Calorimeters A User’s Guide

1. CalorimetersA User’s Guide Elizabeth Dusinberre, Matthew Norman, Sean Simon October 28, 2006

2. Particles Each collision creates multiple particles (Z, W, H) that immediately decay. Generally decay to quarks, photons, and leptons. e, ,  survive the process Quarks hadronize to jets of particles, mostly composed of ,, protons, neutrons A detector only sees these particles!

3. Detector Components

4. Basic Concept Q: What is a Calorimeter? A: A Calorimeter is a device that measures all the energy released during an event 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. Bomb Calorimeter

5. CMS

6. ATLAS

7. CMS Calorimeter Arrangement

8. Calorimeter Images

9. Jet and Particle Response Single Particle Response: Calorimeter Shower Parton Reconstruction • Jets are primarily +,-, 0 •  Ratio between particles ~ 1:1:1 • 0 energy in ECAL • +/- energy in HCAL What causes E/E? a = noise, pileup, radioactivity b = sampling fluctuations c = “quality factor” Most of this talk is about single particle response.

10. EM & Hadronic Showers EM showers Dominated by bremsstrahlung and e± pair production at high energies. Bremsstrahlung Hadronic showers Dominated by succession of inelastic hadronic interactions. At high energies, these interactions are characterized by multiparticle production and particle emission from decay of excited nuclei. Pair production

11. Radiation Length & Molière Radius 100GeV e- hitting Liquid Krypton • Radiation Length X0 • Mean distance traversed by • high energy e± • 7/9 mean free path for pair • production for high energy  Showershape for Photon Component e+ e- component 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.

12. When e- has Ec left then ionization and bremsstrahlung are equal. Simple cascade model Ein e-1Ein e-2Ein Ionization e-3Ein e-4Ein Shower stops when e-nEin ≤ Ec Shower Max @ nX0 = X0ln(Ein/Ec)

13. Design Considerations Radiation Hard Energy in LHC Beam ~ 75 kg TNT equivalent High Pt products vent a fair proportion of the energy into the detector Detector absolutely must be radiation hard! Large - Coverage In 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. Fast Bunch crossing time at LHC ~ 25nsec Calorimeter must react on this time scale, otherwise the events blend together. Electronics must be top notch Containment Ideally, you want total particle containment in the Calorimeter Constructed to minimize leakage of particles out the back. 5) Granularity Measure of how fine the resolution is. For ECAL: Molière Radius For HCAL: Varies - One optimization - angle between jets for 1TeV Higgs -> ZZ -> jj + stuff

14. 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 Compensating Calorimeters

15. Separate ECAL and HCAL E.g. CMS PbWO4 and Brass One combined Calorimeter E.g. ATLAS Liquid Argon Calorimetry near the beamline Calorimeter Design Concepts

16. 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

17. Hadronic Calorimeter • 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 • 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

18. CMS Radiation and Interaction Lengths 26 X0 in ECAL, after that the photons and electrons from the initial event have deposited their energy 8-10 interaction lengths by the end of the HCAL to measure most of the energy from strongly interacting particles

19. 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

20. 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

21. 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 Missing ET

22. 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)