  The finite-volume thermodynamics of a massive integrable QFT is described in
terms of a grand canonical ensemble of loops immersed in a torus and
interacting through scattering factors associated with their intersections. The
path integral of the loops is evaluated explicitly after decoupling the
pairwise interactions by a Hubbard-Stratonovich transformation. The HS fields
are holomorphic fields depending on the rapidity and can be expanded in
elementary oscillators. The torus partition function is expressed as certain
expectation value in the Fock space of these oscillators. In the limit where
one of the periods of the torus becomes asymptotically large, the effective
field theory becomes mean field type. The mean field describes the
infinite-volume thermodynamics which is solved by the Thermodynamical Bethe
Ansatz.
