Detection methods in particle physics

Transcription

1 Detection methods in particle physics in most modern experiments look for evidence of quite rare events - creation of new particles - decays particles have short life times and move rapidly need detectors to track particles (if charged) and/or measure energies (charged/neutral) Acknowledgement: CERN academic training lectures

2 Early particle detectors The cloud chamber The track of a positron observed in a cloud chamber. The positron was discovered by Carl Anderson in The Wilson Expansion chamber invented by Charles Wilson (1910) for study the formation of water droplets that make clouds a vessel filled with moist air the temperature could be lowered rapidly by pulling a piston that caused the air to expand (the air becomes supersaturated) the water vapour would condense into droplets on dust particles - or a track of a charged particle that traverses the chamber at the right moment. the tracks could be photographed Wilson won the Nobel Prize in physics in

3 Early particle detectors - Photographic emulsion The tracks show the paths of charged particles in the nuclear emulsion. The incident particle called τ at the time comes to rest at the point P and disintegrates into three π-mesons, labelled a, b and c. One of the π-mesons (a) makes an interaction in the emulsion. The mass of the so called τ-particle was determined to 970 electron masses. This particle was later identified as the charged K-meson. Nuclear emulsion techniques were developed by Cecil Powell. By lifting a stack of emulsion plates to a high altitude tracks of charged particles - cosmic rays - could be observed. Cecil Frank Powell won the Nobel Prize in physics in

4 Bubble chambers The bubble chamber invented in 1952 Donald Glaser - a sealed container filled with liquefied gas - constructed so pressure can be reduced quickly - liquid kept superheated by high pressure - sudden expansion causes liquid to boil along the track of charged particle BNL s 7-foot bubble chamber Gargamelle at CERN

5 particles follow circular orbits perpendicular to the magnetic field p 0.3 B R GeV / T m c B magnetic field R radius of curvature Very detailed measurements possible

6 Disadvantages - low repetition rate (1-20 Hz) - analysis of film complicated Who can analyse pictures like that?

7 Electronic detectors Modern detectors are electronic - provide information about particle traces on timescales << μs The information is analysed by computers. ionization measurement track reconstruction particle identification wire chambers calorimeters energy measurement Cherenkov detectors particle identification detector systems

8 To be detected a particle must interact with the detector material! - strong interaction if hadron - weak interaction if ν new particles may be produced - some charged - a charged particle - ionizes - radiates electromagnetic processes the basis of most detectors for charged particles photons are also detected directly via the EM interaction for neutral particles the only means of detection are indirect

9 Interactions of photons with matter photo-electric effect dominates for E<400 kev γ + atom ion + e Compton scattering photon scatters off a quasi-free atomic electron dominates for 1 MeV<E<5 MeV γ + e γ + e pair-production dominates for E>5 MeV + γ + nucleus e + e + nucleus

10 L K Mass absorption coefficient for photons in lead. The contribution from the photo-electric effect increases at energies corresponding to the binding energies of the various atomic shells.

11 The electromagnetic interaction processes μ = 0 P μ ρ

12 8.9 cm 1.8 cm 0.6 cm

13 High-energy e _ or e + also initiate showers through the same processes.

14 Photon induced electromagnetic shower in each step # particles sharing the energy increases energy/particle decreases for electrons the brems process competes with ionization at low energies ionization dominates. relativistic electrons primarily loose energy by radiation The radiation energy loss of an electron traversing a thickness dx of material de E dx x = X 0 X 0 E = E0e

15 The energy at which radiation losses equal ionization losses is called the critical energy E crit 600 Z MeV Z = charge of material As the energy of the electrons falls below the critical energy - the electrons cease to radiate - they mostly loose energy by ionization - the development of the shower stops Typical critical energies: H 2 O ~ 90 MeV Pb ~ 7 MeV

16 Ionization losses of charged particles When a charged particle travels through a medium it transfers energy to the medium by inelastic collisions with atomic electrons ionization excitation of the constituent atoms We do not measure directly the energy losses of particles in matter but rather the effects of ionization. It turns out that ionization / cm = const. energy loss (combination of many effects)

17 2 1 β (valid for charged particles other than electrons) ) (1 2 ln 1 ) ( 4 1 β β β β α π ρ I c m z A Z N c m c dx de e A e h α = the fine-structure constant minimum-ionising particle 1-2 MeVg -1 cm 2 loss independent of particle mass

18 Cosmic rays:

19 Which is which? μ π K p p = mγβ If p is known, e m can be determined from de/dx Mean energy loss (measured in a drift chamber)

20 The showering property is used in energy measurement. Calorimeters a block of material of sufficient thickness - a particle interacts and deposits all of its energy - eventually all the energy converts into heat - a small fraction ionization scintillation light ~E Cherenkov radiation electromagnetic calorimeters hadronic calorimeters total absorption calorimeters (CsI, BGO, Pb-glass) sandwich calorimeters (Pb-Sc, Pb-liqAr, )

21 Shower initiated by a high energy photon (electron) - energy dissipated by bremsstrahlung and pair production - as depth increases the number of secondaries increases but their energy decreases - the multiplication process stops when energies fall below E c ionization only Shower reaches a maximum and declines - containment within some 10X 0 - depth for containment increases logarithmically with energy - lateral shower width determined by multiple scattering of low energy electrons and photons Cloud chamber photo of an EM shower

22 High energy hadrons interact with nuclei to produce secondary 0 hadrons ( n, p, K, π ±, π...) - a shower develops (qualitatively similar to the EM shower) - two components (hadronic, electromagnetic) - multiplication continues until pion threshold reached Hadronic shower dimensions governed by nuclear absorption length 1 λa σ inel Ex. Fe, λ a =16.6 cm About 10λ is needed for longitudinal containment. Energy resolution δe E = 30 80% E( GeV ) (Hadronic) cf. (EM) δe E = 3 10% E( GeV )

23 A total absorption calorimeter: The crystal photon spectrometer of the heavy ion experiment ALICE at CERN which contains 3584 lead tungstate (PbWO 4 ) crystals. One crystal is 18 centimetre long and weighs some 750 grams. Photo: Antonio Saba. Photographer. fermilabyrinth/games/ghostbustin/ NuTeV calorimeter: a sandwich of iron, scintillation counters and tracking chambers (~3x3x30 m).

24 Plastic scintillator Inorganic scintillators BGO crystal

25 Scintillating mechanism in organic scintillators: Excited modes of molecules which de-excite by emission of UV light. The UV light is then transformed into visible light by so-called wave-length shifters that are added to the material. Scintillating mechanism in inorganic scintillators: Loosely coupled electron-hole pairs known as excitons can migrate through the crystal and be captured by impurity centers where they de-excite under the emission of light.

26 Light in the scintillator propagates via internal reflection. Sometimes lightguides are used to provide coupling to an optical sensor. A cosmic ray detector: 2 scintillators in coincidence

27 The 10 PMT for the IceCube array. Picture from micro.magnet.fsu.edu

28 Cherenkov radiation When a charged particle moves through matter with a speed larger than the speed of light, coherent radiation is emitted at a characteristic angle w.r.t. the direction of motion. The Penn State Breazeale reactor Cherenkov condition v > c n ct cos θ = n = βct 1 βn No. of photons per unit distance N( λ) dλ = 1 2πα(1 β n ) 2 2 dλ 2 λ n = index of refraction mostly UV

29 Cherenkov Radiation Spherical waves emitted along the particle trajectory. slow fast

30 Threshold Counter 1 cos θ = 1 β βthr = βn 1 n Only particles with velocities above a threshold give light. Momentum measured elsewhere velocity ~ mass Ring imaging Counter Measure the Cherenkov angle β

31 Ring Imaging Detectors

32 Single wire proportional chamber electrons drift towards the anode wire E field close to wire high enough for electrons to ionize further avalanche typical wire Ø ~ few tens of μm G. Charpak, F. Sauli, J.C. Santiard Multi-wire proportional chamber many thin wires, few mm apart known position of the wires allows to reconstruct tracks first electronic device allowing high-statistics particle physics experiments position resolution ~1 mm

33 Drift chambers similar to proportional chambers wires few cm apart can cover large area measure time between arrival of the avalanche on wire and a trigger time, t º, signalling the passage of the primary particle t º measured by a separate detector resolution ~0.1 mm y = v drift t v drift the drift velocity in gas

34 Semiconductor microstrip detectors Thin electrode strips (typical distance ~20 μm) High purity Ge or Si crystal Passing charged particle produces electron-hole pairs. Resolution=strip width and distance:~0.01 mm

35 Construction of the drift chamber for BaBar experiment at SLAC (CP violation)

36 Silicon strip detector for CLEO (decay of c/b quarks.)

37 Detector systems Typical particle physics experiments involve simultaneous detection measurement identification of many particles several types of detection techniques complex detector systems

38 The CDF detector at the TEVATRON

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43 Search for the Higgs at LHC inelastic collisions / s Search for extremely rare events like: p + p H + X H + μ + μ + e + + e Maybe 1 event in but in total 10 8 particles in the detector. (per beam crossing ~20 collisions, beam crossing every 25 ns)