Radiative Width of the Hoyle State from γ-Ray Spectroscopy

The cascading 3.21 and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in 12 C , excited by the 12 C ( p , p ′ ) reaction at 10.7 MeV proton energy. From the proton- γ − γ triple coincidence data, a value of Γ rad / Γ = 6.2 ( 6 ) x 10 − 4 was obtained for the radiative branching ratio. Using our results, together with Γ E 0 π / Γ from Eriksen et al. [Phys. Rev. C 102, 024320 (2020)] and the currently adopted Γ π ( E 0 ) values, the radiative width of the Hoyle state is determined as Γ rad = 5.1 ( 6 ) x 10 − 3 eV . This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.The project was supported by the Australian Research Council Discovery Grants No. DP140102986 and No. DP170101673 and by the Research Council of Norway, Grant No. 263030. T. K., B. A., and A. E. S. acknowledge the hospitality of the University Oslo during the experiments. A. C. L. gratefully acknowledges funding from ERC-STG-2014 under Grant Agreement No. 637686, and support from the “ChETEC” COST Action (CA16117), supported by COST (European Cooperation in Science and Technology)