A polaron-type model is proposed to describe the deformation in the su
perfluid He-4 ground state, due to the presence of a (3)Ke atom. The i
nteraction matrix element of the model is expressed in terms of the st
ructure factor of the superfluid on the basis of the hypernetted chain
approximation. Both the use of an intermediate polaron coupling theor
y (rather than perturbation theory) and the determination of the preci
se interaction matrix elements are crucial ingredients to obtain an ad
equate theoretical description based on a polaron-type model. A genera
lization of Feynman's path-integral technique is used for the treatmen
t of intermediate coupling. The present work solves i.a. an outstandin
g problem regarding the He-3 effective mass in superfluid He-4. The th
eoretical result (an effective mass of 23123m(3) as compared to the ex
perimental value of 2.3m(,) where m(3) is the bare He-3 mass) confirms
that our formalism accurately describes the deformation cloud around
the He-3 atom. Also the difference between the specific heat of the mi
xture and the specific heat of pure superfluid He-4, and the diffusion
of He-3 in the superfluid, are accurately explained as the result of
the interaction of He-3 with the excitations of the superfluid.