Quartet condensation induced by the isovector pairing force.

The phenomenon of quartet condensation in the ground state of an isovector pairing Hamiltonian for an even-even $N=Z$ system is investigated. For this purpose we follow the evolution of the ground state from an unperturbed regime up to a strongly interacting one in a formalism of collective pairs. These pairs are those resulting from the diagonalization of the pairing Hamiltonian in a space of two particles coupled to isospin $T=1$. The ground state is found to rapidly evolve from a product of distinct $T=0$ quartets, each one formed by two of the above pairs, to a condensate of identical quartets built only with the pair corresponding to the lowest energy. This finding establishes a link between the complicated structure of the exact ground state and the simple approximation scheme of the Quartet Condensation Model. The mechanism at the basis of this quartet condensation turns out to be the same which is responsible for the development of a pair condensate in the ground state of a like-particle pairing Hamiltonian.