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. Identification: What are they?
Birds:
Size
Shape
Colour
Sound
Behaviour
Particles
Size
Shape
Colour
Sound
Behaviour
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
Incoming electron, positron or photon
Electron-positron Pair
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
Calorimeters
Tracking B

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.