Two-Photon Production of Four-Quark States Up to the $J/ψ$ Energy

Abstract

Evidence for an explicitly exotic state with isospin 2 and spin-parity 2^+ near the $ρρ$ threshold and nontrivial complementary indications of the unusual quark composition of the $f_0(980)$ and $a_0(980)$ states obtained from the reactions of two-photon formation of neutral meson resonances are discussed, together with puzzling phenomena in the channels $γγ\toρ^0ϕ$ and $γγ\toρ^0ρ^0$ at high energies.