  S-wave pairing in neutron matter is studied within an extension of correlated
basis function (CBF) theory to include the strong, short range spatial
correlations due to realistic nuclear forces and the pairing correlations of
the Bardeen, Cooper and Schrieffer (BCS) approach. The correlation operator
contains central as well as tensor components. The correlated BCS scheme of
Ref. [Nucl. Phys. A363 (1981) 383], developed for simple scalar correlations,
is generalized to this more realistic case. The energy of the correlated pair
condensed phase of neutron matter is evaluated at the two--body order of the
cluster expansion, but considering the one--body density and the corresponding
energy vertex corrections at the first order of the Power Series expansion.
Based on these approximations, we have derived a system of Euler equations for
the correlation factors and for the BCS amplitudes, resulting in correlated non
linear gap equations, formally close to the standard BCS ones. These equations
have been solved for the momentum independent part of several realistic
potentials (Reid, Argonne v_{14} and Argonne v_{8'}) to stress the role of the
tensor correlations and of the many--body effects. Simple Jastrow correlations
and/or the lack of the density corrections enhance the gap with respect to
uncorrelated BCS, whereas it is reduced according to the strength of the tensor
interaction and following the inclusion of many--body contributions.
