  Electron quasiparticles play a crucial role in simplifying the description of
many-body physics in solids with surprising success. Conventional Landau's
Fermi-liquid and quasiparticle theories for high-temperature superconducting
cuprates have, however, received skepticism from various angles. A
path-breaking framework of electron fractionalization has been established to
replace the Fermi-liquid theory for systems that show the fractional quantum
Hall effect and the Mott insulating phenomena; whether it captures the
essential physics of the pseudogap and superconducting phases of cuprates is
still an open issue. Here, we show that excitonic excitation of optimally doped
Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ with energy far above the
superconducting-gap energy scale, about 1 eV or even higher, is unusually
enhanced by the onset of superconductivity. Our finding proves the involvement
of such high-energy excitons in superconductivity. Therefore, the observed
enhancement in the spectral weight of excitons imposes a crucial constraint on
theories for the pseudogap and superconducting mechanisms. A simple
two-component fermion model which embodies electron fractionalization in the
pseudogap state well explains the change, pointing toward a novel route for
understanding the electronic structure of superconducting cuprates.
