Dimesoatoms production in high energy collisions
  The production of two meson electromagnetic bound states and free meson pairs $\pi^+\pi^-$, $K^+K^-$, $\pi^+ K^{\mp}$ in relativistic collisions has been considered. It is shown that making use of the exact Coulomb wave function for dimesoatom (DMA) allows one to calculate the yield of any nS state with desired accuracy. The relative probabilities of production of DMA and meson pairs in the free state are estimated. The amplitude of DMA transition from 1S to 2P state, which is essential for the pionium Lamb shift measurements, has been obtained. 
L. Afanasyev, S. Gevorkyan, O. Voskresenskaya
hep-ph