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Particle Identification

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1 Particle Identification
dE/dx measurement Time of flight Cherenkov detectors Transition radiation detectors Particle Identification This last lecture is devoted to particle identification. This means that we will discuss various methods which allow us to distinguish different particle species, like pions, kaons, protons etc. (Sometimes these methods open a window to new, quite unexpected physics) The first technique is the measurement of the specific energy loss. We have already discussed the basic facts of dE/dx in the first lecture on Monday, so that we can be rather short here. Also the discussion of the Time of Flight technique will be short, since it has rather limited importance in high energy physics. The main part of the lecture will therefore deal with Cherenkov and transition radiation detectors. K K p p p p m m CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

2 Why particle ID ? Who is who ? 1 K + 2 p in final state DELPHI
A ‘charmless’ B decay: 1 K + 2 p in final state Who is who ? CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

3 But: Large fluctuations + Landau tails !
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 ! CERN Summer Student Lectures 2003 Particle Detectors 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) 1 wire 4 wires 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 ! CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

5 Nsamples = 338, wire spacing 4 mm
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 ! CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

6 dE/dx can also be used in Silicon detectors
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] CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

7 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

8 Example: CERN NA49 Heavy Ion experiment
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. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

9 NA49 combined particle ID: TOF + dE/dx (TPC)
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) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

10 Interaction of charged particles
Remember energy loss due to ionisation… There are other ways of energy loss ! e - q 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

11 ‘saturated’ angle (b=1)
Cherenkov detectors Cherenkov radiation Cherenkov radiation is emitted when a charged particle passes a dielectric medium with velocity threshold ‘saturated’ angle (b=1) Number of emitted photons per unit length and unit wavelength interval CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

12 Number of detected photo electrons
Cherenkov detectors Energy loss by Cherenkov radiation small compared to ionization (1%) Number of detected photo electrons DE = E2 - E1 is 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

13 Particle ID with 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

14 (J. Seguinot, T. Ypsilantis, NIM 142 (1977) 377)
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 = C5F12 gas p/h p/K/p K/p  minimize sq  maximize Np.e. Detect N photons (p.e.)  CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

15 Principle of operation of a RICH detectors
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 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

16 The mirror cage of the DELPHI Barrel RICH (288 parabolic mirrors)
Cherenkov detectors The mirror cage of the DELPHI Barrel RICH (288 parabolic mirrors) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

17 “Marriage” of mirror cage and central detector part
Cherenkov detectors “Marriage” of mirror cage and central detector part of the DELPHI Barrel RICH. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

18 (E. Schyns, PhD thesis, Wuppertal University 1997)
Cherenkov detectors Performance of DELPHI RICH (barrel) in hadronic Z decays Liquid radiator gas radiator p p K p p K (E. Schyns, PhD thesis, Wuppertal University 1997) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

19 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 Only high energy e± will emit TR. Identification of e± CERN Summer Student Lectures 2003 Particle Detectors 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

21 Transition radiation detectors
TR Radiators: stacks of CH2 foils are used hydrocarbon foam and fiber materials Low Z material preferred to keep re-absorption small (Z5) R D R D R D R D sandwich of radiator stacks and detectors  minimize re-absorption 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

22 ATLAS Transition Radiation Tracker
A prototype endcap “wheel”. X-ray detector: straw tubes (4mm) (in total ca. !) Xe based gas TRT protoype performance Pion fake rate at 90% electron detection efficiency: p90 = 1.58 % CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

23 Particle Identification
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 …. e± identification p/K separation CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

24 and put everything together !
Detector Systems Let’s find some tools … and put everything together ! CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

25 Detector Systems Geometrical concepts
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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

26 Magnetic field configurations:
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) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

27 Typical arrangement of subdetectors high precision  low precision
Detector Systems Typical arrangement of subdetectors high precision  low precision Low density  high density 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. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

28 Some practical considerations before building a detector
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. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

29 Radiation damage to materials
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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

30 Detector Systems Christian Joram CERN Summer Student Lectures 2003
Particle Detectors Christian Joram

31 Detector Systems Christian Joram CERN Summer Student Lectures 2003
Particle Detectors Christian Joram

32 Detector Systems Christian Joram CERN Summer Student Lectures 2003
Particle Detectors Christian Joram

33 Detector Systems Christian Joram CERN Summer Student Lectures 2003
Particle Detectors Christian Joram

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