Four-body correlations in nuclei

Low-energy spectra of 4n nuclei are described with high accuracy in terms of four-body correlated structures ("quartets"). The states of all N≥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 4n nuclei, four-body degrees of freedom are more important and more general than usually expected.