In this Review paper in p.1464, top left: Rev.Mod.Phys.80:1455-1515,2008 -- Color superconductivity in dense quark matter

It shows a figure of quark momentum of quark $u,d,s$, in unpairing and pairing phase, such that

$$p_d >p_u>p_s$$

however, we know: $$ m_u<m_d<m_s$$

so why isnt that $$p_u >p_d>p_s$$ if momentum is inverse proportional to the quark mass? What is going on here for these quark momentum?

enter image description here


We usually consider electrically neutral matter (because charged matter cannot exist in a macroscopic system -- the Coulomb energy diverges with the volume). In principle $uds$ matter with a deficit of strange quarks (because of the heavier strange quark) can be neutralized by electrons, but it is energetically favorable to neutralize by an excess of $d$ quarks (also negatively charged). The reason is that reference state, neutral matter with equal Fermi momenta for $u,d,s$ quarks, has a large Fermi momentum for down quarks, but zero Fermi momentum for electrons. A small shift in the chemical potentials then leads to a large shift of the down quark abundance, but only a small shift in the electron abundance.


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