When a brane and antibrane meet, they do not annihilate directly to energy. Instead they first fragment into shards. These shards are smaller branes and antibranes; they occupy two fewer dimensions than the original ones did. For instance, if the initial brane and antibrane spanned seven spatial dimensions (a D7-brane and antibrane), they fragment into many D5-branes and antibranes. These shards, in turn, annihilate into D3-branes and antibranes and thence into D1-branes. Only at that point do they vanish altogether.
The cascade of brane-antibrane annihilation tends to remove large branes, which easily find their antibrane doppelgangers and so annihilate. Smaller branes, such as the D3 and D1 varieties, have greater difficulty bumping into antibranes in the vastness of nine-dimensional space. Lisa Randall of Harvard University and Andreas Karch of the University of Washington generalized our results to include nine expanding dimensions. This process may help explain why most branes, like ours, tend to have fairly few dimensions.
—C.B. and F.Q.