The Hoyle state of carbon-12 unmasked

The early, starless universe held only hydrogen and helium (with perhaps a trace of lithium) synthesized in the Big Bang. All other elements were forged in stellar furnaces. Carbon has a special status: Its fusion with helium nuclei (α particles) leads to nitrogen, oxygen, and eventually to the chemistry of life. But forming carbon is no simple task. In 1954 Fred Hoyle realized that after two α particles fuse to form beryllium-8 there is a finite chance that a third will join them in a three-body resonance. That near-ground-state resonance—now called the Hoyle state—usually falls apart but occasionally lasts long enough to decay to the ground state of ¹²C. The smoking-gun gamma ray from the decay was detected a few years later. A German–American group has performed new ab initio lattice calculations, including third-order corrections in chiral effective field theory, that reveal the lowest-energy configurations of the ¹²C nucleus. In the ground state, the α particles retain their individuality but cluster tightly in a compact triangle, which seems reasonable for the lowest energy. The Hoyle state was more surprising in that the α particles were in neither a tight cluster nor a straight chain. Instead they form an open obtuse angle, somewhat like a water molecule. The numerical results dovetail well with the available experimental data. Next on the group's agenda is a higher-resolution lattice. (E. Epelbaum et al., Phys. Rev. Lett. 109, 252501, 2012.)—Stephen G. Benka