  Doppler and Sisyphus cooling of $^{174}$YbOH are achieved and studied. This
polyatomic molecule has high sensitivity to physics beyond the Standard Model
and represents a new class of species for future high-precision probes of new
T-violating physics. The transverse temperature of the YbOH beam is reduced by
nearly two orders of magnitude to $< 600 \, \mu$K and the phase-space density
is increased by a factor of $>6$ via Sisyphus cooling. We develop a full
numerical model of the laser cooling of YbOH and find excellent agreement with
the data. We project that laser cooling and magneto-optical trapping of
long-lived samples of YbOH molecules are within reach and these will allow a
high sensitivity probe of the electric dipole moment (EDM) of the electron. The
approach demonstrated here is easily generalized to other isotopologues of YbOH
that have enhanced sensitivity to other symmetry-violating electromagnetic
moments.
