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Particle Detection and Identification

Particle Detection and Identification. Roger Barlow Particle Physics Masterclass Manchester, March 20 th 2008. Studying Particles. Detection: Where are they? Very small Too small to see Identification: What are they? The particle-spotters’ guide. Detecting particles.

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Particle Detection and Identification

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  1. Particle Detection and Identification Roger Barlow Particle Physics Masterclass Manchester, March 20th 2008

  2. Studying Particles • Detection: Where are they? Very small Too small to see • Identification: What are they? The particle-spotters’ guide

  3. Detecting particles Rule 1: You can’t detect neutral particles, only charged particles. Rule 2: You can only detect charged particles if they’re moving – quite fast. Rule 3: Even then the signals are small and need amplifying Rule 4: You can only detect charged particles with ‘long’ lifetimes, i.e. >~1 ns. That basically means e, , , K, p

  4. Small but with a big kick Charged Particle Excited electron Electric Field What next? Two options ATOM

  5. Option 1: Excited to a higher level Photon Drops back

  6. Many atoms – many photons Scintillation particle Light • Good for measuring • Timing • Energy loss • Bad for measuring • position Collected and amplified by photomultiplier

  7. Option 2: Excited all the way out Positive ion Free electron

  8. Tracking detectors Electrons=charge=current Wire in a gas Big field near wire Amplification through avalanche process Good for position Wire At ~1 kV Geiger counter Multiwire chambers Drift chambers

  9. Tracking Chambers + - - + - + - + - + - + - + + - - + - +

  10. Summary so far We can detect a fast charged particle in all sorts of ways, based on • Scintillation • Ionisation What next?

  11. Birds: Size Shape Colour Sound Behaviour Particles Size Shape Colour Sound Behaviour Identification: What are they? electron Hadron (pi, K) muon proton positron

  12. What Charge is it? + or - ? Apply a magnetic field Particle curves to right or left depending on its charge Bonus: faster particles curve less Bend depends on momentum This measures momentum and direction

  13. Tracking B Path of a charged particle A measured point

  14. Spotting electrons/positrons Intersperse • Sensitive material – scintillators or tracking chambers • Dense material – sheets of iron or lead (or …) Electrons and positrons shower rapidly e- e-   e+ e- Hadrons shower more slowly Collide with protons/neutrons and produce more hadrons Muons don’t shower No strong interaction Bonus: photons convert to electrons and then shower Bonus: size of shower gives the energy

  15. Calorimeters Electron-positron Pair Incoming electron, positron or photon Shower of secondary particles • Count number of secondary particles in showerenergy of incoming particle

  16. Spotting muons Do not interact much • No shower in calorimeter • Penetrate through shielding Muon detector = charged particle detector put where other charged particles would be screened out Muon in Muon out Absorber

  17. Spotting hadrons Anything that is not a muon or an electron is a hadron (pion, kaon, proton) Telling the difference is possible but more complicated and less reliable…

  18. Parts of a Detector Muon chambers B Tracking Calorimeters

  19. DELPHI Detector

  20. Another detector: BaBar

  21. Yet another detector: ATLAS

  22. What about quarks? u,d,s,c,b,t • Never been seen directly • Manifest as jets of hadrons Bonus: gluons look almost just like quarks

  23. Quarks are jets e+ e- q q Many tracks Mostly hadrons Hadrons collimated into jets Jets back to backs

  24. Conclusion Elementary particles are very small BUT we can detect them Lots of different techniques – no single best method New ideas evolving all the time Yesterday’s detectors look primitive compared to today’s sophisticated and ingenious devices Tomorrow’s will be even better.

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