Four-body correlations in nuclei

Abstract

Low-energy spectra of 4$n$ nuclei are described with high accuracy in terms of four-body correlated structures ("quartets"). The states of all $N\ge Z$ nuclei belonging to the $A=24$ isobaric chain are represented as a superposition of two-quartet states, with quartets being characterized by isospin $T$ and angular momentum $J$. These quartets are assumed to be those describing the lowest states in ${}^{20}$Ne ($T_z$=0), ${}^{20}$F ($T_z$=1) and ${}^{20}$O ($T_z$=2). We find that the spectrum of the self-conjugate nucleus ${}^{24}$Mg can be well reproduced in terms of $T$=0 quartets only and that, among these, the $J$=0 quartet plays by far the leading role in the structure of the ground state. The same conclusion is drawn in the case of the three-quartet $N=Z$ nucleus ${}^{28}$Si. As an application of the quartet formalism to nuclei not confined to the $sd$ shell, we provide a description of the low-lying spectrum of the proton-rich ${}^{92}$Pd. The results achieved indicate that, in 4$n$ nuclei, four-body degrees of freedom are more important and more general than usually expected.

Comments: 5 pages, 4 figures