A long-standing crucial question with atomic nuclei is whether or not α clustering occurs there. An α particle (helium-4 nucleus) comprises two protons and two neutrons, and may be the building block of some nuclei. This is a very beautiful and fascinating idea, and is indeed plausible because the α particle is particularly stable with a large binding energy. However, direct experimental evidence has never been provided. Here, we show whether and how α(-like) objects emerge in atomic nuclei, by means of state-of-the-art quantum many-body simulations formulated from first principles, utilizing supercomputers including K/Fugaku. The obtained physical quantities exhibit agreement with experimental data. The appearance and variation of the α clustering are shown by utilizing density profiles for the nuclei beryllium-8, -10 and carbon-12. With additional insight by statistical learning, an unexpected crossover picture is presented for the Hoyle state, a critical gateway to the birth of life.

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetricN/Zratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10⁻²¹ s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of²⁸O and²⁷O through their decay into²⁴O and four and three neutrons, respectively. The²⁸O nucleus is of particular interest as, with theZ = 8 andN = 20 magic numbers^1,2, it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both²⁷O and²⁸O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of²⁸O from a²⁹F beam is consistent with it not exhibiting a closedN = 20 shell structure.

Multiparticle-multihole states around O16 such as the famous 02+ state in O16 are investigated with a shell-model calculation in the full p-sd model space. It is found that the modification of the N=Z=8 shell gap due to correlation energy is unexpectedly large, which plays an essential role in accounting for those states. We point out that correlation energy affects the shell gap in the opposite way between O16 and Ni56, giving its mechanism in terms of the difference of the L-S and j-j closures.

We perform large-scale shell-model calculations of $\beta$-decay properties for neutron-rich nuclei with $13\le Z\le 18$ and $22\le N\le 34$, taking the first-forbidden transitions into account. The natural-parity and unnatural-parity states are calculated in the $0\hslash \omega$ and $1\hslash \omega$ model spaces, respectively, within the full $sd+pf+sdg$ valence shell. The calculated $\beta$-decay half-lives and $\beta$-delayed neutron emission probabilities show good agreement with the experimental data. The first-forbidden transitions make a non-negligible contribution to the half-lives of $N\gtrsim 28$ nuclei. The low-lying Gamow-Teller strengths of even-even nuclei are considerably larger than those of the neighboring odd-$A$ and odd-odd nuclei, strongly affecting the half-lives and neutron emission probabilities. It is shown that this even-odd effect is caused by the $J^{\pi }=1^{+}$ proton-neutron pairing interaction. We derive a formula to represent the positions of the Gamow-Teller giant resonances from the calculated strength distributions.

We clarify the origin of the anomalously hindered $E2$ decay from the ${4}_{1}^{+}$ level in $^{44}\mathrm{S}$ by performing a novel many-body analysis in the shell model. Within a unified picture about the occurrence of isomerism in neutron-rich sulfur isotopes, the ${4}_{1}^{+}$ state is demonstrated to be a $K=4$ isomer dominated by the two-quasiparticle configuration $$\nu${}{\mathrm{$\Omega${}}}^{$\pi${}}=1/{2}^{$-${}}$\bigotimes${}$\nu${}{\mathrm{$\Omega${}}}^{$\pi${}}=7/{2}^{$-${}}$. The ${4}_{1}^{+}$ state in $^{44}\mathrm{S}$ is a new type of high-$K$ isomer which has significant triaxiality.

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