  The structure of a quantized vortex in a Bose-Einstein Condensate is
investigated using the projection method developed by Peierls, Yoccoz, and
Thouless. This method was invented to describe the collective motion of a
many-body system beyond the mean-field approximation. The quantum fluctuation
has been properly built into the variational wave function, and a vortex is
described by a linear combination of Feynman wave functions weighted by a
Gaussian distribution in their positions. In contrast to the solution of the
Gross-Pitaevskii equation, the particle density is finite at the vortex axis
and the vorticity is distributed in the core region.
