  High-momentum components of nuclei are essential for understanding the
underlying inter-nucleon correlations in nuclei. We perform the comprehensive
analysis for the origin of the high-momentum components of $^4$He in the
framework of Tensor-optimized High-momentum Antisymmetrized Molecular Dynamics
(TO-HMAMD), which is a completely variational approach as an $ab$ $initio$
theory starting from the bare nucleon-nucleon ($NN$) interaction. The
analytical derivations are provided for the nucleon momentum distribution of
the Antisymmetrized Momentum Dynamics (AMD) wave functions, with subtraction of
center-of-mass motion. The nucleon momentum distribution for $^4$He is
calculated by applying a new expansion technique to our $ab$ $initio$ wave
function, and agrees with the values extracted from experimental data up to the
high-momentum region. Fine-grained analysis is performed for the high-momentum
components in $^4$He with respect to different nucleon correlations.
Contributions from tensor, central with short-range, and many-body correlations
are extracted from the nucleon momentum distributions. The manifestation of
tensor correlation around 2 fm$^{-1}$ region is explicitly confirmed by
comparing the momentum distributions predicted using different types of $NN$
interactions with and without the tensor force.
