One-arm exponents of the high-dimensional Ising model

We study the probability that the origin is connected to the boundary of the box of size $n$ (the one-arm probability) in several percolation models related to the Ising model. We prove that different universality classes emerge at criticality. - For the FK-Ising measure in a box of size $n$ with wired boundary conditions, we prove that this probability decays as $1\mathrm{/}n$ in dimensions $d>4$, and as $1\mathrm{/}{n}^{1+o(1)}$ when $d=4$. - For the infinite volume FK-Ising measure, we prove that this probability decays as $1\mathrm{/}{n}^2$ in dimensions $d>6$, and as $1\mathrm{/}{n}^{2+o(1)}$ when $d=6$. - For the sourceless double random current measure, we prove that this probability decays as $1\mathrm{/}{n}^{d-2}$ in dimensions $d>4$, and as $1\mathrm{/}{n}^{2+o(1)}$ when $d=4$. Additionally, for the infinite volume FK-Ising measure, we show that the one-arm probability is $1\mathrm{/}{n}^{1+o(1)}$ in dimension $d=4$, and at least $1\mathrm{/}{n}^{3\mathrm{/}2}$ in dimension $d=5$. This establishes that the FK-Ising model has upper-critical dimension equal to $6$, in contrast to the Ising model, where it is known to be less or equal to $4$, thus solving a conjecture of Chayes, Coniglio, Machta, and Shtengel.