  Theory of doped Mott insulators is revisited in the light of recent
understanding on the singular self-energy structure of the single-particle
Green's function. The unique pole structure in the self-energy induces the
high-temperature superconductivity in the anomalous part, while it generates
Mott gap and pseudogap in the normal part. Here, we elucidate that
fractionalization of electrons, which is exactly hold in the Mott insulator in
the atomic limit, more generally produces the emergent Mott-gap fermion and
dark (hidden) fermions. It does not require any spontaneous symmetry breaking.
The two gaps are the consequences of the hybridization of these two fermions
with quasiparticles. We further propose that the Mott-gap fermion and dark
fermions are the fermionic component of Frenkel- and Wannier-type excitons,
respectively, which coexist in the doped Mott insulator. The Bose-Einstein
condensation of the Frenkel-type excitons allowed without spontaneous symmetry
breaking holds a key for understanding the unique pole structure and the
pseudogap through the instantaneous hybridization between the fractionalized
quasiparticle and the dark fermion in analogy with the Mott gap. We argue that
the high-$T_{\rm c}$ superconductivity is ascribed to the dipole attraction of
the Wannier-type excitons. The gap formation mechanism is compared with that
caused by conventional spontaneous symmetry breaking known over condensed
matter and elementary particle physics including quantum chromodynamics. We
propose a theoretical framework and discuss experimental tests to analyze this
idea and concept.
