  We perform coupled-cluster calculations of ground- and dipole excited-state
properties of the 8He halo nucleus with nucleon-nucleon and three-nucleon
interactions from chiral effective field theory, both with and without explicit
delta degrees of freedom. By increasing the precision in our coupled-cluster
calculations via the inclusion of leading order three-particle three-hole
excitations in the cluster operator, we obtain a ground-state energy and a
charge radius that are consistent with experiment, albeit with a slight
under-binding. We also investigate the excited states induced by the electric
dipole operator and present a discussion on the Thomas-Reiche-Kuhn and cluster
sum rules. Finally, we compute the electric dipole polarizability, providing a
theoretical benchmark for future experimental determinations that will study
this exotic nucleus.
