  Using the density-matrix renormalization group in combination with the
Chebyshev polynomial expansion technique, we study the two-hole excitation
spectrum of the one-dimensional Hubbard model in the entire filling range from
the completely occupied band (n=2) down to half-filling (n=1). For strong
interactions, the spectra reveal multiplon physics, i.e., relevant final states
are characterized by two (doublon), three (triplon), four (quadruplon) and more
holes, potentially forming stable compound objects or resonances with finite
lifetime. These give rise to several satellites in the spectra with largely
different spectral weights as well as to different continua. The complex
multiplon phenomenology is analyzed by interpreting not only local and
k-resolved two-hole spectra but also three- and four-hole spectra for the
Hubbard model and by referring to effective low-energy models. In addition, a
filter-operator technique is presented which allows to extract specific
information on the final states at a given excitation energy. While multiplons
composed of an odd number of holes do neither form stable compounds nor
well-defined resonances unless a nearest-neighbor density interaction V is
added to the Hamiltonian, the doublon and the quadruplon are well-defined
resonances. The k-resolved four-hole spectrum at n=2 represents an interesting
special case where a completely stable quadruplon turns into a resonance by
merging with the doublon-doublon continuum at a critical wave vector. For all
fillings with n > 1, the doublon lifetime is strongly k-dependent and is even
infinite at the Brillouin zone edges as demonstrated by k-resolved two-hole
spectra. This can be traced back to the "hidden" charge-SU(2) symmetry of the
model which is explicitly broken off half-filling and gives rise to a massive
collective excitation, even for arbitrary higher-dimensional but bipartite
lattices.
