1. Particle Physics LECTURE 7
Georgia Karagiorgi Department of Physics, Columbia University

2. Experimental Methods in Particle Physics
How we study particles in the “lab”
Last time!

3. Today’s agenda Review of particle properties
Passage of particles through matter
Ionization
Scintillation
Cherenkov radiation
Particle detectors
Modern detectors
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4. Particle detection
We exploit primarily the type of interaction that a particle most typically undergo:
Particles
Type of interaction
Signature
neutrinos
usually none
missing energy
electrons
ionization, electromagnetic
track and electromagnetic shower
muons
ionization
penetrating track
p, K, p
ionization, hadronic
track and hadron shower
photons
electromagnetic
electromagnetic shower
neutrons, K0
hadronic
hadron shower
Last time!

5. Particle detection
Last time!

6. Light detection Last time!
A photomultiplier tube (PMT) is a commonly used instrument used to detect visible photons.
Basis of operation:
photoelectric effect
 single photons converted to electrons and multiplied to a measurable electronic signal
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7. A note about cross-section
A measure of interaction probability in particle physics
“Effective area of collision”
Used to calculate predicted interaction rates
N(E) = F(E) × s(E)
interaction rate (events/sec)
cross-section, or probability of interaction (cm2)
flux of incoming particles (particles/cm2/sec)
Last time!

8. Cherenkov detector
Last time!

9. Cherenkov detector
Last time!

10. Ionization detector Detection method and choice of material
Induced electrical signal on anode can be measured to estimate number of drift electrons  E lost to ionization

11. Ionization detector
Geiger counter

12. Time Projection Chamber (TPC)
Exploits ionization energy losses of charged particles
Electrons are drifted onto a fine grained plane of wires, and the particle trajectories can be mapped out, along with their ionization energy loss, dE/dx.

13. Time Projection Chamber (TPC)
Example: ICARUS liquid argon TPC detector at Gran Sasso Lab, Italy

14. Scintillation detectors
Light output and type of particles/radiation

15. Scintillation detectors
Emitted light depends on detector material. Usually in the visible to UV range.
Sometimes requires the use of wavelength-shifting materials to shift UV light to visible, so it can be efficiently measured by commonly used photomultiplier tubes.

16. Scintillation detectors

17. Application of scintillation detectors
Cosmic ray muon detection

18. Cosmic ray muon detection
incident muon

19. Cosmic ray muon detection
Measurement of the muon lifetime
Measure tdecay (difference between muon signal and decay signal in the second scintillator paddle) of a sample N0 of low energy muons
Fit the data to an exponential curve of the form:
N(t) = N0e -t/T
where T = muon lifetime

20. Cosmic ray muon detection
Measurement of the muon lifetime
Measure tdecay (difference between muon signal and decay signal in the second scintillator paddle) of a sample N0 of low energy muons
Fit the data to an exponential curve of the form:
N(t) = N0e -t/T
where T = muon lifetime
Lifetime T:
T = 2.14μs
Tth = μs

21. Modern particle detectors: @ LHC

22. Why LHC? Large proton accelerator complex
Long-lived charged particles can be accelerated to high momenta using electromagnetic fields.
e+, e−, p, p̅, µ±(?) and Au, Pb & Cu nuclei have been accelerated so far...
Why accelerate particles?
High beam energies ⇒ high ECM ⇒ more energy to create new particles
Higher energies probe physics at shorter distances De-Broglie wavelength:
e.g. 20 GeV/c probes a distance of 0.01 fm.
An accelerator complex uses a variety of particle acceleration techniques to reach the final energy.

23. Particle accelerators around the world

24. Head-on collisions at the LHC

25. The CMS detector
15 m
22 m

26. A slice of CMS

27. Event signatures: the beauty of a magnetic field

28. Solenoid magnet Solenoid
Field direction along beam axis. Homogeneous field inside the coil. Need surrounding iron structure to capture the 'return field'.
CMS: I = 20 kA, B = 4T. Superconducting (4K).

29. Guess what event this is?

30. Guess what event this is?

31. What about “dark matter” ?
Does not interact through weak, electromagnetic, or strong force
But it does interact, very very weakly
We need very “quiet” (cryogenic, deep underground) detectors to see dark matter interactions
Scattering off of nuclei  transfer of kinetic energy

32. The XENON dark matter experiment

33. The XENON dark matter experiment

34. Modern particle detectors: @ South Pole

35. Modern particle detectors: @ South Pole

36. Modern particle detectors: @ South Pole

37. Modern particle detectors: @ South Pole

38. What are we trying to measure?

39. IceCube detector @ South Pole
A neutrino detector
Neutrino telescope
Looks for extra- terrestrial sources of neutrinos; like taking an “X-ray” picture of the Universe

40. IceCube detector @ South Pole
Made up of strings of >5,000 basketball- sized photon detectors called digital optical modules, or DOMs

41. IceCube detector @ South Pole
Made up of strings of >5,000 basketball- sized photon detectors called digital optical modules, or DOMs

42. IceCube detector @ South Pole
How does IceCube “see” neutrinos?
A neutrino interacts with an atom of ice, and produces a muon. The muon radiates blue light that is detected by the DOMs. The direction and intensity of the light is used to determine where the neutrino was coming from in the Universe.

43. IceCube detector @ South Pole
South pole  maps out neutrinos from the northern sky:

44. IceCube detector @ South Pole
Used to study extra-galactic, extremely high-energy sources.
Neutrinos, being nearly massless and without charge, are ideal messengers. They carry directional information. Charged particles are bent by magnetic fields, neutrons decay before reaching the earth and high-energy photons are absorbed.
E.g. gamma ray bursts: hadronic or electromagnetic origin?

45. IceCube detector @ South Pole
Can only be done at the South Pole !
Vast, extremely clean ice; it’s there for free!
Recall, detector constraints: cost and detector technology