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center o bass

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$$n_i = g_i e^{\mu_i/T} \int \frac{d^3p}{(2\pi)^3} e^{-E_i/T}$$

and an equilibrium number density

$$n_i^{(0)} = g_i \int \frac{d^3p}{(2\pi)^3} e^{-E_i/T},$$

from which it follows that the equilibrium number density, has the same form as number density of non-equilibrium just with a chemical potential of zero.

Question: Why does the chemical potential vanish for the equilibrium case?