From closed to open strings: the tensionless route in Kalb-Ramond background and noncommutativity

Abstract: We study tensionless bosonic strings propagating in the presence of a constant Kalb-Ramond background and show how closed strings undergo a transition into open strings in this regime. The tensionless (Carrollian) limit induces a universal gluing between the worldsheet oscillators, which we generalize to include the effects of a constant $B$-field. We derive the modified mixed boundary conditions, construct the corresponding gluing matrix and obtain the generalized induced vacuum as a squeezed boundary state. This vacuum continuously emerges from the closed string vacuum through a Bogoliubov transformation, providing a precise realization of the closed-to-open string transition. We further extend the analysis to toroidal compactifications, and show that the mechanism of worldsheet Bose-Einstein condensation persists unaltered. In the second part of the paper, we give a unified symplectic derivation of the open string noncommutative parameter. We show that the boundary symplectic form alone determines the noncommutative geometry, and that in the tensionless limit the $B$-field term becomes the unique surviving contribution. Its inverse produces a noncommutative parameter similar to the Seiberg-Witten result, but arising intrinsically from the Carrollian worldsheet dynamics. Our results establish a coherent picture in which the emergence of open strings, boundary noncommutativity, and generalized vacuum structure all arise naturally from the worldsheet geometry of null strings in a $B$-field background.