Emergent universality in the two - neutron halo structure of 22 C

The structure of the two-neutron halo C is investigated by means of a renormalized zero-range three-body model, with interactions in the s-wave channel, and a finite-range model with twoand three-body forces provided by the hyperspherical adiabatic expansion method. In both models the halo wave function in configuration space is obtained by using as inputs the two-body scattering lengths and the two-neutron separation energy. The halo-matter density is computed for C with different three-body forces and low-energy parameters, with two-neutron separation energy within the range 50 keV S2n 1000 keV. The halo-neutron density depends weakly on the neutron-C scattering length as long as its absolute value is larger than the neutron-neutron one. The halo-neutron density is then analyzed by means of the root-mean-square radius, the probability density, and also the geometry, taking into account the angle between the two Jacobi coordinates. The results of finite-range and zero-range two-neutron-core models are compared. The effects in the halo structure of short-range and long-range three-body forces are studied, and the emergent universal behavior of the halo-neutron density and its geometry is pointed out.