The vacuum, in particle physics language, is the ground state of a theory. A metastable vacuum is a local minimum of the theory but not the true ground state. If a system is prepared initially in a metastable state it will eventually decay through tunneling to the true ground state.
The role of metastable states in particle physics has expanded considerably with the realization that supersymmetric field theories can be phenomenologically viable even when no ground state breaks supersymmetry spontaneously. These scenarios require long-lived metastable states that do not preserve supersymmetry.
In work appearing in Physical Review D, Keith Dienes and Brooks Thomas of the University of Arizona study decays of metastable ground states of supersymmetric theories with multiple “photons” (a theoretical extension of our usual notion) and matter that is charged with respect to two distinct photons. The authors show that these theories can be arranged to have multiple metastable states allowing for complex decay patterns to lower energy states. The authors find a rich array of possibilities, including collapsing decays to the lowest energy state, cascades through multiple intermediate energy metastable states, and more surprisingly perhaps, states into which decays are not possible. – Ansar Fayyazuddin