Ramond-Ramond couplings of non-BPS D-branes

We study how non-BPS type II D-branes couple to R-R potentials. Upon tachyon condensation the couplings we find give rise to the Wess-Zumino action of BPS D-branes. In a seminal series of papers, Sen has shown how to obtain an exact conformal field theory description of non-BPS D-branes (see [1] for two of the main papers and two recent reviews). In this setup, a non-BPS D(p − 1)-brane remains after tachyon condensation on a Dp–anti-Dp-brane pair. In type II theories the resulting object still has a tachyonic mode on its worldvolume, but performing the orientifold to type I stabilizes the non-BPS brane. In a related development, D-brane charges have been shown to take values in appropriate K-theory groups of space-time. A major result is that all lower-dimensional D-branes can be considered in a unifying manner as non-trivial excitations on the appropriate configuration of higher-dimensional branes. For type IIB this was demonstrated by Witten in [2], where all branes are built from sufficiently many D9–anti-D9 pairs. As to type IIA, Horava outlined the construction of lower branes from non-BPS D9-branes [3]. As is well-known, type II BPS D-branes couple to Ramond-Ramond gauge fields through the Wess-Zumino action [4] (see [5] for string computations checking the form of this action):