High precision probe of the fully sequential decay width of the Hoyle state in ${}^{12}$C

Abstract

The decay path of the Hoyle state in ${}^{12}$C ($E_x=7.654\text{MeV}$) has been studied with the ${}^{14}\text{N}(\text{d},\hat{I}{\pm }_2{)}^{12}\text{C}(7.654)$ reaction induced at $10.5\text{MeV}$. High resolution invariant mass spectroscopy techniques have allowed to unambiguously disentangle direct and sequential decays of the state passing through the ground state of ${}^8$Be. Thanks to the almost total absence of background and the attained resolution, a fully sequential decay contribution to the width of the state has been observed. The direct decay width is negligible, with an upper limit of $0.043\%$ ($95\%$ C.L.). The precision of this result is about a factor $5$ higher than previous studies. This has significant implications on nuclear structure, as it provides constraints to $3$-$\hat{I}\pm$ cluster model calculations, where higher precision limits are needed.