstruction.

Track Reconstruction Track Reconstruction ……

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Track Reconstruction Track Reconstruction ……

Track Fitting

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ConclusionsConclusions

•There’s a lot to tracking and vertexing and ofcourse we’ve just scratched the surface

•It is an absolutely crucial part of any modernhigh energy physics collider experiment

•The only way to get good tracking andvertexing is to really understand your detector