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

The decay path of the Hoyle state in $^{12}$C ($E_x=7.654textrm{MeV}$) has been studied with the $^{14}textrm{N}(textrm{d},alpha_2)^{12}textrm{C}(7.654)$ reaction induced at $10.5textrm{MeV}$. 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 $3$-$alpha$ cluster model calculations, where higher precision limits are needed.