Who plays in the Hoyle band?

The Hoyle state is an excited, resonant state of ¹²C. It is produced via the fusion of 3 α particles, and strongly influences the rate of the triple-alpha process in stars. The Hoyle state was discovered over 50 years ago in 1957. Recently, significant experimental and theoretical effort has been invested to understand, in detail, its unorthodox structure. In particular, identification of excitations of the Hoyle state has been a key goal. Of the recent experimental results, two approaches aimed at elucidating the first excitation of the Hoyle state point to an I^π=2⁺ resonance between 9.5 and 10 MeV. The first approach used is that of inelastic excitation using beams of protons and particles while the second approach used ¹²C (γ,α₀) resonance reactions. These two distinct approaches produce consistent results within approximately one standard deviation. As for the structure of the Hoyle state, recent experimental results have now ruled out some long-standing predictions for its shape (i.e. a linear α chain), but distinguishing between an α gas and an α-particle condensate requires still further precision measurements