Particle theory

Abstract:

Data from the Pierre Auger Observatory are analyzed to search for anisotropies near the direction of the Galactic Centre at EeV energies. The exposure of the surface array in this part of the sky is already significantly larger than that of the fore-runner experiments. Our results do not support previous findings of localized excesses in the AGASA and SUGAR data. We set an upper bound on a point-like flux of cosmic rays arriving from the Galactic Centre which excludes several scenarios predicting sources of EeV neutrons from Sagittarius A. Also the events detected simultaneously by the surface and fluorescence detectors (the 'hybrid' data set), which have better pointing accuracy but are less numerous than those of the surface array alone, do not show any significant localized excess from this direction. © 2006 Elsevier B.V. All rights reserved.

Abstract:

We calculate energies and tensions of closed k-strings in (2+1)-dimensional SU(N) gauge theories with N=4,5,6,8. When we study the dependence of the ground state energy on the string length, we find that it is well described by a Nambu-Goto (NG) free bosonic string for large lengths. At shorter lengths we see deviations which we fit, and this allows us to control the systematic error involved in extracting the tension. We compare the resulting string tensions with Casimir scaling, which we find to be lower than our data by 1% − 4%. Extrapolating our results to N = ∞ we see that our data fits more naturally to 1/N rather than 1/N 2 corrections. Finally, we see that the full spectrum of the k-string states falls into sectors that belong to particular irreducible representations of SU (N).

Abstract:

We investigate numerically the phase structure of the Twisted Eguchi-Kawai (TEK) model in four dimensions. In the numerical simulations of the zero-temperature TEK model (using a symmetric twist) we observe the existence of new phases that break its ZN4 symmetry at intermediate lattice couplings and for large SU(N) gauge groups. This effect can be explained by the contribution of diagonal configurations with collapsed eigenvalues, which are particular cases of “generalised fluxons”. We also investigate finite-temperature versions of the TEK model using anisotropic lattice couplings, where in particular we find van Baal fluxons contributing at large anisotropies.

Abstract:

For ☎ there is a first order bulk transition that cleanly separates the strong and weak coupling regimes of SU( ) lattice gauge theories with the plaquette action. We find that in this case the calculated string tension can be readily fitted throughout the weak coupling region by a standard 3-loop perturbative expression modified by lattice spacing corrections of the expected form. While our fits demand the presence of the latter, they are not constraining enough to tell us which of the various bare coupling schemes is a ‘good’ one, in the sense that terms in the -function beyond 3-loops are indeed negligible (in the relevant range of scales). To resolve this ambiguity we work in SU(3), using the Schrodinger Functional coupling scheme as a benchmark, and find that the Parisi mean-field improved coupling scheme matches it very well. Using the latter scheme, we have fitted the values of the string tension ✡ that have been calculated for ☎, to obtain ✡✚ ★ for , where the first error is statistical and the second is our estimate of the systematic error from all sources.