  Integrating seminal ideas of London, Feynman, and Feenberg, this paper
continues the development of an ab initio theory of the lambda transition in
liquid 4He. The theory is based on variational determination of a trial density
matrix suitable for description of both normal and superfluid phases, which
includes the effects both of temperature-dependent dynamical correlations and
of statistical correlations corresponding to thermal phonon-roton and
quasiparticle/quasihole excitations - all at the level of two-point
descriptors. A full functional minimization of the associated free energy is
performed through solution of a coupled set of Euler-Lagrange equations.
Violation of particle-hole exchange symmetry emerges as a crucial aspect of the
lambda transition, along with broken gauge symmetry. In conjunction with a
semi-phenomenological study in which renormalized effective masses are
introduced for quasiparticle/quasihole excitations, the results suggest that a
quantitative description can be achieved once the trial density matrix is
modified to include backflow effects and allow for ab initio treatment of
important variations in effective masses.
