Antihydrogen-Hydrogen annihilation at sub-kelvin temperatures

Annihilation cross-section of ultra low-energy atomic antihydrogen ($% E\le10^{-5}$ a.u.) on atomic hydrogen are calculated within a quantum mechanical coupled channels approach. The results differ from the extrapolations of semiclassical models to low energies. The main features of the observables are found to be determined by a family of $H\bar{H}$ nearthreshold states.