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Homework Help: Conservation of Lepton Generation Number

  1. Apr 10, 2007 #1
    1. The problem statement, all variables and given/known data
    "Discuss the evidence for the law of conservation of lepton generation number. What is the evidence that neutrinos of different generations are distinct?" [8 marks]

    2. Relevant equations

    3. The attempt at a solution
    I understand the concept of lepton generation numbers and that within the standard model, if neutrinos have zero mass (although we know they're now massless), that each lepton number law is a separate conservation law, but what particular evidence is there that this is the case, apart from say the decay of a W-boson to a lepton and its neutrino (which in itself might only be showing that lepton number is conserved, rather than each lepton generation number)?

    Also, for the second part of the question about the 3neutrinos being distinct what evidence is there to show this? I thought that because of neutrino oscillations, the 3 different neutrino types are not distinct since their flavour can change as they propagate?

    Thanks very much
  2. jcsd
  3. Apr 10, 2007 #2


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    The effects of oscillation can be minimized by placing the detector close to the source of the neutrinos. Check out experiments to measure oscillation - there wouldn't be much point in looking for oscillation if there weren't evidence for distinct neutrino types, now would there?
  4. Apr 10, 2007 #3


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    If you had for exampel more muon events than electron events in a scattering process, that would indicate that there is two flavours of neutrinos. etc

    In the 1950s scientists began to wonder if the neutral particles emitted in pion- and muon decays were the same neutrino and antineutrino as was observed in beta decay. In the beginning of the 1960s, Melvin Schwartz and Leon Lederman build a giant spark chamber consisted of aluminum. If the neutrinos from pion decay ( pion -> moun + neutrino) were different from the ones created in beta decay, they would only see tracks from muons generated by neutrinos. Their first experiment showed only tracks from muons and none from electrons. Schwartz said “if there had been only one kind of neutrino, there should have been as many electron-type as muon-type events”. After this, particle physics started to differ between muon- and electron neutrinos in 1962.

    In 1977, the discovery of the tau lepton was made, and scientists began hunting for the tau neutrino. It took some time to invent a beem of tau neutrinos intense enough to detect them. The experimental proof for their existence came in 2000. Millions of traces from particles were examine on a special 3 dimensional photographic film (at DONUT experiment, Fermilab), and four events could only be interpreted as tau neutrinos that collided with nucleus and produced tau leptons.

    [From my "essay" on neutrinos]
  5. Apr 10, 2007 #4

    Meir Achuz

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    "Discuss the evidence for the law of conservation of lepton generation number."
    Muons do not decay to e + gamma.

    "What is the evidence that neutrinos of different generations are distinct?"
    The Lederman, Schwartz, Steinberg experiment described above.
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