  We investigate multiparticle excitation effect on a collective density
excitation as well as a single-particle excitation in a weakly interacting
Bose--Einstein condensate (BEC). We find that although the weakly interacting
BEC offers weak multiparticle excitation spectrum at low temperatures, this
multiparticle excitation effect may not remain hidden, but emerges as
bimodality in the density response function through the single-particle
excitation. Identification of spectra in the BEC between the single-particle
excitation and the density excitation is also assessed at nonzero temperatures,
which has been known to be unique nature in the BEC at absolute zero
temperature.
