Mixing of partial waves near B^*B^{̄ *} threshold in e⁺e^- annihilation

We consider the production of B^*B^{̄ *} meson pairs in e⁺e^- annihilation near the threshold. The rescattering due to pion exchange between the mesons results in a mixing between three partial wave amplitudes: two P-wave amplitudes with the total spin of the meson pair S=0 and S=2 and an F-wave amplitude. The mixing due to pion exchange with a low momentum transfer is calculable up to c.m. energy E≈15-20 MeV above the threshold. We find that the P-F mixing is numerically quite small in this energy range, while the mixing of the two P-wave amplitudes is rapidly changing with energy and can reach of order one at such low energies.