  Based on the Hubbard model description of the doped Mott insulator, we prove
that there must exist a spinless collective excitation of charge $2e$ and of
energy $U-2\mu$ peaked at momentum $(\pi/a,\pi/a)$ in the non-superconducting
state due to doping. Such a collective excitation arises from the electron pair
correlation. Its existence is related to the breakdown of an approximate SU(2)
particle-hole symmetry, from which an effective field theory of the new
excitations is constructed.
