  We propose that the "hidden order parameter" in URu$_2$Si$_2$ is a helicity
order which must arise, if the Pomeranchuk criteria for the spin-antisymmetric
Landau parameters with respect to the stability of a Fermi liquid state are
violated. In a simple model, we calculate the specific heat, linear and
nonlinear magnetic susceptibilities and the change of transition temperature in
a magnetic field with such an order parameter, and obtain quantitative
agreement with experiments in terms of two parameters extracted from the data.
The peculiar temperature dependence of the NMR linewidth and the nature of the
loss of excitations in the ordered phase seen by neutron scattering are also
explained and experiments are suggested to directly confirm the proposed order
parameter.
