Functional Integral Construction of the BRST Invariant String Ground State
Classical and Quantum Gravity
Institute of Physics
The authors use the Euclidean functional integral prescription to construct the ground-state wavefunction of the closed bosonic string including ghost contributions. They work in 26 dimensions where the sole contribution to the wavefunction comes from the string and ghost classical actions as well as the tachyonic centre of mass motion. They show that the ghost action must include a surface term to be consistent with the ghost boundary conditions. The result obtained represents the usual string plus ghost ground state and is BRST invariant.
D. Birmingham and C.G. Torre, “Functional integral construction of the BRST invariant string ground state,” Classical and Quantum Gravity, vol. 4(5), 1987, p. 1149.