  Fractionalized phases are studied in a low energy theory of exciton bose
condensate in a multi-band insulator. It is shown that U(1) gauge theory with
either fractionalized boson or fermion can emerge out of a single model
depending on the coupling constants. Both the statistics and spin of the
fractionalized particles are dynamically determined, satisfying the
spin-statistics theorem in the continuum limit. We present two mutually
consistent descriptions for the fractionalization. In the first approach, it is
shown that fractionalized degree of freedom emerges from reduced phase space
constrained by strong interaction and that the U(1) gauge field arises as a
collective excitation of the low energy modes. In the second approach,
complimentary descriptions are provided for the fractionalization based on
world line picture of the original excitons. The emergent gauge structure is
identified from the fluctuating web of exciton world lines which, in turn,
realizes the string net condensation in a space-time picture.
