Calorimeters A User’s Guide Elizabeth Dusinberre, Matthew Norman, Sean Simon October 28, 2006.

CalorimetersA User’s Guide

Elizabeth Dusinberre, Matthew Norman, Sean Simon

October 28, 2006

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Particles

Each collision creates multiple particles (Z, W, H) that immediately decay.Generally decay to quarks, photons, and leptons.e, , survive the processQuarks hadronize to jets of particles, mostly composed of ,, protons, neutrons

A detector only sees these particles!

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Detector Components

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Basic ConceptQ: What is a Calorimeter?

A: A Calorimeter is a device that measures all the energy released during an event

Bomb Calorimeter

Calorimeters used in chemistry experiments depend upon a layer of water to absorb all thermal energy from a reaction.

Particle Physics does not have the same luxury because the amount of water needed to intercept all outgoing particles would be too large for there to be a meaningful temperature change.

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CMS

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ATLAS

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CMS Calorimeter Arrangement

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Calorimeter Images

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Jet and Particle Response

What causes E/E?

a = noise, pileup, radioactivityb = sampling fluctuationsc = “quality factor”

Single Particle Response:Calorimeter Shower

Parton Reconstruction

Jets are primarily +,-, 0

Ratio between particles ~ 1:1:10 energy in ECAL+/- energy in HCAL

Most of this talk is about single particle response.

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EM & Hadronic ShowersEM showers

Dominated by bremsstrahlung and e± pair production at high energies.

Hadronic showersDominated by succession of inelastic hadronic interactions. At high energies, these interactions are characterized by multiparticle production and particle emission from decay of excited nuclei.

Bremsstrahlung

Pair production

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Radiation Length & Molière Radius

Radiation Length X0

•Mean distance traversed by high energy e±

•7/9 mean free path for pair

production for high energy

Molière Radius RM

RM = X0 Es/Ec

EM shower shapes scale longitudinally with X0 and laterally with RM

90% of energy is within cylinder of radius RM.

Photon Component e+ e- component

100GeV e- hitting Liquid Krypton

Showershape for

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Ionization

Simple cascade model

Ein

e-1Ein

e-2Ein

e-3Ein

e-4Ein

Shower stops when e-nEin ≤ Ec

Shower Max @ nX0 = X0ln(Ein/Ec)

When e- has Ec left thenionization and bremsstrahlungare equal.

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Design Considerations

1) Radiation HardEnergy in LHC Beam ~ 75 kg TNT equivalentHigh Pt products vent a fair proportion of the energy into the detectorDetector absolutely must be radiation hard!

2) Large - CoverageIn order to get the best efficiency, you want the calorimeter to cover as much space as possible.However, it is practically impossible to measure energies at high eta (close to the beamline)Planning needed to reduce the number of “cracks” in the detector.

3) FastBunch crossing time at LHC ~ 25nsecCalorimeter must react on this time scale, otherwise the events blend together.Electronics must be top notch

• ContainmentIdeally, you want total particle containment in the CalorimeterConstructed to minimize leakage of particles out the back.

5) GranularityMeasure of how fine the resolution is.For ECAL: Molière RadiusFor HCAL: Varies - One optimization - angle between jets for 1TeV Higgs -> ZZ -> jj + stuff

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Compensating Calorimeters

• Hadronic showers have both hadronic (π±,p) and electromagnetic (, π0) components

• A good hadronic calorimeter will respond equally to both components (e/h ≈ 1, important for measureing jet energies)

• Without effort, e/h is often more like 1.2-1.5• Various ways to improve e/h:

– adjust relative thickness of absorber and active layers– shielding active layers with low Z material to stop soft

photons– Offline methods, etc

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Calorimeter Design Concepts

• Separate ECAL and HCAL– E.g. CMS PbWO4 and Brass

• One combined Calorimeter– E.g. ATLAS Liquid Argon

• Calorimetry near the beamline

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Electromagnetic Calorimeter

• Measures energy of photons and electrons

– Scintillating crystals, liquid scintillator

– Collect photons using photomultiplier tubes or photodiodes

• Few enough nuclear interaction lengths that strongly interacting particles don’t deposit much of their energy

• Close to the beam, so should be radiation hard

• Example: CMS Ecal – Lead Tungstate (PbWO4) crystals– Radiation length = .89 cm– Interaction length = 19.5 cm– 26 X0 or about 23 cm deep

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Hadronic Calorimeter

•• Example: CMS Hcal

– Absorbers are brass and steel plates

– Active layers are scintillating plastic– Interaction length of steel ~ 17cm– Not compensating calorimeter, e/h

~ 1.45– 7-10 I deep

•Measures energy from quarks, gluons, and neutrinos•Often are sampling calorimeters

–made up of alternating layers of absorber and active layers–Absorbers are dense materials like lead, copper or stainless steel–Active layers are scintillators or ionizable materials

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CMS Radiation and Interaction Lengths

26 X0 in ECAL, after that thephotons and electrons from theinitial event have deposited their energy

8-10 interaction lengthsby the end of the HCAL tomeasure most of the energyfrom strongly interactingparticles

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Combined ECAL and HCAL• Example: ATLAS Liquid

Argon Calorimeter– Cryogenically cooled liquid

argon ionized when charged particles pass through it

– Electrons and hadrons shower in lead or stainless steel absorbers

– Liquid argon radiation length = 14 cm

– Liquid argon interaction length = 84 cm

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CMS Very Forward Calorimeter

• Uses quartz fiber calorimetry– Fibers of quartz are

embedded in tungsten

– Quartz fibers are very radiation hard

• Detects Cherenkov radiation from very forward jets

• Important for calculating MET

• Similar to detector used at RHIC

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Missing ET

• Neutrinos do not interact with any of the subdetectors of a collider experiment

• There may also be non interacting exotic particles discovered at the LHC

• MET = - ET

• Good energy resolution, a hermetic detector, and an e/h close to one are extremely important when looking for MET

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References

Green, Dan, High PT Physics at Hadron Colliders, Cambridge University Press, 2005

Kleinknecht, Konrad, Detectors for Particle Radiation, Cambridge University Press, 1998

http://www.fys.uio.no/elg/alice/dirPapers/NIM_A_550_2005_169-184.pdf

CMS TDR Volume 1 and CMS TDR Volume 2

ATLAS TDR

CMS Outreach Webpage : http://cmsdoc.cern.ch/cms/outreach/html/index.shtml

ATLAS Outreach Webpage: http://atlasexperiment.org/

Review of Particle Physics (PDG)