The question of which effective string theory describes the confining flux-tube has been a subject of extensive investigation during the last decade. We extracted by means of numerical simulations using lattice techniques the spectrum of confining SU(N) flux-tubes that wind around a spatial torus of variable length L, in 2+1 dimensions, in the fundamental representation of colour and for several combinations of parity and longitudinal momentum. We compare the energies of the lowest 30 states to the free string Nambu-Goto model and to recent results on the universal properties of effective string actions. Our most recent and useful calculations are in SU(6) at a very small lattice spacing, which we check to be very close to the large-N continuum limit. We find that the energy levels are remarkably close to the predictions of the free string Nambu-Goto model, even well below the critical length at which the expansion of the Nambu-Goto energy in powers of 1/L^2 diverges. In this talk I will present these findings without spending too much time on lattice technical details. The main aim of this talk will be to provide convincing evidence that Nambu-Goto string is the best known description for the confining fundamental flux-tube in 2+1 dimensions. The spectrum of 3+1 dimensional SU(N) closed flux-tubes has also been investigated. I will provide only a brief summary of our findings most of which resemble those in 2+1 dimensions apart from some states with particular quantum numbers which have large deviations from Nambu-Goto. Finally, I would like to present our very recent results on the spectrum of 2+1 dimensional closed flux-tubes in higher (than the fundamental) irreducible-representations of colour with N-alities k=1, 2, 3. For these N-alities we have found that the low-lying flux-tube spectrum can be well described by Nambu-Goto irrespective of what the representation is.
The Particle Physics Theory seminar is a weekly series of talks reflecting the diverse interests of the group. Topics include analytic and numerical calculations based on the Standard Model of elementary particle physics, theories exploring new physics, as well as more formal developments in gauge theories and gravity..