Universality of Brunnian ($N$-body Borromean) four and five-body systems

Abstract

We compute binding energies and root mean square radii for weakly bound systems of $N=4$ and $5$ identical bosons. Ground and first excited states of an $N$-body system appear below the threshold for binding the system with $N-1$ particles. Their root mean square radii approach constants in the limit of weak binding. Their probability distributions are on average located in non-classical regions of space which result in universal structures. Radii decrease with increasing particle number. The ground states for more than five particles are probably non-universal whereas excited states may be universal.