The Spectrum of the 2d Black Hole

or Does the 2D black hole have tachyonic or W–hair? Abstract We solve the equations of motion of the tachyon and the discrete states in the background of Witten's semiclassical black hole and in the exact 2D dilaton-graviton background of Dijkgraaf et al. We find the exact solutions for weak fields, leading to conclusions in disagreement with previous studies of tachyons in the black hole. Demanding that a state in the black hole be well behaved at the horizon implies that it must tend asymptotically to a combination of a Seiberg and an anti-Seiberg c = 1 state. For such a state to be well behaved asymptotically, it must satisfy the condition that neither its Seiberg nor its anti-Seiberg Liouville momentum is positive. Thus, although the free-field BRST cohomologies of the underlying SL(2, R R)/U(1) theory is the same as that of a c = 1 theory, the black hole spectrum is drastically truncated: There are no W ∞ states, and only tachyons with x-momenta |p tach | ≤ |m tach | are allowed. In the Minkowski case only the static tachyon is allowed. The black hole is stable to the back reaction of these remaining tachyons, so they are good perturbations of the black hole, or " hair ". However, this leaves only 3 tachyonic hairs in the black hole and 7 in the exact solution! Such sparse hair is clearly irrelevant to the maintenance of coherence during black hole evaporation.