Let $U_q(mathfrak{g})$ be a quantum affine algebra with an indeterminate $q$ and let $mathscr{C}_{mathfrak{g}}$ be the category of finite-dimensional integrable $U_q(mathfrak{g})$-modules. We write $mathscr{C}_{mathfrak{g}}^0$ for the monoidal subcategory of $mathscr{C}_{mathfrak{g}}$ introduced by Hernandez-Leclerc. In this paper, we associate a simply-laced finite type root system to each quantum affine algebra $U_q(mathfrak{g})$ in a natural way, and show that the block decompositions of $mathscr{C}_{mathfrak{g}}$ and $mathscr{C}_{mathfrak{g}}^0$ are parameterized by the lattices associated with the root system. We first define a certain abelian group $mathcal{W}$ (resp. $mathcal{W}_0$) arising from simple modules of $ mathscr{C}_{mathfrak{g}}$ (resp. $mathscr{C}_{mathfrak{g}}^0$) by using the invariant $Lambda^infty$ introduced in the previous work by the authors. The groups $mathcal{W}$ and $mathcal{W}_0$ have the subsets $Delta$ and $Delta_0$ determined by the fundamental representations in $ mathscr{C}_{mathfrak{g}}$ and $mathscr{C}_{mathfrak{g}}^0$ respectively. We prove that the pair $( mathbb{R} otimes_mathbb{Z} mathcal{W}_0, Delta_0)$ is an irreducible simply-laced root system of finite type and the pair $( mathbb{R} otimes_mathbb{Z} mathcal{W}, Delta) $ is isomorphic to the direct sum of infinite copies of $( mathbb{R} otimes_mathbb{Z} mathcal{W}_0, Delta_0)$ as a root system.
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