A Solution of the Scalar Nonet Mass Puzzle

We present a short review dedicated to low-lying meson states. We present all meson nonets, which consist from up, down and strange light quarks. We consider the scalar nonet as a basic nonet. We work in the framework of the massless Nambu – Jona-Lasinio UR(3) × UL(3) quark model. The collective meson states are described through quark-antiquark pairs, whose condensates lead simultaneously to spontaneous breaking of the chiral and the flavour symmetry. We present an explanation of the inverse mass hierarchy of the low-lying nonet of the scalar mesons. The proposed explanation is based on symmetry principles. It is shown that, due to the flavour symmetry breaking, two isodoublets of K∗0 (700) mesons play the role of Goldstone bosons. It is also proven that there exists a solution with degenerate masses of the a0(980) and f0(980) mesons and a zero mass of the f0(500) meson. Short description of the physical properties of other meson nonets is provided. In particular a unique mass relations among the different nonets, which are experimentally confirmed, are presented.