Critically-bound four-body molecules

Abstract

The $(p,d,\bar{p},\bar{d})$ molecule, with a proton, a deuteron and their antiparticles, is stable againt spontaneous dissociation, but none of its three-body subsystems are stable. This molecule should be built by combining two atoms, for instance a protonium $(p\bar{p})$ and its heavier analogue $(d\bar{d})$. Most other four-body molecules have at least one stable three-body subsystem and thus can be built by adding the constituents one by one.