Lectures on Integrable probability: Stochastic vertex models and symmetric functions

TL;DR

This paper develops integral formulas for multi-point q-moments and correlation functions in a stochastic higher spin six vertex model, connecting it to known integrable probabilistic systems within the KPZ universality class.

Contribution

It introduces a unified approach using symmetric rational functions and Yang-Baxter identities to analyze the higher spin six vertex model and its degenerations.

Findings

Derived integral representations for multi-point q-moments and correlation functions.

Connected the model's formulas to known KPZ class systems in certain limits.

Extended results to the inhomogeneous higher spin six vertex model.

Abstract

We consider a homogeneous stochastic higher spin six vertex model in a quadrant. For this model we derive concise integral representations for multi-point q-moments of the height function and for the q-correlation functions. At least in the case of the step initial condition, our formulas degenerate in appropriate limits to many known formulas of such type for integrable probabilistic systems in the (1+1)d KPZ universality class, including the stochastic six vertex model, ASEP, various q-TASEPs, and associated zero range processes. Our arguments are largely based on properties of a family of symmetric rational functions (introduced in arXiv:1410.0976) that can be defined as partition functions of the higher spin six vertex model for suitable domains; they generalize classical Hall-Littlewood and Schur polynomials. A key role is played by Cauchy-like summation identities for these functions, which are obtained as a direct corollary of the Yang-Baxter equation for the higher spin six vertex model. These are lecture notes for a course given by A.B. at the Ecole de Physique des Houches in July of 2015. All the results and proofs presented here generalize to the setting of the fully inhomogeneous higher spin six vertex model, see arXiv:1601.05770 for a detailed exposition of the inhomogeneous case.