$^{22}$C: $S$-Wave Two-Neutron Halo

A dripline nucleus $^{22}$C is studied in a Borromean three-body model of $^{20}$C+n+n. The valence neutrons, interacting via a realistic potential, are constrained to satisfy the Pauli exclusion principle. We obtain ample results supporting that $^{22}$C is an ideal $S$-wave two-neutron halo: The ground state is bound by about 500 keV, the neutron and proton root mean square radii are 4.0 and 2.4 fm, and the two neutrons are predominantly in $(s_{1/2})^2$ orbits. The binding mechanism of $^{22}$C is discussed. One- and two-body density distributions elucidate the halo character as well as the correlated motion of the neutrons. The reaction cross sections of $^{22}$C+$^{12}$C collisions are predicted