Speaker
Description
We present a comprehensive study of the spectrum of the open confining string (open flux tube) in 3+1 dimensions for several $\mathrm{SU}(N_c)$ gauge groups, with a focus on the large-$N_c$ limit and the possible emergence of world-sheet axion states. Specifically, we analyze the excitation spectra of flux tubes formed between static quark-antiquark pairs for $N_c = 3, 4, 5$, and $6$. Our study includes a detailed analysis of a wide range of radial excitations and eight irreducible representations, classified by the angular momentum quantum number $\Lambda$, charge conjugation and parity $\eta_{CP}$, and the reflection symmetry $\epsilon$ for $\Lambda = 0$. To extract the spectrum, we employ an anisotropic lattice action, smearing techniques, a diverse basis of operators, and solve the generalized eigenvalue problem. We compare our results to the predictions of the Nambu--Goto string model to identify potential discrepancies that may signal new physical phenomena. Notably, we observe clear evidence for a massive axion-like excitation on the world-sheet, with a mass matching that of the axion previously identified in the closed flux-tube spectrum. This strongly supports the conjecture that the axion is an intrinsic world-sheet excitation of the QCD string.