Invisibility of the integers for the discrete Gaussian chain via a Caffarelli-Silvestre extension of the discrete fractional Laplacian

The Discrete Gaussian Chain is a model of interfaces $\Psi : \mathbf{Z} \to \mathbf{Z}$ governed by the Hamiltonian $$ H(\Psi)= \sum_{i\neq j} J_\alpha(|i-j|) |\Psi_i -\Psi_j|^2 $$ with long-range coupling constants $J_\alpha(k)\asymp k^{-\alpha}$. For any $\alpha\in [2,3)$ and at high enough temperature, we prove an invariance principle for such an $\alpha$-Discrete Gaussian Chain towards a $H(\alpha)$-fractional Gaussian process where the Hurst index $H$ satisfies $H=H(\alpha)=\frac {\alpha-2} 2$. This result goes beyond a conjecture by Fr\"ohlich and Zegarlinski \cite{frohlich1991phase} which conjectured fluctuations of order $n^{\tfrac 1 2 (\alpha-2) \wedge 1}$ for the Discrete Gaussian Chain. More surprisingly, as opposed to the case of $2D$ Discrete Gaussian $\Psi : \mathbf{Z}^2 \to \mathbf{Z}$, we prove that the integers do not affect the {\em effective temperature} of the discrete Gaussian Chain at large scales. Such an {\em invisibility of the integers} had been predicted by Slurink and Hilhorst in the special case $\alpha_c=2$ in \cite{slurink1983roughening}. Our proof relies on four main ingredients: (1) A Caffareli-Silvestre extension for the discrete fractional Laplacian (which may be of independent interest) (2) A localisation of the chain in a smoother sub-domain (3) A Coulomb gas-type expansion in the spirit of Fr\"ohlich-Spencer \cite{FS} (4) Controlling the amount of Dirichlet Energy supported by a $1D$ band for the Green functions of $\mathbf{Z}^2$ Bessel type random walks Our results also have implications for the so-called {\em Boundary Sine-Gordon field}. Finally, we analyse the (easier) regimes where $\alpha\in(1,2) \cup (3,\infty)$ as well as the $2D$ Discrete Gaussian with long-range coupling constants (for any $\alpha>\alpha_c=4$).