Weinberg sum rules, four-quark condensates and chiral symmetry restoration

The purpose of this talk is to raise the interest in four-quark condensates and to propose a specific four-quark condensate as an order parameter of chiral symmetry breaking. Studying the in-medium change of this four-quark condensate will give a clue where one can expect the restoration of chiral symmetry as a function of temperature and baryo-chemical potential.