  We demonstrate a unidirectional motion of a quasiparticle without an explicit
symmetry breaking along the space-time coordinate of the particle motion. This
counterintuitive behavior originates from a combined action of two intrinsic
asymmetries in the other two directions. We realize this idea with the
magnon-driven motion of a magnetic domain wall in thin films with interfacial
asymmetry. Contrary to previous studies, the domain wall moves along the same
direction regardless of the magnon-flow direction. Our general symmetry
analysis and numerical simulation reveal that the odd order contributions from
the interfacial asymmetry is unidirectional, which is dominant over
bidirectional contributions in the realistic regime. We develop a simple
analytic theory on the unidirectional motion, which provides an insightful
description of this counterintuitive phenomenon.
