  We present a detailed theoretical characterization of the two fundamental
collective resonances underlying the xenon giant dipole resonance (GDR). This
is achieved consistently by two complementary methods implemented within the
framework of the configuration-interaction singles (CIS) theory. The first
method accesses the resonance states by diagonalizing the many-electron
Hamiltonian using the smooth exterior complex scaling technique. The second
method involves a new application of the Gabor analysis to wave-packet
dynamics. We identify one resonance at an excitation energy of 74 eV with a
lifetime of 27 as, and the second at 107 eV with a lifetime of 11 as. Our work
provides a deeper understanding of the nature of the resonances associated with
the GDR: a group of close-lying intrachannel resonances splits into two
far-separated resonances through interchannel couplings involving the 4d
electrons. The CIS approach allows a transparent interpretation of the two
resonances as new collective modes. Due to the strong entanglement between the
excited electron and the ionic core, the resonance wave functions are not
dominated by any single particle-hole state. This gives rise to plasma-like
collective oscillations of the 4d shell as a whole.
