Detectors

1. Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors

2. Why do we accelerate particles ? • (1) To take existing objects apart • 1803 J. Dalton’s indivisible atom • atoms of one element can combine with atoms of other element to make compounds, e.g. water is made of oxygen and hydrogen (OH) • 1896 M. & P. Curie find atoms decay • 1897 J. J. Thomson discovers electron • 1906 E. Rutherford: gold foil experiment • Physicists break particles by shooting other particles on them

3. Why do we accelerate particles ? • (2) To create new particles • 1905 A. Einstein: energy is matter E=mc2 • 1930 P. Dirac: math problem predicts antimatter • 1930 C. Anderson: discovers positron • 1935 H. Yukawa: nuclear forces (forces between protons and neutrons in nuclei) require pion • 1936 C. Anderson: discovers pion muon • First experiments used cosmic rays that are accelerated for us by the Universe • are still of interest as a source of extremely energetic particles not available in laboratories

4. Generating particles • Before accelerating particles, one has to create them • electrons: cathode ray tube (think your TV) • protons: cathode ray tube filled with hydrogen • It’s more complicated for other particles (e.g. antiprotons), but the main principle remains the same

5. Basic accelerator physics • Lorentz Force: F = qE + q(vB) • magnetic force: perpendicular to velocity, no acceleration (changes direction) • electric force: acceleration

6. Accelerators: Cockroft-Walton • A (series of) voltage gap(s) • Maximum energy of a single gap is 200 kV, limited by discharge • CW accelerator at Fermilab: 750 kV

7. Accelerators: Van de Graaf • Van de Graaf generator: an electrostatic machine which uses a moving belt to accumulate very high voltages on a hollow metal globe 1: metallic sphere 2: electrode connected to 1 3: upper roller 4: belt (positive side) 5: belt (negative side) 6: lower roller 7: lower electrode (ground) 8: spherical device, used to discharge the main sphere 9: spark

8. Surfing the electromagnetic wave • Charged particles ride the EM wave • create standing wave • use a radio frequency cavity • make particles arrive on time • Self-regulating: • slow particle  larger push • fast particle  small push

9. Surfing the electromagnetic wave

10. How to create a standing wave ? • Klystron (S. & R. Varian) • electrons flow into cavity, excite eigen modes • creates standing electromagnetic waves • A similar device (magnetron) found in your microwave oven 325 MHz Klystron for Proton Driver Linac (Fermilab)

11. Cyclotron • 1929 E.O. Lawrence • The physics: centripetal force mv2/r = Bqv • Particles follow a spiral in a constant magnetic field • A high frequency alternating voltage applied between D-electrodes causes acceleration as particles cross the gap • Advantages: compact design (compared to linear accelerators), continuous stream of particles • Limitations: synchronization lost as particle velocity approaches the speed of light the world largest cyclotron at TRIUMF (520 MeV protons)

12. Synchrotron • The idea: both magnetic field strength and electric field frequency are synchronized with the traveling particle beam • particle trajectories confined to a thin vacuum beamline  no large magnets, expandable • synchrotron radiation limits its use for electrons • Currently, accelerators of this type provide highest particle energies in the world

13. Summary on accelerator types • Electrostatic accelerators • acceleration tube: breakdown at 200 keV • Cockroft-Walton: improves to 800 keV • AC driven accelerators • linear: cavity design and length critical • circular accelerators: • cyclotron: big magnet, non-relativistic • synchrotron: vacuum beamline, expandable, small magnets and cavities • synchrotron radiation large for light particles

14. Hadron vs electron colliders

15. Large Electron-Positron collider • Location: CERN (Geneva, Switzerland) • accelerated particles: electrons and positrons • beam energy: 45104 GeV, beam current: 8 mA • the ring radius: 4.5 km • years of operation: 19892000

16. Tevatron • Location: Fermilab (Batavia, IL) • accelerated particles: protons and anti-protons • beam energy: 1 TeV, beam current: 1 mA • the ring radius: 1 km • in operation since 1983

17. Large Hadron Collider • Location: CERN (Geneva, Switzerland) • accelerated particles: protons • beam energy: 7 TeV, beam current: 0.5 A • the ring radius: 4.5 km • scheduled start: 2007

18. Future of accelerators • International Linear Collider: 0.53 TeV • awaiting directions from LHC findings • political decision of location • Very Large Hadron Collider (magnet development ?): 40200 TeV • Muon Collider (source ?) 0.54 TeV • lepton collider without synchrotron radiation • capable of producing many more Higgs particles compared to an e+e collider

19. Conclusions • Motivation for particle acceleration • understand matter around us • create new particles • Particle accelerator types • electrostatic: limited energy • AC driven: linear or circular • Modern accelerators • TeVatron, LHC • accelerators to come: ILC, VLHC, muon collider…

20. Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors

21. Detectors and particle physics • detectors allow one to detect particles  • experimentalists study their behavior • new particles are found by direct observation or by analyzing their decay products • theorists predict behavior of (new) particles • experimentalists design the particle detectors

22. Overview of particle detectors • What do particle detectors measure ? • spatial location • trajectory in an EM field  momentum • distance between production and decay point  lifetime • energy • momentum + energy  mass • flight times • momentum/energy + flight time  mass

23. Natural particle detectors • A very common particle detector: the eye • detected particles: photons • sensitivity: high (single photons) • spatial resolution: decent • dynamic range: excellent (11014) • energy range: limited (visible light) • energy discrimination: good • speed: modest (~10 Hz, including processing)

24. Photographic paper • 1895 W. C. Röntgen: sensitivity to high energy photons (X-rays) invisible to the eye • working medium: emulsion • Properties: • detected particles: photons • sensitivity: good • spatial resolution: very good • dynamic range: good • no online recording • no speed resolution

25. The Geiger counter • 1908 H. Geiger • passing charge particles ionize the gas • ions (electrons) drift towards cathode (anode) • cause an electric pulse, can be heard in a speaker • Properties: • detected particles: charged particles (electrons, ,…) • sensitivity: single particles • spatial resolution: none (detector size) – can be fixed • dynamic range: none – can be fixed • speed: high (determined by charge drift velocity)

26. The cloud chamber • 1911 C. T. R. Wilson (1927 Nobel Prize) • the first tracking detector (tracking=many spatial measurements per particle) • Principle of operation: • an air volume is saturated with water vapor • pressure lowered to generate super-saturated air • charge particles cause saturation of vapor into small droplets  can be observed as a “track” • photographs allow longer inspection

27. The cloud chamber • Properties: • detected particles: charged particles (electrons, ,…) • sensitivity: single particles • spatial resolution: excellent • dynamic range: good • as particle slows down, droplets occur closer to each other • if placed inside a magnet, can observe curled trajectories • speed: limited (need time to recover the super-saturated state)

28. Photographic emulsions • Rarely used in modern experiments due to principal restrictions: • cannot be read out electronically • used to need a lot of technicians looking at photographs by eye – inefficient, boring, and error prone • today using pattern recognition software (think OCR) • cannot be used online • One advantage is excellent spatial resolution (<1 m) • Were used in the -neutrino discovery (DONUT, 2000)

29. Modern detector types • Tracking detectors • detect charged particles • principle of operation: ionization • two basic types: gas and solid • Scintillators • sensitive to single particles • very fast, useful for online applications • Calorimeters • measure particle energy • usually measure energy of a bunch of particles (“jet”) • modest spatial resolution • Particle identification systems • recognize electrons, charged pions, charged kaons, protons

30. Tracking detectors • A charged track ionizes the gas • 10—40 primary ion-electron paris • multiplication 3—4 due to secondary ionization • typical amplifier noise 1000 e— • the initial signal is too weak to be effectively detected ! • as electrons travel towards cathode, their velocity increases • electrons cause an avalanche of ionization (exponential increase) • The same principle (ionization + avalanche) works for solid state tracking detectors • dense medium  large ionization • more compact  put closer to the interaction point • very good spatial resolution

31. Calorimetry • The idea: measure energy by total absorption • also measure location • the method is destructive: particle is stopped • detector response proportional to particle energy • As particles traverse material, they interact producing a bunch of secondary particles (“shower”) • the shower particles undergo ionization (same principle as for tracking detectors) • It works for all particles: charged and neutral

32. Electromagnetic calorimeters • Electromagnetic showers occur due to • Bremsstrahlung: similar to synchrotron radiation, particles deflected by atomic EM fields • pair production: in the presence of atomic field, a photon can produce an electron-positron pair • excitation of electrons in atoms • Typical materials for EM calorimeters: large charge atoms, organic materials • important parameter: radiation length

33. Hadronic calorimeters • In addition to EM showers, hadrons (pions, protons, kaons) produce hadronic showers due to strong interaction with nuclei • Typical materials: dense, large atomic weight (uranium, lead) • important parameter: nuclear interaction length • In hadron shower, also creating non detectable particles (neutrinos, soft photons) • large fluctuation and limited energy resolution

34. Muon detection • Muons are charged particles, so using tracking detectors to detect them • Calorimetry does not work – muons only leave small energy in the calorimeter (said to be “minimum ionization particles”) • Muons are detected outside calorimeters and additional shielding, where all other particles (except neutrinos) have already been stopped • As this is far away from the interaction point, use gas detectors

35. Detection of neutrinos • In dedicated neutrino experiments, rely on their interaction with material • interaction probability extremely low  need huge volumes of working medium • In accelerator experiments, detecting neutrinos is impractical – rely on momentum conservation • electron colliders: all three momentum components are conserved • hadron colliders: the initial momentum component along the (anti)proton beam direction is unknown

36. Multipurpose detectors • Today people usually combine several types of various detectors in a single apparatus • goal: provide measurement of a variety of particle characteristics (energy, momentum, flight time) for a variety of particle types (electrons, photons, pions, protons) in (almost) all possible directions • also include “triggering system” (fast recognition of interesting events) and “data acquisition” (collection and recording of selected measurements) • Confusingly enough, these setups are also called detectors (and groups of individual detecting elements of the same type are called “detector subsystems”)

37. Generic HEP detector

38. D detector at Fermilab • D detector is one of two large multipurpose detectors at Fermilab (another one is CDF) • name = one of six intersection points

39. D: fairly typical HEP detector

40. D: tracking system (1) • Vertex detector: Silicon Microstrip Tracker • four layers of silicon detectors intercepted with twelve disks + (recent addition) Layer 0

41. D: tracking system (2) • Outer tracking detector: Central Fiber Tracker • sixteen double layers of scintillating fibers

42. D: calorimeter • Liquid argon / uranium calorimeter, consisting of central and two end calorimeters

43. D: outer muon system • The outermost part of the detector, surrounds the whole thing • Proportional Drift Tubes, Mini Drift Tubes • Central (Forward) muon SCintillators

44. D: other elements • Magnet: a central solenoid magnet (2 T) and outer toroid magnet • Luminosity scintillating counters • Central and forward preshower • Forward proton detector (Roman pots) • Data acquisition, trigger system, …

45. Conclusions • Particle detectors follow simple principles • detectors interact with particles • most interactions are electromagnetic • imperfect by definition but have gotten pretty good • crucial to figure out which detector goes where • Three main ideas • track charged particles and then stop them • stop neutral particles • finally find the muons which are left

46. Detectors 1. Accelerators 2. Particle detectors overview 3. Tracking detectors

47. Gas detectors • As a charged particle crosses a gas volume, it creates ionization • electrons get kicked out of atoms • the rest of atom becomes electrically charged (ion) • In absence of external field, ions and electrons recombine back to neutral atoms • electrons drift to anode • ions drift to cathode E = V/r ln(b/a)

48. Ionization • Affected by many factors • gas temperature • gas pressure • electric field • gas composition • Important parameters: • ionization potential • mean free path • Some gases eat up electrons (“quenchers”)

49. Ionization as a function of energy • Ionization probability gas dependant • General features: • threshold (~20 eV) • fast turn on • maximum (~100 eV) • soft decline eV

50. Mean free path • Average distance an electron travels before it hits an atom – determined by gas density • At ambient pressure (1013 hPa), air density is 2.71019 molecules/ccm, and mean free path is 68 m • At high vacuum (10—3…10—7 hPa), mean free path is 0.1…1000 m