  Macro-orbital representation of a particle (detailed account given in
cond-mat/0603784) has been used to develop the microscopic theory of a system
of interacting bosons. It concludes that: (i) below certain temperature (say,
$T_{\lambda}$), particles assume a state of (q, -q) bound pairs, (ii) the
$\lambda-$transition is a consequence of inter-particle quantum correlations
clubbed with zero-point repulsion and inter-particle attraction and represents
an onset of the order-disorder of particles in their $\phi-$space followed
simultaneously by their BEC as (q, -q) bound pairs in a state of q = $q_o =
\pi/d$ and K = 0, (iii) particles at $T \le T_{\lambda}$ acquire collective
binding which locks them at <k> = 0, <r> = $\lambda/2$ and $\Delta\phi = 2n\pi$
(with n = 1, 2, 3, ...), (iv) the entire system assumes mechanical strain in
inter-particle bonds and behaves like a single macroscopic molecule, (v) there
exists an energy gap between the superfluid and normal fluid phases of the
system, (vi) the $\lambda-$transition represents the twin phenomena of broken
gauge symmetry and phase coherence, (vii) the system does not have p = 0
condensate, (viii) a new kind of quantum quasi-particle "omon" (a phononlike
wave of the oscillations of the momentum coordinates of particles) exists in
superfluid phase, etc. It explains the properties of He-II, including the
origin of quantized vortices, critical velocities, logarithmic singularity of
specific heat, etc. at quantitative level and provides microscopic foundation
to two fluid theory, $\Psi-$theory, idea of macroscopic wave function, etc. The
framework of the theory can unify the physics of interacting bosons and
fermions.
