From braids to transverse slices in reductive groups
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.