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Questions regarding "Thermal History of the Early Universe"

  1. Dec 4, 2015 #1

    Buzz Bloom

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    I do not have any education in particle physics, and I am trying to read the paper
    http://www.helsinki.fi/~hkurkisu/cosmology/Cosmo6.pdf .
    I would much appreciate some help regarding the specific questions below.

    The following is from the first page of the article.
    I gather that g is a count of the different varieties of particles of different types. There are some oddities in the table I can't figure out, and I hope someone will be able to explain them to me.
    1. Under Quarks, g = 2⋅2⋅3. I get that one "2" is for the particle and its antiparticle, and the "3"is for 3 colors. What is the other "2"? Is it +1/2 and -1/2 spin values?
    2. For each of τ, μ, and e, g = 2⋅2=4, making 12 altogether. One "2" corresponds to the particle and antiparticle. What is the other "2" Is it +1/2 and -1/2 spin values?
    3. For Electroweak gauge bosons, g=3. Is this because there are 3 spin values: +1, 0, and -1?
    4. Under Electroweak gauge bosons, there is a single line for γ, the photon. Why is g = 2 for the photon?
    5. Also, what is the photon mass value in parentheses, "< 6×10-17 eV"?
    Last edited: Dec 4, 2015
  2. jcsd
  3. Dec 4, 2015 #2


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    The thing you are missing in both your first questions is polarisation. Both particle and anti-particle come with two spin possibilities.

    Same as for the quarks and leptons. It is the two polarisation directions.

    Most likely because it is zero according to theory and that is the upper bound.
  4. Dec 4, 2015 #3

    Buzz Bloom

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    Hi Orodruin:

    Thanks a lot for your prompt and useful answers.

    I have been updating the post, and I think I added a question after your answer post. Please look at the revised questions #3.

  5. Dec 4, 2015 #4


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    It is from 2006, so the experimental upper limit on the photon mass is a bit outdated. See the current status. Most of those values are a bit model-dependent, but they are all extremely tiny.
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