The surprising thing is that the light scalar meson problem, arising 50 years ago from the linear sigma model (LSM) with spontaneously broken chiral symmetry, has become central in the nonperturbative quantum chromodynamics (QCD), because it has been made clear that LSM could be the low energy realization of QCD. First, we review briefly signs of four-quark nature of light scalars. Then we show that the light scalars are produced in the two-photon collisions via four-quark transitions in contrast to the classic P wave tensor qq mesons that are produced via two-quark transitions γγ → qq. Thus we get new evidence of the four-quark nature of the lower scalar states.Outline 1. Introduction 2. Evidence for the four-quark nature of light scalar mesons i) Normal (qq) and inverted (q 2q2 ) mass spectra ii) The φ(1020) meson radiative decays about light scalars iii) Chiral shielding of the σ(600) meson in ππ → ππ 3. Light scalar manifestations in γγ collisions i) Prediction of the four-quark model. New stage of high statistics measurements, the Belle data ii) Dynamics of the σ(600) and f 0 (980) production in γγ → ππ iii) Dynamics of the a 0 (980) production in γγ → π 0 η 4. Future trends: the σ(600), f 0 (980) and a 0 (980) investigations in γγ → KK and in γγ * collisions 5. Summary
Introduction.The scalar channels in the region up to 1 GeV became a stumbling block of QCD. The point is that both perturbation theory and sum rules do not work in these channels because there are not solitary resonances in this region.At the same time the question on the nature of the light scalar mesons is major for understanding the mechanism of the chiral symmetry realization, arising from the confinement, and hence for understanding the confinement itself.2. Evidence for the four-quark nature of light scalar mesons. i) Normal (qq) and inverted (q