  We employ Statistical Field Theory techniques for coarse-graining the
steady-state properties of Active Ornstein-Uhlenbeck particles. The computation
is carried on in the framework of the Unified Colored Noise approximation that
allows an effective equilibrium picture. We thus develop a mean-field theory
that allows to describe in a unified framework the phenomenology of scalar
Active Matter. In particular, we are able to describe through spontaneous
symmetry breaking mechanism two peculiar features of Active Systems that are
(i) The accumulation of active particles at the boundaries of a confining
container, and (ii) Motility-Induced Phase Separation (MIPS).
\textcolor{black}{We develop a mean-field theory for steric interacting active
particles undergoing to MIPS and for Active Lennard-Jones (ALJ) fluids.}
\textcolor{black}{Within this framework}, we discuss the universality class of
MIPS and ALJ \textcolor{black}{showing that it falls into Ising universality
class.} We \textcolor{black}{thus} compute analytically the critical line
$T_c(\tau)$ for both models. In the case of MIPS, $T_c(\tau)$ gives rise to a
reentrant phase diagram compatible with an inverse transition from liquid to
gas as the strength of the noise decreases. \textcolor{black}{However, in the
case of particles interacting through anisotropic potentials, } the field
theory acquires a $\varphi^3$ term that, \textcolor{black}{in general, cannot
be canceled performing the expansion around the critical point.} In this case,
the \textcolor{black}{Ising} critical point might \textcolor{black}{be
replaced} by a first-order phase transition \textcolor{black}{region}.
