Nuclear and Particle Physics 4b Physics of the Quark Gluon Plasma

Transcription

1 Nuclear and Particle Physics 4b Physics of the Quark Gluon Plasma Goethe University Frankfurt GSI Helmholtzzentrum für Schwerionenforschung Lectures and Exercise Summer Semester

2 Organization Language: English Lecture: Wednesday 13:00-15:00 Phys Marks / examination only if required / desired Seminar presentation schein Oral Exam grade Office hours: tbd on demand 2

3 Info: and Website Website: 3

4 Measurements in heavy ion collisions Beams Interaction of particle with matter Particle detectors Measurement of charged particles Charge and momentum deflection in the magnetic field Mass (particle identification) - specific energy loss - velocity (flight time, Cherenkov radiation,...) - Total energy - penetration calorimetry (measurement of muons) Measurement of neutral particles Reconstruction from measurements of the decay products calorimetry photon conversion Examples from real experiments 4

5 Beams 5

6 Beams 6

7 Beams 7

8 Criteria for beam energy 8

9 e+e- vs pp colliders 9

10 history 10

11 General demands on particle detectors 11

12 Decay length 12

13 Interaction of Particle Detectors particles with matter Energy loss by ionization (heavy particles) Interaction of electrons Energy loss by Ionization Gas detectors Semiconductors Scintillation counters Bremsstrahlung Cherenkov effect Transition radiation Interaction of photons Photo-effect Compton scattering Pair Production Electromagnetic Calorimeters Hadronic Calorimeters (Detection of neutral particles: neutrons and neutrinos) 13

14 Momentum Measurements For fixed B and q the momentum p is proportional to the radius of curvature 14

15 Forward Spectrometers Deflection in (y-z) plane 15

16 16

17 ALICE 17

18 Momentum Resolution Worsen momentum resolution 18

19 Magnets for 4 Detectors Solenoid + Large homogeneous field inside - Weak opposite field in return yoke - Size limited by cost - Relatively large material budget Examples: Delphi: SC, 1.2 T, 5.2 m, L 7.4 m L3: NC, 0.5 T, 11.9 m, L 11.9 m CMS: SC, 4 T, 5.9 m, L 12.5 m Toroid - Non-uniform field + Field always perpendicular to p + Rel. large fields over large volume + Rel. low material budget - Complex structural design Example: ATLAS: Barrel air toroid, SC, ~1 T, 9.4 m, L 24.3 m 19

20 Solenoidal and Toroidal fields at colliders Deflection in (x-y) plane 20

21 Momentum is determined by the track curvature 1/ in B field 21

22 Sagitta method Measure the sagitta of the track Simple case of 3 measurements: 22

23 Sagitta/radius obtained by a circle fit through measurement points along the track with point resolution x for each point Resolution: degrades linearly with momentum Improves linearly with B field Improves quadratically with radial extension of detector 23

24 Tracker Technologies 2 major technologies are used for tracking detectors Gaseous detectors Ionization in gas (creation of electron-ion pair) O(100 e/cm) Not sufficient to create significant signal height above noise for standard amplitudes Gas amplification needed to reach sufficient signal height above noise Silicon detectors Ionization (creation of electron-ion pair) in solid state material O(100 e/ m) No amplification needed 24

25 Gas detectors 25

26 Ionization chamber no gas gain, charges move in electric field and induce signal in electrodes. 2 electrodes form parallel plate capacitor. Proportional Counter gas amplification with a gas gain in vicinity of wire Multi-wire proportional chamber MWPC planar arrangement of proportional counters without separating walls Drift chamber needs well defined drift field introduction of additional field wires in between anode wires. number of anode wires can be reduced wrt MWPC at improved spatial resolution (but affected by diffusion) Time Projection Chamber 3-dimensional measurement of a track (mostly) cylindrical detector central HV cathode + MWPCs at the end-caps of the cylinder electrons drift in homogeneous electric fields towards MWPC, where arrival time and point and amount of charge are continuously sampled + complete track determination good momentum measurement + relatively few wires (mechanical advantage) + since also charge is measured: particle identification via de/dx + drift parallel to B transverse diffusion suppressed - drift time: relatively long - tens of microseconds - large data volume 26

27 Semiconductor detectors 27

28 Position measurement with semiconductor detectors segmentation of readout electrodes into strips, pads,pixels -tracking of particles close to primary vertex before multiple scattering -identification of secondary vertexes Micro-strip detectors principle and segmentation typical pitch m width of charge distribution = 10 m signal in 300 m Si: = e for minimum ionizing particles order 100 channels/cm2 Silicon drift detectors potential inside wafer, analog to gaseous drift chambers: charge carriers drifting in well-defined E-field measurement of drift time position of ionizing track typical drift time: a few s for 5-10 cm Pixel detectors principle: like micro-strips, but 2-dimensional segmentation of p+ contacts: 'pixel` each pixel connected to bias voltage and readout electronics + 2-dim information like double-sided micro-strip, but more simultaneous hits per detector + low capacity and thus low noise good S/N - large number of read-out channels expensive, large data volume - pixel contacts are complicated typical pixel areas 2000 m2 order 5000 channels/cm2 28

29 Interaction of Particle Detectors particles with matter Energy loss by ionization (heavy particles) Interaction of electrons Energy loss by Ionization Gas detectors Semiconductors Scintillation counters Bremsstrahlung Cherenkov effect Transition radiation Interaction of photons Photo-effect Compton scattering Pair Production Electromagnetic Calorimeters Hadronic Calorimeters (Detection of neutral particles: neutrons and neutrinos) 29

30 Particle Identification 30

31 Special Signatures 31

32 Energy loss by ionization 32

33 Energy loss by ionization at low energies / velocities decrease as approx. ~1/ 2 up to > 1 broad minimum at = MeV cm2/g `minimally ionizing particle' logarithmic rise and `Fermi plateau' caused by growth of the transverse component of the E field of the particle with γ more pronounced for gases (50%) than for solids (10%) very low velocities (v < v ) electron cannot be treated this way 33

34 Interaction of electrons Energy loss by ionization 34

35 Energy Loss de/dx Relativistic rise: due to Landau tail large overlap many measurements of de/dx likelihood 35

36 Homework Compute the range of 4 MeV α -particles in air and tissue. 36

37 Bremsstrhalung 37

38 38

39 Interaction of photons with matter 39

40 Photo Effect If E is high enough (Eγ > BeK binding energy on K-shell) the photo-effect will pass almost only on these electrons K(L,M)Edges: drop where K(L,M)-electrons are no longer available 40

41 Compton Scattering Compton Scattering Scattered photon energy Reflected electron energy We assumed: 1) scattering on free electron (E γ>>be) 2) electron is in the rest Alberica Toia Distribution of energy transferred to electrons 41

42 Pair Production 42

43 Total absorption cross-section Total cross-sections: σ = σf + σc + σp Multiply by number of atoms per volume unit N: μ= Nσ= N a ρσ A Na Avogadro constant, A atomic mass, ρ material density μ total absorption coefficient inverse value of mean free path of photon at material di = -μ I dx Equation for decreasing of photon number: I e x I0 43

44 Electron Photon Showers Combined effect of pair production and brammstrhalung for high energy photons - photon (E0) converts in matter into e+ e- pair (E0/2) - e+ and e- then emit energetic bremmstrhalung photons creation of an electromagnetic shower continues until the energy of the e+e- produced drops below the critical energy Ec when they lose energy by atomic collisions rather than bremmstrhalung Number of cascade particles increases geometrically: N(t) = 2t Mean energy of particles ε is: (t ) E E t 2 N (t ) Multiplication continues up to critical energy EC Maximal number of particles NMAX at deepness tmax N MAX ~ N MAX ~ E N (t MAX ) EC E N (t MAX ) EC tmax ~ ln Є Radiation length of material X0 : distance over which electron energy is reduced E E/e due to radiation loss only /2 1/4 1/8 Width of electromagnetic shower Moliere radius R0 ~ cm (about 90% of shower energy is inside) t N(t) ε(t)/e 44

45 EM Calorimeter 2 different types of detectors homogeneous calorimeter (lead glass, lead tungstate, barium fluoride) Sampling calorimeter (scintillator + absorber) as hadronic calorimeter ( 10 times the interaction length) Energy resolution Optical fiber collects light Bremsstrahlung effective cross-section decreases with 1 / m4 only applicable for the measurement of e ±, γ 45

46 Cherenkov effect 46

47 Cherenkov detectors 47

48 /K/p separation with several Cherenkov thresholds 48

49 49

50 Ring Imaging Cherenkov (RICH) 50

51 Example: K/p separation at p=200gev 51

52 Example: CERES RICH event display 52

53 Homework Compute the threshold energies an electron and a proton must possess in light water to emit Cherenkov radiation. An electron moving in water emits Cherenkov radiation in a cone making an angle of 40o with electron s direction of motion. Compute the number of photons emitted per centimeter by the electron. 53

54 DIRC: Detection of Internally Reflected Cherenkov light 54

55 Transition Radiation 55

56 Principle of a Transition Radiation Detector 56

57 Time of Flight 57

58 The time a relativistic particle, traveling at velocity v, covers a path of length L is: where E and pc are the particle energy and momentum where t0=l/c is the time taken by a particle traveling at the speed of light 58

59 Time of flight method 59

60 60

61 Resistive parallel chambers 61

62 Multi-gap resistive plate chambers 62

63 ALICE TOF: large coverage and high granularity Particle ID in high multiplicity environment... - Large array to cover whole ALICE barrel: 160 m2-100 ps time resolution - Highly segmented: 160,000 channels of size 2.5 x 3.5 cm2 to cope with very high multiplicity events TOF 63

64 Muon Measurements Momentum measurement: easy Track reconstruction in the magnetic field Identification: difficult de / dx: difficult to separate pions (mμ mπ) Time of Flight: difficult to separate pions (mμ mπ) Cherenkov: difficult to separate pions (mμ mπ) TR: TR not produced by muons Calorimetry: no showers production by muons Identification of decay products: rejected because muon to long-lived (τ = 2.2 microseconds) Absorber technology: muons penetrating more than other particles (except neutrinos) absorber thickness of several 64

65 Comparison different PID methods for K/ separation 65

66 Decay Particles Invariant Mass Combinatorial background Event topology 66

67 Invariant Mass 67

68 Event topology Impact Parameter: Prolongation of a track to the primary vertex. Distance between primary vertex and prolongation is called impact parameter. If this number is large the probability is high that the track comes from a secondary vertex. 68

69 Impact parameter resolution 69

70 Examle: ALICE Silicon Tracker 70

71 ALICE: A Large Ion Collider Experiment 71

72 ATLAS: A Toroidal LHC ApparatuS 72

73 ATLAS: A Toroidal LHC ApparatuS 73

74 CMS: Compact Muon Solenoid 74

75 CMS: Compact Muon Solenoid 75

76 Homework ALICE Collaboration, Performance of the ALICE Experiment at the CERN LHC, ALICE Collaboration, Particle identification in ALICE: a Bayesian approach, 76

77 Toroidal fields 77

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79 Principle of a Transition Radiation Detector 79

80 80