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.
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