Infinitesimal isometries of connection metric and generalized moment map equation

TL;DR

This paper explores the conditions under which a connection on a principal bundle remains invariant under certain diffeomorphisms, linking this invariance to a generalized moment map equation involving the curvature and a section of the adjoint bundle.

Contribution

It establishes a precise criterion connecting connection invariance under diffeomorphisms to a generalized moment map equation, extending understanding of symmetries in connection metrics.

Findings

Connection A is invariant under a local diffeomorphism group if and only if the generalized moment map equation holds.

The Lie algebra of fiber-preserving Killing fields is characterized for compact, connected, semisimple groups.

The paper provides a new perspective on the symmetry properties of connection metrics in principal bundles.

Abstract

Let $(M,g)$ be a smooth Riemannian manifold, $K$ a compact Lie group and $p:P\to M$ a principal $K$-bundle over $M$ endowed with a connection $A$. Fixing a bi invariant inner product on Lie algebra $\mathfrak{k}$ of $K$, the connection $A$ and metric $g$ define a Riemannian metric $g_A$ on $P$. Let $\tilde {X}$ be the horizontal lift of vector field $X$ on $M$ and, let $\xi^\nu$ be the vertical field associated with section $\nu\in A^0(\mathrm{ad}( P))$ of the adjoint bundle. It is proved that the connection $A$ is invariant under the 1-parameter group of local diffeomorphism generated by $\tilde{ X}+\xi^\nu$ if and only if $X$ and $\nu$ satisfy the generalized moment map equation $\iota_XF_A=-\nabla^A\nu$. The Lie algebra of fiber preserving Killing fields of $(P,g_A)$ is studied, in the case where $K$ is compact, connected and semisimple.