Classical and Quantum aspects of Bosonic String Field Theory

This thesis presents several developments in bosonic string field theory (SFT), addressing both formal and physical aspects of the theory. We construct a gauge-invariant kinetic term for SFT on target spaces with boundaries by introducing boundary terms and new degrees of freedom localized on the boundary, ensuring a well-defined variational principle and restoring gauge invariance. The construction is illustrated through explicit examples in open and closed string sectors, including the linear dilaton background. Open–closed SFT is reformulated in the coalgebraic language, allowing a systematic derivation of the nilpotent structure that governs quantum consistency within the Batalin-Vilkovisky (BV) framework. This formalism is applied to describe D-brane backreaction in the large-N limit, allowing us to derive the classical closed SFT action corresponding to the new background without branes. A systematic procedure for adding stubs to generic classical and quantum SFTs is developed, accompanied by a proof of invariance of tree- and loop-level amplitudes under such deformations. Finally, SFT methods are used to study short bulk RG flows in two-dimensional CFTs with D-branes. By computing the on-shell disk action for slightly relevant bulk perturbations, we determine the shift in the g-function and the string coupling constant at next-to-leading order. This provides an alternative approach to computations traditionally requiring higher-loop conformal perturbation theory