From braids to transverse slices in reductive groups

TL;DR

This paper explores the geometric structure of conjugacy classes in reductive groups, generalizing Steinberg's slices, and establishes criteria for their properties and Poisson structures, connecting classical and quantum group theories.

Contribution

It introduces a unified framework for slices in reductive groups based on Weyl group elements, providing new criteria and linking to Poisson geometry and quantum groups.

Findings

Established a criterion for cross sections using roots.

Connected slices to Deligne-Garside factors in braid monoids.

Identified conditions under which the Semenov-Tian-Shansky bracket reduces to a Poisson structure.

Abstract

In 1965, Steinberg's study of conjugacy classes in connected reductive groups led him to construct an affine subspace parametrising regular conjugacy classes, which he noticed is also a cross section for the conjugation action by the unipotent radical of a Borel subgroup on another affine subspace. Recently, generalisations of this slice and its cross section property have been obtained by Sevostyanov in the context of quantum group analogues of W-algebras and by He-Lusztig in the context of Deligne-Lusztig varieties. Such slices are often thought of as group analogues of Slodowy slices. In this paper we explain their relationship via common generalisations associated to Weyl group elements and provide a simple criterion for cross sections in terms of roots. In the most important class of examples this criterion is equivalent to a statement about the Deligne-Garside factors of their powers in the braid monoid being maximal in some sense. Moreover, we show that these subvarieties transversely intersect conjugacy classes and determine for a large class of factorisable r-matrices when the Semenov-Tian-Shansky bracket reduces to a Poisson structure on these slices.