Alireza Sharifian, Andreas Athenodorou, Pedro Bicudo
In this study, we conduct a thorough examination of the spectrum of the open confining string in 3 + 1 dimensions, commonly referred to as the open flux tube, across various gauge groups of S U ( N c ) . Our primary objective is to explore its behavior as we approach the large- N c limit and the identification of possible world-sheet axion states. Specifically, we undertake a detailed analysis of the associated spectrum for N c = 3 , 4, 5, 6. This marks the first systematic investigation of the open flux-tube spectrum within the context of the large- N c limit. More specifically, we analyze the spectra of flux tubes that form between a static quark-antiquark pair, considering a significant number of radial excitations and eight irreducible representations characterized by the quantum numbers of angular momentum Λ , charge conjugation and parity η C P , and the reflection symmetry ε for Λ = 0 . To this purpose we employ a diverse set of suitable operators, an anisotropic action, and smearing techniques, and solve the generalized eigenvalue problem. We compare our findings with predictions from the Nambu-Goto string model to assess potential tensions indicative of novel phenomena such as the existence of axionlike state along the flux-tube world sheet. Notably, we provide undoubted evidence of the existence of a massive axionlike particle with the same mass as the corresponding axion extracted within the context of closed flux tube. This strengthens the conjecture that the axion is a property of the world sheet of the QCD string.