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Rolling tachyons for separated brane-antibrane systems

We consider tachyon condensation between a D-brane and an anti-D-brane in superstring theory, when they are separated in their common transverse directions. A simple rolling tachyon solution, that describes the time evolution of the process, is studied from the point of view of boundary conformal field theory. By computing the boundary beta-functions of the system, one finds that this theory is conformal, hence corresponds to an exact solution of the string theory equations of motion. By contrast, the time-reversal-symmetric rolling tachyon is not conformal. These results put constraints on the space-time effective actions for the system.