Relativistic limit on the $AdS_3 \times S^3 \times S^3 \times S^1$ string worldsheet
Emanuele Maria Cattaneo, Davide Polvara, Alessandro Sfondrini
TL;DR
The paper derives and tests the relativistic limit of the worldsheet S-matrix for AdS$_3\times$S$^3\times$S$^3\times$S$^1$ strings with mixed RR/NSNS flux, completing the S-matrix bootstrap with dressing factors for both fundamental and bound states. It identifies two families of massive bound states arising from each sphere, computes their S-matrices in the relativistic limit, and demonstrates consistency with the two-sphere fusion structure and with the relativistic limit of the Frolov–Sfondrini dressing factors. A competing construction based on a fictitious minimal-mass particle is analyzed and found not to reconcile with the Frolov–Syndrini proposal, highlighting the physical relevance of the two-sphere picture. The results provide a concrete relativistic benchmark for dressing factors in this AdS$_3$ background and clarify how bound-state fusion operates in a mixed-flux, two-sphere environment with nontrivial automorphisms and central extensions. This advances understanding of integrable structure, bound-state spectra, and potential connections to QSC frameworks for this class of theories.
Abstract
We study the relativistic limit of the worldsheet S matrix of $AdS_3 \times S^3 \times S^3 \times S^1$ strings in the presence of mixed Ramond-Ramond (RR) and Neveu-Schwarz-Neveu-Schwarz (NS-NS) flux, and complete the S-matrix bootstrap including the dressing factors for fundamental particles and bound states. Our results agree with the relativistic limit of the recent proposal arXiv:2512.07721.
