Effective String Theories (EST) of Yang-Mills Flux Tubes

This chapter explains the concept of \emph{Effective String Theories}(EST), and their success in explaining the results that Yang-Mills flux tubes behave, to a high degree of accuracy, like Bosonic Strings(BST). It describes EST's of Lüscher and Weisz, and their principal conclusions. It then discusses the Polchinski-Strominger EST's. which are valid in all dimensions. It then describes the works by Drummond, and, the author and Peter Matlock, which extended the analysis to $R^{-3}$ order and showed, that even at that order the spectrum is that of BST. The chapter analyses the issues of string momentum in higher orders. It discusses at length the powerful covariant calculus, to systematically construct EST's to arbitrary orders. The most general actions to $R^{-7}$ order are shown to be governed by just two parameters. The works of Aharony and collaborators on the spectrum of EST's, both in static and conformal gauges,to $R^{-5}$ order, and their results that even at that order the ground state energy remains the same as that of BST, but the excited spectrum gets corrected for $D\,>\,3$, are explained. It discusses the simulation results for excited states. It also discusses the AdS-CFT approaches and thickness of flux tubes.Recent works by Ambjorn, Makeenko, and Sedrakyan on the path-integral approaches to this issue are also discussed and compared with the other approaches. It concludes with remarks on the significance of the results for QCD.