# What was above and below the QCD chiral symmetry breaking temperature?

Above a critical temperature in the Universe, there was probably a phase of unbroken approximate QCD chiral symmetry. Mathematically, the symmetry breaking is triggered when the operators $$\bar{\psi}{\psi}$$ for the quark fields $$\psi$$ acquired a nonzero vacuum expectation value (vev). This vev is called a quark condensate. For me this is a very mathematical description and I failed to translate it into a physical picture. Can someone explain what is really going on above and below this transition temperature in physical terms? My feeling is that above the transition temperature quarks where free and below it, the quarks and antiquarks became trapped to form mesons.

Chiral symmetry breaking is seen to happen in lattice QCD since a critical temperature $$T_c\approx 170\ MeV$$. Beyond this point the symmetry is restored and, when the chemical potential is also considered, there could be a critical end-point (CEP) beyond which a quark-plasma forms and we have a deconfined phase. Please, note that, as for today, we lack any experimental evidence for the existence of the CEP. This is a cross-over point marking the beginning of a first order phase transition. It means that, below $$T_c$$ there is the regime of chiral symmetry breaking of the existence of hadronic matter as we know. You can check the diagram I posted in my aforementioned answer.