The phenomenon of core excitation in the breakup of a two-body halo nucleus
is investigated. We show that this effect plays a significant role in the
reaction dynamics and, furthermore, its interference with the valence
excitation mechanism has sizable and measurable effects on the breakup angular
distributions. These effects have been studied in the resonant breakup of 11Be
on a carbon target, populating the resonances at 1.78 MeV (5/2+) and 3.41 MeV
(3/2+). The calculations have been performed using a recently extension of the
DWBA method, which takes into account the effect of core excitation in both the
structure of the halo nucleus and in the reaction mechanism. The calculated
angular distributions have been compared with the available data [Fukuda et
al., Phys. Rev. C70,054606]. Although each of these resonances is dominated by
one of the two considered mechanisms, the angular patterns of these resonances
depend in a very delicate way on the interference between them. This is the
first clear evidence of this effect but the phenomenon is likely to occur in
other similar reactions.Comment: 5 pages, 2 figures, (Version to appear in Physical Review Letters
