Comments on Unstable Branes

TL;DR

This paper discusses the existence and properties of unstable branes in type II string theories and M-theory, proposing new unstable branes and analyzing their tensions, symmetries, and supergravity solutions.

Contribution

It introduces the concept of unstable M4 and M3 branes as sphalerons and compares their tensions with unstable D-branes, expanding the understanding of unstable branes in string theory.

Findings

Unstable NS4 branes exist in type II string theories.

Unstable M4 and M3 branes are proposed as sphalerons in M-theory.

Supergravity solutions for unstable Mp branes are presented.

Abstract

We argue that type II string theories contain unstable NS4 branes, which descend from a conjectured unstable M4 brane of M-theory. Assuming that an M2 brane can arise in M5 brane/anti-brane annihilation, the unstable M4 brane, and also an unstable M3 brane, must exist as sphalerons. We compare the tensions of the unstable NS4 branes, M4 brane, and related type II unstable D-branes, and present 11d supergravity solutions for unstable Mp branes for all p. We study the Z_2 gauge symmetry on the worldvolume of unstable branes, and argue that it can never be unbroken in the presence of lower brane charge.