The Hoyle family: the search for alpha-condensate states in light nuclei

Our present understanding of the structure of the Hoyle state in ¹²C and other near-threshold states in α-conjugate nuclei is reviewed in the framework of the α-condensate model. The ¹²C Hoyle state, in particular, is a candidate for α-condensation, due to its large radius and α-cluster structure. The predicted features of nuclear α-particle condensates are reviewed along with a discussion of their experimental indicators, with a focus on precision break-up measurements. Two experiments are discussed in detail, firstly concerning the break-up of ¹²C and then the decays of heavier nuclei. With more theoretical input, and increasingly complex detector setups, precision break-up measurements can, in principle, provide insight into the structures of states in α-conjugate nuclei. However, the commonly-held belief that the decay of a condensate state will result in Nα-particles is challenged. We further conclude that unambiguously characterising excited states built on α-condensates is difficult, despite improvements in detector technology.