  Momentum-conserving quasiparticle collisions in two-dimensional Fermi gases
give rise to a large family of exceptionally long-lived excitation modes. The
lifetimes of these modes exceed by a factor $(T_F/T)^2\gg 1$ the conventional
Landau Fermi-liquid lifetimes $\tau\sim T_F/T^2$. The long-lived modes have a
distinct angular structure, taking the form of $\cos m\theta$ and $\sin
m\theta$ with odd $m$ values for a circular Fermi surface, with relaxation rate
dependence on $m$ of the form $m^4\log m$, valid at not-too-large $m$. In
contrast, the even-$m$ harmonics feature conventional lifetimes with a weak $m$
dependence. The long-time dynamics, governed by the long-lived modes, takes the
form of angular (super)diffusion over the Fermi surface. Altogether, this leads
to unusual long-time memory effects, defining an intriguing transport regime
that lies between the conventional ballistic and hydrodynamic regimes.
