  We study a two-species PASEP, in which there are two types of particles,
"heavy" and "light," hopping right and left on a one-dimensional lattice of $n$
cells with open boundaries. In this process, only the "heavy" particles can
enter on the left of the lattice and exit from the right of the lattice. In the
bulk, any transition where a heavier particle type swaps places with an
adjacent lighter particle type is possible. We generalize combinatorial results
of Corteel and Williams for the ordinary PASEP by defining a combinatorial
object which we call a rhombic alternative tableau that gives a combinatorial
formula for the stationary probabilities for the states of this two-species
PASEP.
