Particle Detection and Identification

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 cant detect neutral particles, only
  charged particles.
• Rule 2 You can only detect charged particles if
  theyre 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. gt1 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
Free electron
Positive ion
8
Tracking detectors Electronschargecurrent
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
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 dont 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
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
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
• Yesterdays detectors look primitive compared to
  todays sophisticated and ingenious devices
• Tomorrows will be even better.