The Hoyle Family: The Search for Alpha-Condensate States in Light Nuclei

Abstract

Our present understanding of the structure of the Hoyle state in 12 C and other near-threshold states in α-conjugate nuclei is reviewed in the framework of the α-condensate model. The 12 C 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 12 C 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 N α-particles is challenged. We further conclude that unambiguously characterising excited states built on α-condensates is difficult, despite improvements in detector technology. This contribution has been presented during the ceremony of the Few-Body Systems Award for young professionals.

Almost medium-free measurement of the Hoyle state direct-decay component with a TPC

“…Breit-Wigner) of resonance line-shapes [17]. A non-zero branching ratio was also predicted in previous studies [3,4] although the ability to determine these as direct-looking rather than pileup on an event-by-event basis was not possible.…”

“…This preferentially populates the center of the Dalitz plot where all the α-particles have similar energies. It is therefore practically identical to the DDE (Direct Decay Energy-sharing) model where the α-particles have identical energies, smeared only by the uncertainty principle [3,5]. The experimental data, shown in Fig.…”

“…When the average nuclear density drops below 1/3 of its normal value, the lowest-energy state is a bosonic cluster of α-particles, a state with some properties similar to a Bose-Einstein condensate. Such a hypothesis has received extensive study in the past decade theoretically, but experimental observables for such an exotic state are extremely difficult to obtain [3]. One relevant observable is the direct 3-body decay of the Hoyle state, i.e.…”

Prospects of searching for unstable nucleus states in relativistic nuclear fragmentation

“…The HS state is the second excitation of the 12 C nucleus (review [2]) at 378 keV above the 3α threshold. The isolation of HS at the beginning of the 12 C excitation spectrum and the width Γ(HS) = 9.3 ± 0.9 eV make it a 3α analogue of 8 Be.…”

“…The HS state is the second excitation of the 12 C nucleus (review [2]) at 378 keV above the 3α threshold. The isolation of HS at the beginning of the 12 C excitation spectrum and the width Γ(HS) = 9.3 ± 0.9 eV make it a 3α analogue of 8 Be. Synthesis of 12 C in the medium of red giants is possible through the merger 3α → α 8 Be → 12 C(0 + 2 ) → 12 C (+2γ or e + e − with a probability of about 10 −4 ).…”