Hydrogen-antihydrogen atomic interaction at subKelvin temperatures

The annihilation cross-section of ultralow-energy atomic antihydrogen (E≤10-5 a.u.) on atomic hydrogen is 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 HH-bar near-threshold metastable states. We show that the presence of such states results in a strong enhancement of nuclear effects in HH-bar scattering, strong isotope effect in DH-bar scattering, oscillatory behavior of the cross-sections as function of the van der Waals constant C6, thought of as a free parameter