Braid groups of type ADE, Garside monoids, and the categorified root lattice

Abstract

We study Artin-Tits braid groups ${\mathbb{B}}_W$ of type ADE via the action of ${\mathbb{B}}_W$ on the homotopy category $\mathcal{K}$ of graded projective zigzag modules (which categorifies the action of the Weyl group $W$ on the root lattice). Following Brav-Thomas, we define a metric on ${\mathbb{B}}_W$ induced by the canonical $t$-structure on $\mathcal{K}$, and prove that this metric on ${\mathbb{B}}_W$ agrees with the word-length metric in the canonical generators of the standard positive monoid ${\mathbb{B}}_W^{+}$ of the braid group. We also define, for each choice of a Coxeter element $c$ in $W$, a baric structure on $\mathcal{K}$. We use these baric structures to define metrics on the braid group, and we identify these metrics with the word-length metrics in the Birman-Ko-Lee/Bessis dual generators of the associated dual positive monoid ${\mathbb{B}}_{W.c}^{\vee }$. As consequences, we give new proofs that the standard and dual positive monoids inject into the group, give linear-algebraic solutions to the membership problem in the standard and dual positive monoids, and provide new proofs of the faithfulness of the action of ${\mathbb{B}}_W$ on $\mathcal{K}$. Finally, we use the compatibility of the baric and $t$-structures on $\mathcal{K}$ to prove a conjecture of Digne and Gobet regarding the canonical word-length of the dual simple generators of ADE braid groups.

40 pages