The spectrum of open confining strings in the large-Nc limit
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 $SU(N_c)$. Our primary objective is to explore its behaviour 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 analyse 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 $η_{CP}$ and the reflection symmetry $ε$ for $Λ=0$. To this purpose we employ a diverse set of suitable operators, an anisotropic action, 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 axion-like state along the flux-tube world-sheet. Notably, we provide undoubted evidence of the existence of a massive axion-like 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.