Strong interactions in pionium ⁎

Pionium is investigated in various pion-pion strong interactions which reproduce well the pion-pion scattering data. It is found that the ground-state pionium wave functions in those realistic pion-pion strong interactions are considerably different from the hydrogen-like one at small distance. One may suggest that some pion-pion interactions may need to be largely improved before applied to the pion-pion atomic system.