A new measurement of the direct alpha-decay width of the Hoyle state in 12C

The so-called Hoyle state in C-12 (7.654 MeV, 0(+)) is an excited state of C-12 characterized by a well-pronounced 3 alpha cluster nature. Many possible cluster configurations, such as a linear-chain arrangement of 3 alpha particles, bent-arm, triangular or gas-like structures were theoretically conjectured within the previous decades, but, also due to the existence of ambiguities in experimental data, it is still impossible to draw a firm conclusion. Its direct alpha-decay partial width is accurately predicted by models but requires better experimental constraints. We performed a high-precision and high-selectivity investigation of its alpha-decay by using the N-14(d,alpha(2))C-12* reaction at 10.5 MeV incident energy with the aim of distinguishing between sequential and direct decay patterns. We are able to significantly improve the state of the art upper limit for direct 3 alpha decays by placing the new limit at 0.043%. Such a low limit allows to give fundamental constraints to theoretical models attempting to describe the structure of the Hoyle state.