Microscopic investigations of the structure of the bosons and the hamiltonian of the interacting boson model

In this paper we aim at a formal understanding of the IBA hamiltonian. We start from the point of view that the s- and d-bosons are the L = 0 and L = 2 members of two correlated protons or neutrons. The IBA bosons are therefore considered to be particle-particle bosons. These can conveniently be calculated within the broken pair approximation which is explained in some detail in the text. We then map the correlated two-particle states thus obtained onto a boson space; this is performed using either Marumori's or Dyson's mapping. We also shortly discuss how through a renormalization procedure we can eliminate the less collective members of the bosons to finally arrive at the IBA hamiltonian.