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How abundant are the different generations of matter in the universe?

  1. Mar 3, 2012 #1
  2. jcsd
  3. Mar 3, 2012 #2
    Well forgetting neutrinos for minute, there are certainly far more first generation fermions than the others because the heavier fermions are unstable. Muons have a lifetime of 2 microseconds or so, and that is the longest of any of them.
    Neutrinos oscillate between flavours, so I am not quite certain what the correct thing to say about them is, but I am pretty sure that if you grab a random neutrino the probability of it being an electron neutrino is much higher than the other flavours (unless you build experiments specifically to watch for the oscillations).
  4. Mar 4, 2012 #3
    How long is the lifespan of a tau electron?

    Is it much less abundant than muons?

    Why aren't antiparticles, dark matter particles, the graviton and the higgs boson on this diagram?
    Last edited: Mar 4, 2012
  5. Mar 4, 2012 #4
    ττ ~ 2.9 x 10-13s - see http://en.wikipedia.org/wiki/Tau_lepton.

    Showing antiparticles would be redundant, as all particles have them (though some particles eg γ are their own antiparticle).

    Dark matter particles and gravitons have not yet actually been discovered in experiments, and as such are outside the current scope of the standard model - they are only postulated by other, as yet unconfirmed, theories. Though cosmological observations provide strong evidence for the existence of CDM, we don't yet know whether it actually exists or, if so, what exactly the particle(s) is(are).

    Nor, so far, has the Higgs been confirmed in experiments, even though it is a key part of the standard model theory. If the recent "hints" at LHC are confirmed, then it would belong in a separate column on the diagram. We don't yet know how many rows this column should have, as different theories predict different numbers of Higgs bosons.
  6. Mar 4, 2012 #5
    also the hypothetical supersymmatric partner particles are not listed because they havent been observed
  7. Mar 4, 2012 #6
    in the Standard Model, the second and third generation of quarks (charm, strange, bottom and top) and charged leptons (muon and tau) are unstable and their life time is much much smaller than the age of the universe. All three generations of quarks and leptons have an abundance (of the same order) in the early universe when they were in thermal equilibrium. The unstable particles decay when the temperature of the universe drops below their mass and stable particles left. So electrons and protons(up and down quarks) are the matter contents of the present universe.

    Neutrinos have a different story. When the universe expanded to a certain point, neutrinos cannot interact with other particles effectively. Then they left in the background of the universe forever. And all three generations of neutrinos exist.

    Anti-protons and positions can not exit because they would annihilate with protons and electrons in the universe and lead to disasters. The reason why no anti-protons/positions today is still an open question for particle cosmologists.

    However, the second and third generations of leptons/quarks and Anti-protons/positions are also constantly produced in the present universe by cosmic ray scattering, supernova etc. But that is negligibly small compared with proton and electrons and once they are produced, they decay in a very short time.

    Dark Matter might be a particle which is not listed in the Standard Model, some one in Supersymmetric Models is possible.
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