  We study the time reversal (T) symmetry breaking of 2d helical fermi liquid,
with application to the edge states of 3d topological band insulators with only
one two-component Dirac fermion at finite chemical potential, as well as other
systems with spin-orbit coupling. The T-breaking Ising order parameter is not
over-damped and the theory is different from the ordinary Hertz-Millis theory
for order parameters at zero momentum. We argue that the T-breaking phase
transition is an 3d Ising transition, and the quasiparticles are well-defined
in the quantum critical regime.
