Particle theory

Abstract:

We use lattice techniques to calculate the continuum string tensions of SU (N) gauge theories in 2 + 1 dimensions. We attempt to control all systematic errors at a level that allows us to perform a precise test of the analytic prediction of Karabali, Kim and Nair. We find that their prediction is within 3% of our values for all N and that the discrepancy decreases with increasing N. When we extrapolate our results to N = ∞ we find that there remains a discrepancy of ≃ 1 %, which is a convincing ∼ 6 σ effect. Thus, while the Karabali-Nair analysis is remarkably accurate at N = ∞, it is not exact. © 2006 Elsevier B.V. All rights reserved.

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).