  Active fluids composed of constituents that are constantly driven away from
thermal equilibrium can support spontaneous currents and can be engineered to
have unconventional transport properties. Here we report the emergence of
(meta-)stable traveling bands in computer simulations of aligning circle
swimmers. These bands are different from polar flocks and we show that they can
be understood as non-dispersive soliton solutions of the underlying non-linear
hydrodynamic equations with constant celerity (phase propagation speed) that is
much larger than the propulsion speed. In contrast to solitons in passive
media, these bands can induce a bulk particle current with a component
perpendicular to the propagation direction, thus constituting a collective Hall
(or Magnus) effect. Traveling bands require sufficiently small orbits and
undergo a discontinuous transition into a synchronized state with transient
polar clusters for large orbital radii.
