Würzburg Seminar on Quantum Field Theory and Gravity

In spacetime manifolds with boundaries, the Gibbons-Hawking-York (GHY) boundary term is essential for ensuring a well-posed variational principle. Deriving the GHY boundary term from the string worldsheet has been a longstanding challenge. 

Recent progress was made in https://arxiv.org/abs/2406.00712 where the authors used the method of images and Tseytlin's off-shell prescriptions to derive the boundary dilaton term in flat spacetime to first order in α′ from the string worldsheet. Building upon this, an upcoming work has successfully applied the same approach to derive the total (bulk + boundary) classical (tree-level) off-shell action from the string worldsheet.

In this talk, I will explain the fundamental concepts of Tseytlin's off-shell sphere prescriptions and the method of images on the worldsheet, and demonstrate how they were usee to derive the GHY boundary term.