  The possibility to effect valley splitting of an electronic current in
graphene represents the essential component in the new field of valleytronics
in such two-dimensional materials. Based on a symmetry analysis of the
scattering matrix, we show that if the spatial distribution of multiple
potential scatterers breaks mirror symmetry about the axis of incoming
electrons, then a splitting of the current between two valleys is observed.
This leads to the appearance of the valley Hall effect. We illustrate the
effect of mirror symmetry breaking in a minimal system of two symmetric
impurities, demonstrating the splitting between valleys via the differential
cross sections and non-vanishing skew parameter. We further discuss the role
that these effects may play in transport experiments.
