Orateur
Description
The second $0_{2}^+$ state of $^{12}$C at an excitation energy of 7.654 MeV, known as the Hoyle state [1], is crucial to understanding how $^{12}$C is formed in stellar nucleosynthesis. Despite recent studies, there is no consensus on the properties of the Hoyle state, with different theoretical models predicting a range of radii and spatial arrangements[2,3].
Experimentally, only a few attempts have been made to measure the radius of the Hoyle state, mostly through inelastic scattering angular cross sections. The most frequently cited study reported a 0.5 fm larger Hoyle state radius than the ground state radius [4] from $^{12}$C + $^{12}$C diffusion at 121.5 MeV. However, the extraction of the Hoyle state radius was based on a simple diffusion model relying on strong assumptions. Moreover, the cross section was measured at large angles, leading to the first minimum, expected at smaller angles, being missed.
To overcome these limitations, a new experiment was conducted at GANIL in 2025 to measure the Hoyle state radius by comparing single and mutual excitation in $^{12}$C + $^{12}$C inelastic scattering using the multi-detector FAZIA [5]. This comparative analysis eliminates many of the assumptions that were previously required, allowing for more accurate comparisons with modern scattering theory that incorporates realistic nuclear potentials.
In this talk, I will present this new experiment as well as the its results.
Bibliography
[1] F. Hoyle, On Nuclear Reactions Occuring in Very Hot STARS.I. the Synthesis of Elements from Carbon to Nickel, Astrophys. J. Suppl. Ser. 1, 121, (1954).
[2] Shen, S., Elhatisari, S., Lähde, T.A. et al., Emergent geometry and duality in the carbon nucleus, Nat Commun, 14, (2023)
[3] Otsuka, T., Abe, T., Yoshida, T. et al., \textit{$\alpha$-Clustering in atomic nuclei from first principles with statistical learning and the Hoyle state character, Nat Commun,13 , (2022)
[4] V. A. Maslov et al., Study of the Diffraction Scattering (^{12}C + ^{12}C)
with the Excitation of the (^{12}C) Exotic State + (0_{2}^{+})
(the Hoyle State), Physics of Particles and Nuclei Letters, 8, (2011)
[5] S Barlini et al., FAZIA: a new performing detector for charged particles, J. Phys.: Conf. Ser, 1561, (2020).