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The Large Hadron Collider Machine, Experiments, Physics Detector Physics 1: Calorimetry

The Large Hadron Collider Machine, Experiments, Physics Detector Physics 1: Calorimetry. Johannes Haller Thomas Schörner-Sadenius Hamburg University Summer Term 2009. THE ATLAS EXPERIMENT. - L ength ~40 m - Diameter ~25 m - Weight ~7000 t - 10 8 channels (2MB/event).

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The Large Hadron Collider Machine, Experiments, Physics Detector Physics 1: Calorimetry

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  1. The Large Hadron ColliderMachine, Experiments, PhysicsDetector Physics 1: Calorimetry Johannes HallerThomas Schörner-Sadenius Hamburg UniversitySummer Term 2009

  2. THE ATLAS EXPERIMENT - Length ~40 m- Diameter ~25 m- Weight ~7000 t- 108 channels (2MB/event) - ‘Inner Detector’ (tracking)- Numerous calorimeters- Large muon system ~40 Nations~150 Institutes~2000 physicists - central: solenoid around ID, Toroids in muon system- End caps: Toroidal fields UHH SS09: LHC

  3. LHC DETECTORS: ATLAS  Solenoid magnet in front of calorimeters  Curvature of tracks in muon system: into paper plane! ATLAS UHH SS09: LHC

  4. UHH SS09: LHC

  5. Transverse slice through CMS detector Click on a particle type to visualise that particle in CMS Press “escape” to exit UHH SS09: LHC

  6. COMPARISON OF SIZES ATLAS CMS UHH SS09: LHC

  7. INSERTION OF CMS MAGNET UHH SS09: LHC

  8. MOTIVATION – Also electrons very important for SM/BSM physics: – and of course you want hadronic jets to be measured with superior resolution (part of signal, SM background to new physics!): Aim is jet energy scale knowledge of 1%.  In order to achieve this, extremely well understood detectors for energy measurements (“calorimeters”) are needed. We haven’t yet discussed in detail, but: – among the very important signatures at the LHC will be photons, e.g. from light Higgs decays via fermion loops: However, the background from QCD jets, from prompt photons etc. is 6 orders of magnitude larger  need 1% mass, Eγ resolution. UHH SS09: LHC

  9. ATLAS: CALORIMETERS ‘Hadronic Tile’ - 463000 scintillating ‘tiles’- 10000 PMTs- granularity 0.1•0.1 - : <1.0, (0.8-1.7)- L=11.4 m, Rout=4.2 m ‘Hadronic LArEndcaps’ - Steel absorber - 4400 channels- 0.1•0.1 / 0.2•0.2- 1-5  ‘EM LAr Accordeon’ - Lead absorber - 174000 channels- 0.025•0.025- : <2.5, <3.2 ‘Forward LAr’ - 30000 ‘rods’, each 1mm- Cell size 2-5cm2 (4 rods)- : <3.1, <4.9- copper / tungsten ‘LAr Pre-Sampler’ Compensates energy loss in front of calorimeters UHH SS09: LHC

  10. BASIC FACTS AND DEFINITIONS (1) • Calorimeter – origin of name: • Instrument for measurement of heat generated by chemical, biological or physical processes. • Energy administered to a well-defined material volume is calculable from temperature change ΔT and specific heat capacity C: • Historic example: • Meitner & Orthmann 1930: Measurement of energy deposited by β decay of 210Bi with a copper calorimeter: 3.3 μW deposited by source of 7.6 Ci! • Measured mean energy ~ mean kinetic energy! • Principle of particle energy measurement is working – Aim: Determination of energy of charge and neutral particles (electrons, photons, hadrons). – Measurement is destructive: Particles loose their total energy in the calorimeter (except for muons) (in contrast to tracking devices). - loss of total energy requires high material density. - Since hadrons and leptons have different energy loss mechanisms, calorimeters are often separated in electromagnetic (EM) and hadronic (HA) parts. – Calorimeters are needed (and useful) for high-energy particles:  calorimeters get ever better for higher-energy particles!  and high-momentum tracks with small curvature don’t allow precise momentum determination. UHH SS09: LHC

  11. BASIC FACTS AND DEFINITIONS (2) – Response ε: average signal / particle energy. Should be linear and same for all particles! – MIP: minimum-ionising particle – deposited energy corresponds to Bethe-Bloch (-dE/dx) minimum. – e,h,π,mip: responses for e,hadrons,π,mpi. - e: response to EM component of hadrons. - h: non-EM response, assumed constant! - e/mip = 1. – Compensation: achieved if e/h = 1! Normally: - h < e: invisible energy in hadronic component! - EM contribution to hadrons increases with energy.  e/π > 1! And non constant! Problem! Try to achieve compensation (later)! Distinguish homogeneous and sampling calorimeters: – Homogenous calorimeters: Full calorimeter material is sensitive to particle and contributes to signal. Often realized as high-density crystals (OPAL, CMS) – Sampling calorimeters: Separate energy absorption and signal generation in passive and active medium. Only fraction of energy deposited in active material  worse resolution, cheaper (H1, ATLAS) CMS crystals, 2×2×23cm3. UHH SS09: LHC

  12. ENERGY LOSS OF ELECTRONS (1) Ionisation: – governed by Bethe-Bloch formula: … can be used for particle identification (later). • Main processes: • Bremsstrahlung: radiation of photons in Coulomb field: • – Depends strongly on nuclear charge Z. – Described by material-dependent radiation length X0: Length after which E has fallen to 1/e. • Scattering and annihilation processes:- Annihilation: e+e- γγ. • Moeller / Bhabha scattering • multiple scattering on nuclei. •  Significant only at low energies. E UHH SS09: LHC

  13. ENERGY LOSS OF ELECTRONS (2) AND MUONS Energy loss of muons: … and of other heavy charged particles: mainly ionisation and atomic excitation. At high energies also radiative losses: For the present-day muon energies, muons are almost always to be treated as MPIs (minimum-ionising particles) with almost constant (small) energy loss. Can be used for calibration purposes! Putting mechanisms together (e in Pb): Definition of critical energy Ecrit: Defined to be the electron energy at which ionisation and Bremsstrahlung losses are equal. UHH SS09: LHC

  14. ENERGY LOSS OF PHOTONS Minimum at 1-5 GeV – Atomic photo effect: Photon absorption and electron emission from atom: – Compton scattering: incoherent scattering of photons on atomic electrons: – Raleygh scattering: coherent scattering of photons on electrons. – Pair creation in fields of nuclei and electrons: – nuclear photo effect. Also different mechanisms: – Beer’s law with mass absorption coefficent μ: UHH SS09: LHC

  15. ELECTROMAGNETIC SHOWERS (1) • Simple model (Heitler): Assume interaction after one X0 (and symmetric energy sharing). • After T X0: 2T particles with energy E0/2T. Continue particle production until E<Ecrit. Then only ionisation left. • Note: cascade stops after tmax generations: •  weak dependence on E! For electrons and photons with high energy, the main processes are: – Bremsstrahlung of new photons. – Pair creation  Ever new EM particles are created; energy loss appears peu-a-peu, not in one step. Formation of an EM shower. Process continues until photons are too low-energy to split. EM shower in lead plates(high dE/dx), interleavedwith detector planes. UHH SS09: LHC

  16. ELECTROMAGNETIC SHOWERS (2) Derive a longitudinal profile function … The power of t describes particle production in the shower, and the exponential describes particle absorption. From that also the position of the shower maximum can be derived (a=-1. for e±, 0.5 for γ): t [X0] As a rule of thumb: 95% of the shower’s energy is containted in Also relevant: Transverse shower profile: Cylinder with Moliere radius RM around axis contains 90% of energy: t [X0] UHH SS09: LHC

  17. ELECTROMAGNETIC SHOWERS (3) Examples for transverse shower profiles of electrons of 100 GeV on liquid Krypton, photon and electron component separately UHH SS09: LHC

  18. ENERGY RESOLUTION (1) Then think about having a sampling calorimeter: sampling fluctuations: Resolution (dominant for sampling calorimeters) depends on sampling fraction f (fraction of energy deposited in active layers) and active layer thickness d: In addition photon statistics in PMTs and Landau fluctuations (high-energy transfer to electrons). The master formula for the energy resolution of a calorimeter is: Term 1/√E (normally dominant): – Stochastic fluctuations in shower development, – sampling fluctuations in sampling calorimeters,– particle statistics. Consider - stochastic fluctuations in particle number (Poisson)- number of track pieces with dE/dx: UHH SS09: LHC

  19. ENERGY RESOLUTION (2) Percentage los (%) – non-linearities: more relevant for higher energies. – inter-calibration between calorimeter cells. – Dead material in front of calorimeter (cables, trackers, magnets, …)  early showering  energy loss (at small E). (compensate with presamplers). Term: ~c/E: electronic noise; can mostly be neglected at present energies. Constant term : – Absorption losses (leakage out of the detector): increase with energy. – Model: Longitudinal more relevant! UHH SS09: LHC

  20. EXISTING EM CALORIMETERS (1) Homogeneous calorimeters: – Absorber = active material – good resolution, limited by homogenity, leakage – spatial (longitudinal) resolution limited – large volumina  large detectors, expensive  only useable for EM calorimeters. – typically scintillator crystals like BGO etc. Example CMS: - Design resolution: PbWO4 crystals, 2×2×2,3cm3. 26 X0 optical length (+3 pre) UHH SS09: LHC

  21. EXISTING EM CALORIMETERS (2) • Example OPAL: • 9440 lead-glass blocks of 26 X0: • projective geometry. UHH SS09: LHC

  22. EXISTING EM CALORIMETERS (3) Example ZEUS: - 12000 channels- /E~0.18/E for electrons- /E~0.35/E for hadrons Example Sampling calorimeters: – Absorber and active material separate. Active medium: gas, liquid, solid. – Absorber with high density  stopping power. – sampling fluctuations  worse resolution. – Signal readout according to active medium: - scintillation light  crystals, optical fibers. - charge  ionisation chambers, MWPCs, semiconductors, … Examples: – Pb – scintillator: Argus, H1-SpaCal – Pb/Fe – scintillator: CDF – Pb – LAr: H1 – Pb – gas: Delphi, Aleph – U – scintillator: ZEUS – U – LAr: D0 – … UHH SS09: LHC

  23. EXISTING EM CALORIMETERS (4) UHH SS09: LHC

  24. EXISTING EM CALORIMETERS (5) Example ATLAS: – Ionisation chambers with liquid argon at 90K. – 1-2 mm Pb/Fe absorbers; minimisation of inactive regions through accordeon shape. – 5×106 e- per GeV – finely segmented: - longitudinal: 6/24/1-12 X0; - transversely: Δη=0.018, Δφ=0.020 – preshower detector in x and y. – LAr is intrinsically radiation hard. – spatial resolution: 5mm / √E. – 5% homogenity in space and angle. UHH SS09: LHC

  25. ATLAS AND CMS UHH SS09: LHC

  26. EXISTING EM CALORIMETERS (6) Resolution for electrons of 10, 20, 30 and 50 GeV: - 45000 channels- /E~0.11/E for electrons- /E~0.50/E for hadrons • Example H1: • LAr calorimeter with copper/steel absorber • Additional calorimeter systems • – Organisation according geometry / expected particle flow. UHH SS09: LHC

  27. COMPARISON EM CALORIMETERS For some recent HEP experiments: UHH SS09: LHC

  28. HADRONIC SHOWERS (1) Description and measurement difficult: – About 7λ needed to contain shower  better use heavy absorbers: Showers initiated by hadrons are different: – nuclear reactions (strong interactions)! Multitude of processes with probabilities to be determined in experiment. – Production of many secondary particles: - EM fraction (π0γγ!) increases with energy - hadronic contribution: π±, n, p, … – equivalent to radiation length X0: nuclear absorption length: – Particle generation down to π threshold. – Number of secundary hadrons rises like ln(E). – Showers are broader than EM ones. – Invisible contribution  worse energy resolution. UHH SS09: LHC

  29. HADRONIC SHOWERS (2) Nuclear absorption length: Many contributions: – Ionisation and excitation (largest contribution to energy loss) - production of slow protons with very high ionisation density (Bethe-Bloch)  saturation effects – charge exchange reactions, pion decays - strong statistical fluctuations - logarithmic energy dependence – invisible component from neutrinos, neutrons, fast muons, … - free binding energy  small kinetic energies of nuclear pieces; partly compensated by capture and successive photon emission. - thrust of heavy nuclei, … – Production of slow neutrons: - energy loss via scattering with protons, or capture in nuclei - contribution to signal depends on passive/active medium (e.q. LAr: no contribution  invisible …) – … UHH SS09: LHC

  30. HADRONIC SHOWERS (3) EM hadronic binding /thrust E slow n Hadron shower signals: – Problem: different efficiencies/resolution for measurement of EM / hadronic part of showers. – In addition, EM fraction energy-dependent:  non-linearities: Relative contributions are function of energy of primary particle  optimisation! UHH SS09: LHC

  31. HADRON SHOWERS (4) Sampling calorimeters: more complicated …: – εe/εmip < 1 (0.6! – “transition effect”)! Quantity difficult to determine experimentally! – Define the sampling fraction: Ratio of energy deposited in active layers to total deposited energy for minimum ionising particles (muons)! – Response to pions: at low E like mips, at higher E (but below 5 GeV or so) like electrons (many neutral pions!)  non-linear! – At high E different non-linearities take over  In the end mostly non-linear response to hadrons!  One cure: Compensation: Aim for εe/εh = 1 ! In addition, the hadronic-shower signals are non-gaussian and have worse resolutions:  The worse the e/h ratio, the worse is also the resolution! (note relevance of energy scale and resolution for jet measurements!) ZEUS! e/h~1! UHH SS09: LHC

  32. COMPENSATION (1) Methods to achieve compensation: – Reducing the EM response: Choose high-Z absorber. Assume e/MIP~0.6, and same response for MIP and for non-EM component  accounted for 40% of invisible energy in the shower. Additional effect from wrapping active material in low-Z foil (stainless steel for ZEUS). e/mip or e/h ratio depends crucially on the foil thickness! Experience: h/e < 1 (non-compensating) mainly because of invisible energy in neutrinos, muons, K and π decays etc.  non-linearities, worse resolution … Compensation: selectively increase deposited energy for hadronic showers. First idea: Nuclear fission  extra energy from non-visible part (nuclear γs, soft neutrons, …) First test: Fabjan & Willis (1977): Although first test promising later many problems – over/under-compensation … many parameters enter! Detailed tuning of exploitation of different shower particles necessary. Response = slope UHH SS09: LHC

  33. COMPENSATION (2) – Boosting the non-EM component: (potentially much more powerful, specifically if response to neutrons is changed.). Depends critically on hydrogen fraction in active material (neutrons scatter elastically with protons which recoil and deposit their energies). – Play with the sampling fraction. The ratio of active to passive material also influences e/h since different response in different materials! Choice of gas! – Offline compensation: Employed for example by H1 Collaboration: Determine the energy density in showers (which is different for EM and HA-dominated parts) and apply weighting functions to the corresponding calorimeter cell energies. UHH SS09: LHC

  34. COMPENSATION (3) Details ZEUS: – Sampling calorimeter uranium / scintillator: Details H1: – No dependence of reconstructed energy of energy of hottest energy left after weighting. – Drastically improved resolution! after before linearity resolution UHH SS09: LHC

  35. HADRON CALORIMETER CMS (1) Details: – Segmented in barrel and endcap from 29 mm copper and 4 mm scintillator. – In barrel about 11 nuclear absorption lengths λ. – Readout via wavelength-shifting fibers: General: – Hadronic calorimeter important for measurement and identification of jets (from quarks and gluons) and of missing transverse energy (MET). – MET important for (for example) W, top, new physics, … – Resolution not so important as for EM calorimeter but need hermeticity (|η|<5). UHH SS09: LHC

  36. HADRON CALORIMETER CMS (2) e/π: non-linear! Resolution for pions E/p ratio for pions UHH SS09: LHC

  37. ATLAS: CALORIMETERS ‘Hadronic Tile’ - 463000 scintillating ‘tiles’- 10000 PMTs- granularity 0.1•0.1 - : <1.0, (0.8-1.7)- L=11.4 m, Rout=4.2 m ‘Hadronic LArEndcaps’ - Steel absorber - 4400 channels- 0.1•0.1 / 0.2•0.2- 1-5  ‘EM LAr Accordeon’ - Lead absorber - 174000 channels- 0.025•0.025- : <2.5, <3.2 ‘Forward LAr’ - 30000 ‘rods’, each 1mm- Cell size 2-5cm2 (4 rods)- : <3.1, <4.9- copper / tungsten ‘LAr Pre-Sampler’ Compensates energy loss in front of calorimeters UHH SS09: LHC

  38. (HADRON) CALORIMETER ATLAS (1) – All in all 10000 read-out channels. So-called Fe/scintillator Tile-Cal: – scintillating tiles (3 mm thickness) perpendicular to the beam interleved with iron slabs. – Readout via wavelength-shifting fibers. – Segmentation via bundeling of fibers  PMTs. UHH SS09: LHC

  39. (HADRON) CALORIMETER ATLAS (2) Excellent linearity < 2%! But danger of punch-through! e/π of about 1.1-1.25 UHH SS09: LHC

  40. OUTLOOK: CALICE FOR THE ILC simulated reconstructed The ILC: – The next generation of e+e- colliders. – Center-of-mass energy of up to 1 TeV!– Expect very dense environment. – Go for “tracking calorimeter”: follow path of particles within the calorimeter! UHH SS09: LHC

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