  Symmetry considerations and a direct, Hubbard-Stratonovich type, derivation
are used to construct a replica field-theory relevant to the study of the spin
glass transition of short range models in a magnetic field. A mean-field
treatment reveals that two different types of transitions exist, whenever the
replica number n is kept larger than zero. The Sherrington-Kirkpatrick critical
point in zero magnetic field between the paramagnet and replica magnet (a
replica symmetric phase with a nonzero spin glass order parameter) separates
from the de Almeida-Thouless line, along which replica symmetry breaking
occurs. We argue that for studying the de Almeida-Thouless transition around
the upper critical dimension d=6, it is necessary to use the generic cubic
model with all the three bare masses and eight cubic couplings. The critical
role n may play is also emphasized. To make perturbative calculations feasible,
a new representation of the cubic interaction is introduced. To illustrate the
method, we compute the masses in one-loop order. Some technical details and a
list of vertex rules are presented to help future renormalisation-group
calculations.
