An integrable approach to macroscopic fluctuation theory for the multispecies SSEP

We study the macroscopic fluctuation theory (MFT) of a multispecies generalization of the symmetric simple exclusion process (mSSEP) on the infinite line, in which particles of $N+1$ species -- including, possibly, a vacancy species -- exchange positions at unit rate. Working with the full, redundant set of coarse-grained densities $\bfrho=\{\rho_0,\dots,\rho_N\}$ keeps the relabelling symmetry of the model manifest throughout. We first extend the argument of Derrida and Gerschenfeld to the multispecies setting, showing that the cumulant generating function of the multispecies current between two regions of an arbitrary graph depends on the boundary densities $ \bfrho_L,\bfrho_R$ and on the fugacities $\bflambda$ only through a single scalar variable $\omega$. We then formulate the MFT saddle-point equations for the mSSEP on the infinite line and show that they define an integrable system: they are of Landau--Lifshitz type, and a Zakharov--Takhtajan gauge transformation recasts them in AKNS form. Solving the resulting linear scattering problem by the inverse scattering method, we recover the cumulant generating function $F(\omega)$ for the multispecies current, as well as the initial and final density profiles conditioned on a prescribed current fluctuation. In particular, we show that $F(\omega)$ coincides with the function obtained by Derrida and Gerschenfeld for the single-species SSEP, now derived for an arbitrary number of species directly from the integrable structure of the multispecies MFT equations.