QCD with massless quarks has chiral symmetry SU(Nf)L×SU(Nf)R. The QCD vacuum spontaneously breaks this to SU(Nf)V via a quark condensate ⟨ψ̄ψ⟩ ≠ 0.
The order parameter follows a Mexican hat potential. Goldstone's theorem predicts Nf²−1 massless bosons (pions), which gain small mass from explicit breaking by quark masses.
At T > Tχ ≈ 150 MeV, chiral symmetry restores: ⟨ψ̄ψ⟩ → 0.