Speaker
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 for understanding how $^{12}$C is formed in stellar nucleosynthesis. Despite extensive studies, the Hoyle state characteristics remain a challenging topic for nuclear structure theories: many theoretical models predict very different radii and spatial arrangements of this state [2,3].
Experimentally, only few attempts have been made in order to measure the radius of the Hoyle state, mostly using 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 and relied on strong assumptions. Moreover, the cross section was measured at large angles, leading to the first minimum expected at a smaller angle being missed.
To get rid of these limitations, a new experiment was conducted at GANIL in 2025 to measure the Hoyle state radius by comparing single- and double- 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 first results.
References :
[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] T. Otsuka, Abe, T., Yoshida, T. et al., $\alpha$-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).
