Search for the Efimov state near the 3$\alpha$ threshold

TL;DR

This study reports a high-precision search for the Efimov state in carbon-12 near the 3-alpha threshold, providing new experimental limits and insights into its structure and astrophysical implications.

Contribution

The paper presents the first high-sensitivity experimental observation of the Efimov state in $^{12}$C near the 3$ extalpha$ threshold, with improved sensitivity and new theoretical analysis.

Findings

Detected 21 probable Efimov state events at 7.458 MeV

Set an upper limit of 0.014% for the Efimov state's alpha-decay width

Found the Efimov state's effect on the triple-$ extalpha$ reaction rate to be significant in astrophysical scenarios

Abstract

A high-precision experiment in search of the predicted Efimov state in $^{12}\mathrm{C}$ at 7.458 MeV excitation energy was performed. Using a state-of-the-art detector system and novel analysis techniques, it was possible to observe the Efimov state at the predicted energy level above the 3$\alpha$ threshold with much better sensitivity in the $^{12} \mathrm{C}$ excitation energy spectrum compared to the existing data. The mutual $^{8} \mathrm{Be}$ resonance (91.84 keV) condition filters out a total of 21 probable Efimov state events around 7.458 MeV. With 2$\sigma$ confidence, it gives an upper limit of 0.014$\%$ for the Efimov state $\alpha$-decay width relative to that of the Hoyle state, which is about an order of magnitude smaller than the latest upper limit found in the literature. This observation was supported by a new penetrability calculation assuming a relatively extended structure of the Efimov state compared to the Hoyle state. The effect of the Efimov state was also explored in a nuclear astrophysical scenario, where the triple-$\alpha$ reaction rate, including both the Hoyle state and the Efimov state, was found to be larger than the allowed limit, while the temperature dependence of the combined rate was found to be compatible with the helium shell flash criterion of the AGB stars.