Background: Stellar carbon synthesis occurs exclusively via the $3\alpha$ process, in which three $\alpha$ particles fuse to form ${}^{12}\mathrm{C}$ in the excited Hoyle state, followed by electromagnetic decay to the ground state. The Hoyle state is above the $\alpha$ threshold, and the rate of stellar carbon production depends on the radiative width of this state. The radiative width cannot be measured directly, and must instead be deduced by combining three separately measured quantities. One of these quantities is the $E0$ decay branching ratio of the Hoyle state, and the current $10\%$ uncertainty on the radiative width stems mainly from the uncertainty on this ratio. The rate of the $3\alpha$ process is an important input parameter in astrophysical calculations on stellar evolution, and a high precision is imperative to constrain the possible outcomes of astrophysical models.

Purpose: To deduce a new, more precise value for the $E0$ decay branching ratio of the Hoyle state.

Method: The $E0$ branching ratio was deduced from a series of pair conversion measurements of the $E0$ and $E2$ transitions depopulating the $0_2^{+}$ Hoyle state and $2_1^{+}$ state in ${}^{12}\mathrm{C}$, respectively. The excited states were populated by the ${}^{12}\mathrm{C}(p,p^{\prime })$ reaction at 10.5 MeV beam energy, and the pairs were detected with the electron-positron pair spectrometer, Super-e, at the Australian National University. The deduced branching ratio required knowledge of the proton population of the two states, as well as the alignment of the $2_1^{+}$ state in the reaction. For this purpose, proton scattering and $\gamma$-ray angular distribution experiments were also performed.

Results: An $E0$ branching ratio of ${\mathrm{\Gamma }}_{\pi }^{E0}/\mathrm{\Gamma }=8.2\left(5\right)\times {10}^{-6}$ was deduced in the current work, and an adopted value of ${\mathrm{\Gamma }}_{\pi }^{E0}/\mathrm{\Gamma }=7.6\left(4\right)\times {10}^{-6}$ is recommended based on a weighted average of previous literature values and the new result.

Conclusions: The new recommended value for the $E0$ branching ratio is about 14% larger than the previous adopted value of ${\mathrm{\Gamma }}_{\pi }^{E0}/\mathrm{\Gamma }=6.7\left(6\right)\times {10}^{-6}$, while the uncertainty has been reduced from 9% to 5%. The new result reduces the radiative width, and hence $3\alpha$ reaction rate, by 11% relative to the adopted value, and the uncertainty to 6.1%. This reduction in width and increased precision is likely to constrain possible outcomes of astrophysical calculations.

• Received 28 May 2020
• Accepted 29 July 2020

DOI:https://doi.org/10.1103/PhysRevC.102.024320