Dynamically generated mesons and chiral restoration

Spontaneous chiral symmetry breaking and its restoration in a QCD medium is one of the most interesting aspects of non-perturbative QCD [1]. One clear sign that chiral symmetry is broken in the QCD vacuum comes from a comparison of the spectra of two quark currents related by a chiral transformation, namely the isovector–vector and isovector–axial-vector current. If the vacuum obeyed chiral symmetry, both spectra would be the same. Instead one finds the ρ/a1 meson in the vector/axial-vector channel [2]. Without much doubt the ρ meson is dominantly a quarkantiquark state [3]. In contrast, we have demonstrated recently [4] that the axial-vector spectrum can be well understood with the proposition [5] that the a1 meson is a dynamically generated state, i.e. that it is dominantly a π-ρ “molecule” or correlation. This is demonstrated in Fig. 1.