Complex sentence: It is possible to analyze the nuclear data in terms of "pure" neutron-antineutron annihilation; this is complicated, even in the simplest case of deuterium, by the presence of the proton nearby, which smears the final state momenta.
Split into simpler sentences: It is possible, however, to analyze the nuclear data in terms of "pure" neutron-antineutron annihilation. This is complicated, even in the simple case of deuterium, by the presence of the nearby proton, which smears the final state momenta.