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

## TL;DR

This study precisely measures the decay pathways of the Hoyle state in carbon-12, confirming it predominantly decays sequentially through beryllium-8, with a negligible direct decay component, thus refining nuclear structure models.

## Contribution

It provides the most precise measurement to date of the decay width of the Hoyle state, especially constraining the direct decay component to less than 0.043%.

## Key findings

- Sequential decay dominates the Hoyle state decay.
- Direct decay contribution is negligible, below 0.043%.
- Results improve constraints for 3-alpha cluster models.

## Abstract

The decay path of the Hoyle state in $^{12}$C ($E_x=7.654\textrm{MeV}$) has been studied with the $^{14}\textrm{N}(\textrm{d},\alpha_2)^{12}\textrm{C}(7.654)$ reaction induced at $10.5\textrm{MeV}$. 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.