  Considering the simple chiral fermion meson model when the chiral symmetry is
explicitly broken, we show the validity of a trace identity -- to all orders of
perturbation theory -- playing the role of a Callan-Symanzik equation and which
allows us to identify directly the breaking of dilatations with the trace of
the energy-momentum tensor. More precisely, by coupling the quantum field
theory considered to a classical curved space background, represented by the
non-propagating external vielbein field, we can express the conservation of the
energy-momentum tensor through the Ward identity which characterizes the
invariance of the theory under the diffeomorphisms. Our ``Callan-Symanzik
equation'' then is the anomalous Ward identity for the trace of the
energy-momentum tensor, the so-called ``trace identity''.
