An improved upper limit on the direct 3α decay of the Hoyle state

The structure of the Hoyle state, a near-threshold 0⁺ state of extreme astrophysical significance in ¹²C has long been investigated. An experiment was performed to measure the branching ratio for the decay of this state directly into 3 α-particles. Such a branching ratio is expected to be a good observable for whether the resonance can be described as a dilute gas of α- particles known as an α-condensate. This experiment gave the best upper limits to date for this direct decay via the improvement of the traditionally used DDΦ model to isotropic decay to the available phase space. The new DDP² model includes three-body penetrabilities and gives a limit of < 0.026% (95% C.L.), a factor of 5 improvement over the previous experimentally obtained limit.