  We study the soft mode along the critical line in the phase diagram with the
tricritical point, using the Nambu--Jona-Lasinio model. At the critical point
with finite quark mass, the ordering density becomes a linear combination of
the scalar, quark number and energy densities, and their susceptibilities
diverge with the same exponent. Based on the conservation law, it is argued
that the divergent susuceptibility of a conserved density must be accompanied
by a critically--slowing hydrodynamic mode. The shift of the soft mode from the
sigma meson to the hydrodynamic mode occurs at the tricritical point on the
critical line.

High Energy Physics - Phenomenology