  An effective field theory for clean electron systems is developed in analogy
to the generalized nonlinear sigma-model for disordered interacting electrons.
The physical goal is to separate the soft or massless electronic degrees of
freedom from the massive ones and integrate out the latter to obtain a field
theory in terms of the soft degrees of freedom only. The resulting theory is
not perturbative with respect to the electron-electron interaction. It is
controlled by means of a systematic loop expansion and allows for a
renormalization-group analysis in a natural way. It is applicable to universal
phenomena within phases, and to transitions between phases, with order
parameters in arbitrary angular-momentum channels, and in the spin-singlet,
spin-triplet, particle-hole, and particle-particle channels. Applications
include ferromagnetic and ferrimagnetic ordering, non-s-wave ferromagnetic
order (magnetic nematics), Fermi-liquid to non-Fermi-liquid transitions, and
universal phenomena within a Fermi-liquid phase.
