Hypertoric varieties, $W$-Hilbert schemes, and Coulomb branches

Abstract

We study transverse equivariant Hilbert schemes of affine hypertoric varieties equipped with a symplectic action of a Weyl group. In particular, we show that the Coulomb branches of Braverman, Finkelberg, and Nakajima can be obtained either as such Hilbert schemes or Hamiltonian reductions thereof. Furthermore, we propose that the Coulomb branches for representations of non-cotangent type are also obtained in this way. We also investigate the putative complete hyperk\"ahler metrics on these objects. We describe their twistor spaces and, in the case when the symplectic quotient construction of the hypertoric variety is $W$-equivariant (which includes Coulomb branches of cotangent type), we show that the hyperk\"ahler metric can be described as the natural $L^2$-metric on a moduli space of solutions to modified Nahm's equations on an interval with poles at both ends and a discontinuity in the middle, with the latter described by a new object: a hyperspherical variety canonically associated to a hypertoric variety.