  The goal of this paper is to provide a combinatorial expression for the
steady state probabilities of the two-species PASEP. In this model, there are
two species of particles, one "heavy" and one "light", on a one-dimensional
finite lattice with open boundaries. Both particles can swap places with
adjacent holes to the right and left at rates 1 and $q$. Moreover, when the
heavy and light particles are adjacent to each other, they can swap places as
if the light particle were a hole. Additionally, the heavy particle can hop in
and out at the boundary of the lattice. Our main result is a combinatorial
interpretation for the stationary distribution at $q=0$ in terms of certain
multi-Catalan tableaux. We provide an explicit determinantal formula for the
steady state probabilities, as well as some general enumerative results for
this case. We also describe a Markov process on these tableaux that projects to
the two-species PASEP, and thus directly explains the connection between the
two. Finally, we give a conjecture that extends our formula for the stationary
distribution to the $q=1$ case, using certain two-species alternative tableau.

Combinatorics