  Recent polarized neutron scattering experiments on $YBa_2 Cu_3 O_7$ have
revealed a sharp spectral peak at the $(\pi,\pi)$ in reciprocal lattice
centered around the energy transfer of $41\ meV$. We offer a theoretical
explanation of this remarkable experiment in terms of a new collective mode in
the particle particle channel of the Hubbard model. This collective mode yields
valuable information about the symmetry of the superconducting gap.
