Dipole Strength Distribution of ^{8}He and Decay Characteristics

The weak binding and spatially extended neutron densities characteristic of drip-line nuclei give rise to a distinctive low-energy dipole response. The drip-line nucleus ^{8}He is the most neutron-rich bound nucleus with a mass-to-charge ratio of A/Z=4. We measure the dipole response of ^{8}He, including for the first time the four-neutron decay channel. A total dipole strength of ∑B(E1)(E^{*}<15  MeV)=0.95(16)e^{2} fm^{2} and a dipole polarizability of α_{D}=0.61(1)  fm^{3} are extracted from the differential Coulomb-excitation cross section and compared to state-of-the-art theoretical calculations employing coupled cluster and three-body approaches. We find that the dipole continuum is dominated, even at high excitation energies well above the 4n decay threshold, by two-neutron emission, pointing to a ^{6}He+2n structure of the excited dipole mode. No indication was found for a 4n final-state correlation, while pronounced nn and ^{6}He-n final-state correlations are apparent.