Half-Branes, Singular Brane Intersections, and Kaluza-Klein reduction

Two subtle aspects of brane intersections are investigated. The first concerns the `half-branes' that arise in discussions of the Hanany-Witten effect, often in the D0/D8 setting. The second involves the validity of seemingly singular classical BPS brane intersections. A study of holomorphic curves in the background of a Kaluza-Klein monopole and the associated reduction to type IIA supergravity sheds light on both issues. Many seemingly singular D2/D6 intersections are shown to lift to smooth configurations of M2-branes in 11-dimensions, and a mechanism is found for certain $Z_2$ confinement effects in type II string theories that eliminates any need for half-branes.