Stochastic six-vertex model in a half-quadrant and half-line open asymmetric simple exclusion process

We consider the asymmetric simple exclusion process (ASEP) on the positive integers with an open boundary condition. We show that, when starting devoid of particles and for a certain boundary condition, the height function at the origin fluctuates asymptotically (in large time τ) according to the Tracy–Widom Gaussian orthogonal ensemble distribution on the τ[superscript 1/3]-scale. This is the first example of Kardar–Parisi–Zhang asymptotics for a half-space system outside the class of free-fermionic/determinantal/Pfaffian models. Our main tool in this analysis is a new class of probability measures on Young diagrams that we call half-space Macdonald processes, as well as two surprising relations. The first relates a special (Hall–Littlewood) case of these measures to the half-space stochastic six-vertex model (which further limits to the ASEP) using a Yang–Baxter graphical argument. The second relates certain averages under these measures to their half-space (or Pfaffian) Schur process analogues via a refined Littlewood identity. Keywords: Kardar–Parisi–Zhang universality class; interacting particle systems; asymmetric simple exclusion process; six-vertex model; integrable probability; Macdonald symmetric functions; Yang–Baxter equationNational Science Foundation (U.S.) (Grant DMS-160790