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In transition from quark-gluon plasma to hadron gas in the early Universe, the value of the effective degrees of freedom $g_{\star}$ decreases abruptly. This seems to me like a sort of decoupling: when quarks and gluons are free, they are in thermal equlibrium, but when they get together to form hadrons they "decouple" and stop contributing to the effective degrees of freedom. Why?

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I found an answer here:

All [hadrons] except pions are nonrelativistic below the QCD phase transition temperature. Thus the only particle species left in large numbers are the pions, muons, electrons, neutrinos, and the photons. For pions, g = 3, so now g∗ = 17.25.

So actually $g_{\star}$ is the effective relativistic degrees of freedom, and hadrons like $p$ or $n$ are not relativistic, whereas quarks like $u$ or $d$ are relativistic.

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