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Register a new userLinear chain structure of four-$alpha$ clusters in $^{16}$O
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We investigate the linear-chain configurations of four-$alpha$ clusters in $^{16}$O using a Skyrme cranked Hartree-Fock method and discuss the relationship between the stability of such states and angular momentum. We show the existence of a region of angular momentum (13-18 $hbar$) where the linear chain configuration is stabilized. For the first time we demonstrate that stable exotic states with a large moment of inertia ($hbar^2/2Theta$ $sim$ 0.06-0.08 MeV) can exist.
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We present a new picture that the $alpha$-linear-chain structure for ${^{12}{rm C}}$ and ${^{16}{rm O}}$ has one-dimensional $alpha$ condensate character. The wave functions of linear-chain states which are described by superposing a large number of Brink wave functions have extremely large overlaps of nearly $100%$ with single Tohsaki-Horiuchi-Schuck-Ropke (THSR) wave functions, which were proposed to describe the $alpha$ condensed gas-like states. Although this new picture is different from the conventional idea of the spatial localization of $alpha$ clusters, the density distributions are shown to have localized $alpha$-clusters which is due to the inter-$alpha$ Pauli repulsion.
Observed well-developed $alpha$ cluster states in $^{16}$O, located above the four $alpha$ threshold, are investigated from the viewpoint of Bose-Einstein condensation of $alpha$ clusters by using a field-theoretical superfluid cluster model in which the order parameter is defined. The experimental energy levels are reproduced well for the first time by calculation. In particular, the observed 16.7 MeV $0_7^+$ and 18.8 MeV $0_8^+$ states with low-excitation energies from the threshold are found to be understood as a manifestation of the states of the Nambu-Goldstone zero-mode operators, associated with the spontaneous symmetry breaking of the global phase, which is caused by the Bose-Einstein condensation of the vacuum 15.1 MeV $0^+_6$ state with a dilute well-developed $alpha$ cluster structure just above the threshold. This gives evidence of the existence of the Bose-Einstein condensate of $alpha$ clusters in $^{16}$O. It is found that the emergence of the energy level structure with a well-developed $alpha$ cluster structure above the threshold is robust, almost independently of the condensation rate of $alpha$ clusters under significant condensation rate. The finding of the mechanism why the level structure that is similar to $^{12}$C emerges above the four $alpha$ threshold in $^{16}$O reinforces the concept of Bose-Einstein condensation of $alpha$ clusters in addition to $^{12}$C.
Recently, we have determined surface distributions of $alpha$ clusters in the ground state of $^{20}mathrm{Ne}$ from $alpha$-transfer cross sections, without investigating the properties of its excited states. In this paper we extend our comprehension of $alpha$-cluster structures in excited states of nuclei through reaction studies. In particular we focus on $^{16}mathrm{O}$, for which attention has been paid to advances of structure theory and assignment regarding $4^+$-resonance states. We study the surface manifestation of the $alpha$-cluster states in both the ground and excited states of $^{16}mathrm{O}$ from the analysis of the $alpha$-transfer reaction $^{12}mathrm{C}(^6mathrm{Li},d)^{16}mathrm{O}$. The $alpha$-transfer reaction is described by the distorted-wave Born approximation. We test two microscopic wave functions as an input of reaction calculations. Then a phenomenological potential model is introduced to clarify the correspondence between cluster-wave functions and transfer-cross sections. Surface peaks of the $alpha$-wave function of $^{16}mathrm{O}(0^+)$ are sensitively probed by transfer-cross sections at forward angles, while it remains unclear how we trace the surface behavior of $^{16}mathrm{O}(4^+)$ from the cross sections. We are able to specify that the $alpha$-cluster structure in the $0_1^+$ and $0_2^+$ states prominently manifests itself at the radii $sim 4$ and $sim 4.5$~fm, respectively. It is remarkable that the $4_1^+$ state has the $^{12}mathrm{C}+alpha$-cluster component with the surface peak at the radius $sim 4$ or outer, whereas the $^{12}mathrm{C}+alpha$-cluster component in the $4_2^+$ state is found not to be dominant. The $4_2^+$ state is difficult to be interpreted by a simple potential model assuming the $^{12}mathrm{C}+alpha$ configuration only.
We have studied gas-like states of $alpha$ clusters around an $^{16}$O core in $^{24}$Mg based on a microscopic $alpha$-cluster model. This study was performed by introducing a Monte Carlo technique for the description of the THSR (Tohsaki Horiuchi Schuck R{o}pke) wave function, and the coupling effect to other low-lying cluster states was taken into account. A large isoscalar monopole ($E0$) transition strength from the ground to the gas-like state is discussed. The gas-like state of two $alpha$ clusters in $^{24}$Mg around the $^{16}$O core appears slightly below the 2$alpha$-threshold e
Collisions of light and heavy nuclei in relativistic heavy-ion collisions have been shown to be sensitive to nuclear structure. With a proposed $^{16}mathrm{O}^{16}mathrm{O}$ run at the LHC and RHIC we study the potential for finding $alpha$ clustering in $^{16}$O. Here we use the state-of-the-art iEBE-VISHNU package with $^{16}$O nucleonic configurations from {rm ab initio} nuclear lattice simulations. This setup was tuned using a Bayesian analysis on pPb and PbPb systems. We find that the $^{16}mathrm{O}^{16}mathrm{O}$ system always begins far from equilibrium and that at LHC and RHIC it approaches the regime of hydrodynamic applicability only at very late times. Finally, by taking ratios of flow harmonics we are able to find measurable differences between $alpha$-clustering, nucleonic, and subnucleonic degrees of freedom in the initial state.
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