  We describe a new path integral approach to strongly correlated fermion
systems, considering the Hubbard model as a specific example. Our approach is
based on the introduction of spin-particle-hole coherent states which
generalize the spin-1/2 coherent states by allowing the creation of a hole or
an additional particle. The action of the fermion system
$S[\gamma^*,\gamma;{\bf\Omega}]$ can be expressed as a function of two
Grassmann variables ($\gamma_\uparrow$,$\gamma_\downarrow$) describing
particles propagating in the lower and upper Hubbard bands, and a unit vector
field ${\bf\Omega}$ whose dynamics arises from spin fluctuations. In the strong
correlation limit, $S[\gamma^*,\gamma;{\bf\Omega}]$ can be truncated to quartic
order in the fermionic fields and used as the starting point of a
strong-coupling diagrammatic expansion in $t/U$ ($t$ being the intersite
hopping amplitude and $U$ the on-site Coulomb repulsion). We discuss possible
applications of this formalism and its connection to the t-J model and the
spin-fermion model.
