Chiral symmetry in matter

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Soft-pion theorems are used to show how chiral symmetry constrains the contributions of low-momentum pions to the quark condensate, the pion decay constant and hadron masses, all of which have been proposed as signals of partial restoration of chiral symmetry in matter. These have contributions of order T² for a pion gas or of order mₚi for cold nuclear matter, which have different coefficients in all three cases, showing that there are no simple relations between the changes to these quantities in matter. In particular, such contributions are absent from the masses of vector mesons and nucleons and so these masses cannot scale as any simple function of the quark condensate. More generally, pieces of the quark condensate that arise from low-momentum pions should not be associated with partial restoration of chiral symmetry.