Quartet structure of N = Z nuclei in a boson formalism: The case of 28Si

The structure of the N=Z nucleus 28Si is studied by resorting to an IBM-type formalism with s and d bosons representing isospin T=0 and angular momentum J=0 and J=2 quartets, respectively. T=0 quartets are four-body correlated structures formed by two protons and two neutrons. The microscopic nature of the quartet bosons, meant as images of the fermionic quartets, is investigated by making use of a mapping procedure and is supported by the close resemblance between the phenomenological and microscopically derived Hamiltonians. The ground state band and two low-lying side bands, a β and a γ band, together with all known E2 transitions and quadrupole moments associated with these states are well reproduced by the model. An analysis of the potential energy surface places 28Si, only known case so far, at the critical point of the U(5)-SU(3)‾ transition of the IBM structural diagram.