An Application of the Hartle-Hawking Prescription to String Theory
Physics Letters B
We use the Hartle-Hawking prescription to construct the ground state wave function of the closed bosonic string in 26 dimensions including ghost contributions. We find that the prescription leads to the sl(2,Image ) vacuum in both the string and ghost sectors. We construct from this state the BRST invariant ground state wave function.
Danny Birmingham, C.G. Torre, An application of the Hartle-Hawking prescription to string theory, Physics Letters B, Volume 194, Issue 1, 30 July 1987, Pages 49-53, ISSN 0370-2693, 10.1016/0370-2693(87)90767-2.