Matter densities of 8B and 8Li in a microscopic cluster model and the proton-halo problem of 8B

The microscopic generator-coordinate method is generalized to the calculation of matter densities in light nuclei having a cluster structure. Multipole components of the proton and neutron densities are calculated for the bound states and narrow resonances of 8B and 8Li, as well as for transitions between these states. The proton densities of 8B and neutron densities of 8Li resemble each other. They suggest that the 2+ and 1+ states belong to a same band while 3+ is of a different nature. For the 2+ ground state, they display long tails which can be related to the slow decrease of the 7Be+p and 7Li+n relative wave functions. The tails of the 8B neutron and 8Li proton densities are much shorter and are sensitive to the choice of the model space. The thicknesses of the proton skin in 8B and of the neutron skin in 8Li are 0.42 and 0.35 fm respectively. The large value of the 8B quadrupole moment is explained without a notion of halo, although the tail of the density contributes by about 20%. Whether the tail of the 8B proton density can help determine the astrophysical S factor for the 7Be(p, γ)8B reaction at zero energy is also discussed. ((orig.))