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Like the title say, does quark color matter in counting contributions in a early universe plasma (QGP), as when adding up the total plasma energy density, or is it just spin? The book I have (Pathria) only does one example and doesn't really firmly say one way or the other. I kind of thought it was to account for all degeneracy but the table in the book only has "spin degeneracy".

E.g.: $u_{total}(T) = X\,u_{\gamma}(T) = \left(2 + 8 + 7/2 \right)\,\frac{u_{\gamma}(T)}{2}$ where the terms are for, photons, gluons, and charm/strange quarks. (I know these together do not make sense, but it is just an example).

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    $\begingroup$ Yes, and if you have a ``2'' for degrees of freedom of a photon you should put 16=2$\times$8 (polarization times color) for gluons. Also, charm plus strange gives $7/8\times 4\times 2=7$ (fermion corrections factor times Dirac=spin$\times$ particle/anti-particle times flavor). $\endgroup$
    – Thomas
    Commented Apr 15, 2013 at 0:06
  • $\begingroup$ That is what I have eventually learned as well - thank you and I would gladly accept your answer if you submitted it. $\endgroup$
    – nate
    Commented Apr 15, 2013 at 8:48

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