Two-proton emission from the ground state of [sup 12]O

The three-body decay [sup 12]O[r arrow]2[ital p]+[sup 10]C was studied following production via single-neutron stripping from a radioactive [sup 13]O projectile. This is the first observation of two-proton emission from an unbound ground state where the one-proton emission channel is energetically closed beyond the lightest case of [sup 6]Be. No evidence for [sup 2]He emission is seen, despite predictions for a large diproton branching ratio. An upper limit of 7% (95% C.L.) is established for this decay branch. The implications of the small diproton branching ratio observed here and seen previously in [sup 6]Be are discussed.