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Particle Identification

This article explores advanced methods for particle identification (PID) using time of flight, Cherenkov detectors, and specific energy loss measurements in high-energy physics. Learn about techniques, resolutions, and applications in particle ID.

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Particle Identification

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  1. dE/dx measurement Time of flight Cherenkov detectors Transition radiation detectors Particle Identification Particle Identification p p K K p p m m Christian Joram

  2. DELPHI Why particle ID ? A ‘charmless’ B decay: 1 K + 2 p in final state Who is who ? Christian Joram

  3. Specific energy loss Particle ID using the specific energy loss dE/dx Simultaneous measurement of p and dE/dx defines mass m, hence the particle identity e m m m p/K separation (2s) requires a dE/dx resolution of < 5% p p p K K K Average energy loss for e,m,p,K,p in 80/20 Ar/CH4 (NTP) (J.N. Marx, Physics today, Oct.78) p p p But: Large fluctuations + Landau tails ! Christian Joram

  4. Specific energy loss (backup) Improve dE/dx resolution and fight Landau tails • Chose gas with high specific ionization • Devide detector length L in N gaps of thickness T. • Sample dE/dx N times (B. Adeva et al., NIM A 290 (1990) 115) 4 wires 1 wire L: most likely energy loss A: average energy loss (M. Aderholz, NIM A 118 (1974), 419) Don’t cut the track into too many slices ! There is an optimum for each total detector length L. • calculate truncated mean, i.e. ignore samples with (e.g. 40%) highest values • Also pressure increase can improve resolution, but reduced rel. rise due to density effect ! Christian Joram

  5. Specific energy loss Example ALPEPH TPC Gas: Ar/CH4 90/10 Nsamples = 338, wire spacing 4 mm dE/dx resolution: 4.5% for Bhabhas, 5% for m.i.p.’s log scale ! linear scale ! Christian Joram

  6. Specific energy loss dE/dx can also be used in Silicon detectors Example DELPHI microvertex detector (3 x 300 mm Silicon) DE (a.u.) log p [GeV/c] DE (a.u.) log p [GeV/c] Christian Joram

  7. Time of flight Particle ID using Time Of Flight (TOF) start stop Combine TOF with momentum measurement Mass resolution TOF difference of 2 particles at a given momentum Dt for L = 1 m path length st = 300 ps p/K separation up to 1 GeV/c Christian Joram

  8. Time of flight Example: CERN NA49 Heavy Ion experiment detail of the grid Small, but thick scint. 8 x 3.3 x 2.3 cm Long scint. (48 or 130 cm), read out on both sides TOF requires fast detectors (plastic scintillator, gaseous detectors), approporiate signal processing (constant fraction discrimination, corrections + continuous stability monitoring. Christian Joram

  9. Time of flight From g conversion in scintillators System resolution of the tile stack L = 15 m Trel. = T / Tp NA49 combined particle ID: TOF + dE/dx (TPC) Christian Joram

  10. q e - Interaction of charged particles Remember energy loss due to ionisation… There are other ways of energy loss ! • A photon in a medium has to follow the dispersion relation schematically ! • For soft collisions + energy and momentum conservation  real photons:  Emission of Cherenkov photons if Christian Joram

  11. Cherenkov radiation Cherenkov radiation is emitted when a charged particle passes a dielectric medium with velocity Cherenkov detectors threshold ‘saturated’ angle (b=1) Number of emitted photons per unit length and unit wavelength interval Christian Joram

  12. Energy loss by Cherenkov radiation small compared to ionization (1%) Cherenkov detectors Number of detected photo electrons DE = E2 - E1is the width of the sensitive window of the photodetector (photomultiplier, photosensitive gas detector...) Example:for a detector with and a Cherenkov angle of one expects photo electrons Christian Joram

  13. Particle ID with Cherenkov detectors Cherenkov detectors Detectors can exploit ... • Nph(b):threshold detector • q(b):differential and Ring Imaging Cherenkov detectors “RICH” (do not measure qC) Threshold Cherenkov detectors principle Example: study of an Aerogel threshold detector for the BELLE experiment at KEK (Japan) Goal: p/K separation bkaon Christian Joram

  14. Cherenkov detectors Ring Imaging Cherenkov detectors (RICH) RICH detectors determine qC by intersecting the Cherenkov cone with a photosensitive plane  requires large area photosensitive detectors, e.g. • wire chambers with photosensitive detector gas • PMT arrays (J. Seguinot, T. Ypsilantis, NIM 142 (1977) 377) . . . . . . . . . . . n = 1.28 C6F14 liquid DELPHI p/K p/K/p K/p n = 1.0018 C5F12 gas p/h p/K/p K/p  minimize sq  maximize Np.e. Detect N photons (p.e.)  Christian Joram

  15. Cherenkov detectors Principle of operation of a RICH detectors DELPHI RICH 2 radiators + 1 photodetector A RICH with two radiators to cover a large momentum range. p/K/p separation 0.7 - 45 GeV/c: DELPHI and SLD spherical mirror C5F12 (40 cm, gas) C4F10 (50 cm, gas) Photodetector TMAE-based C6F14 (1 cm, liquid) (W. Adam et al. NIM A 371 (1996) 240) Two particles from a hadronic jet (Z-decay) in the DELPHI gas and liquid radiator + hypothesis for p and K Christian Joram

  16. Cherenkov detectors The mirror cage of the DELPHI Barrel RICH (288 parabolic mirrors) Christian Joram

  17. Cherenkov detectors “Marriage” of mirror cage and central detector part of the DELPHI Barrel RICH. Christian Joram

  18. Performance of DELPHI RICH (barrel) in hadronic Z decays Cherenkov detectors Liquid radiator gas radiator p p K p p K (E. Schyns, PhD thesis, Wuppertal University 1997) Christian Joram

  19. Transition radiation detectors Only high energy e± will emit TR. Identification of e± Transition radiation detectors (there is an excellent review article by B. Dolgoshein (NIM A 326 (1993) 434)) TR predicted by Ginzburg and Franck in 1946 Electromagnetic radiation is emitted when a charged particle traverses a medium with a discontinuous refractive index, e.g. the boundaries between vacuum and a dielectric layer. medium vacuum A (too) simple picture electron A correct relativistic treatment shows that… (G. Garibian, Sov. Phys. JETP63 (1958) 1079)  Radiated energy per medium/vacuum boundary Christian Joram

  20. Transition radiation detectors  Number of emitted photons / boundary is small Need many transitions  build a stack of many thin foils with gas gaps  X-rays are emitted with a sharp maximum at small angle  TR stay close to track  Emission spectrum of TR Typical energy:  photons in the keV range • Simulated emission spectrum of a CH2 foil stack Christian Joram

  21. TR Radiators: stacks of CH2 foils are used hydrocarbon foam and fiber materials Low Z material preferred to keep re-absorption small (Z5) Transition radiation detectors sandwich of radiator stacks and detectors  minimize re-absorption R D R D R D R D TR X-ray detectors: • Detector should be sensitive for 3  Eg  30 keV. • Mainly used: Gaseous detectors: MWPC, drift chamber, straw tubes… • Detector gas:sphoto effect  Z5 gas with high Z required, e.g. Xenon (Z=54) Intrinsic problem: detector “sees” TR and dE/dx dE/dx 200 e- TR (10 keV) 500 e- Pulse height (1 cm Xe) Discrimination by threshold t Christian Joram

  22. ATLAS Transition Radiation Tracker A prototype endcap “wheel”. X-ray detector:straw tubes (4mm) (in total ca. 400.000 !) Xe based gas TRT protoype performance Pion fake rate at 90% electron detection efficiency: p90 = 1.58 % Christian Joram

  23. Summary: A number of powerful methods are available to identify particles over a large momentum range. Depending on the available space and the environment, the identification power can vary significantly. A very coarse plot …. Particle Identification e± identification p/Kseparation Christian Joram

  24. Detector Systems Let’s find some tools … and put everything together ! Christian Joram

  25. Detector Systems Detector Systems Remember: we want to have info on... • number of particles • event topology • momentum / energy • particle identity Can’t be achieved with a single detector !  integrate detectors to detector systems Geometrical concepts Fix target geometry Collider Geometry “Magnet spectrometer” “4p Multi purpose detector” traget tracking muon filter N S barrel endcap endcap beam magnet calorimeter (dipole) • Limited solid angle dW coverage • rel. easy access (cables, maintenance) • “full” dW coverage • very restricted access Christian Joram

  26. Detector Systems collider geometry cont. Magnetic field configurations: solenoid toroid B B Imagnet coil Imagnet + Large homogenous field inside coil - weak opposite field in return yoke - Size limited (cost) - rel. high material budget Examples: • DELPHI (SC, 1.2T) • L3 (NC, 0.5T) • CMS (SC, 4T) + Rel. large fields over large volume + Rel. low material budget - non-uniform field - complex structure Example: • ATLAS (Barrel air toroid, SC, 0.6T) Christian Joram

  27. Detector Systems Typical arrangement of subdetectors Low density high density high precision  low precision high granularity  low granularity track density 1/r2 m+ e- g p vertex location (Si detectors)  main tracking (gas or Si detectors)  particle identification  e.m. calorimetry  magnet coil  hadron calorimetry / return yoke  muon identification / tracking  ATLAS and CMS require high precision tracking also for high energetic muons  large muon systems with high spatial resolution behind calorimeters. Christian Joram

  28. Detector Systems Some practical considerations before building a detector Find compromises and clever solutions … • Mechanical stability, precision  distortion of resolution (due multiple scattering, conversion of gammas) • Hermeticity  routing of cables and pipes • Hermeticity  thermal stability • Hermeticity  accessibility, maintainability • Compatibility with radiation … and always keep an eye on cost Composites are very interesting candidates, e.g. glass or carbon fiber reinforced epoxy materials. Christian Joram

  29. Detector Systems Radiation damage to materials Radiation levels in CMS Inner Tracker (0 < z < 280 cm) (=J/Kg) no damage moderate damage destruction H. Schönbacher, M. Tavlet, CERN 94-07 Christian Joram

  30. Detector Systems Christian Joram

  31. Detector Systems Christian Joram

  32. Detector Systems Christian Joram

  33. Detector Systems Christian Joram

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