Nature of light scalars through photon-photon collisions

The surprising thing is that arising almost 50 years ago from the linear sigma model (LSM) with spontaneously broken chiral symmetry, the light scalar meson problem has become central in the nonperturbative quantum chromodynamics (QCD) for 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 $q\bar q$ mesons that are produced via two-quark transitions $\gamma\gamma\to q\bar q$. Thus we get new evidence of the four-quark nature of these states.