Particle theory

Abstract:

The standard Big Bang cosmology predicts the existence of an, as yet undetected, relic neutrino background, similar to the photons observed in the cosmic microwave background. If neutrinos have mass, then such relic neutrinos are a natural candidate for the dark matter of the universe, and indeed were the first particles to be proposed for this role. This possibility has however been increasingly constrained by cosmological considerations, particularly of large-scale structure formation, thus yielding stringent bounds on neutrino masses, which have yet to be matched by laboratory experiments. Another probe of relic neutrinos is primordial nucleosynthesis which is sensitive to the number of neutrino types (including possible sterile species) as well to any lepton asymmetry. Combining such arguments with the experimental finding that neutrino mixing angles are large, excludes the possibility of a large asymmetry and disfavours new neutrinos beyond those now known.

Abstract:

We use the Lorentzian AdS/CFT prescription to find the poles of the retarded thermal Green's functions of N=4 SU(N) supersymmetric Yang-Mills theory in the limit of large N and large 't Hooft coupling. In the process, we propose a natural definition for quasinormal modes in an asymptotically AdS spacetime, with boundary conditions dictated by the AdS/CFT correspondence. The corresponding frequencies determine the dispersion laws for the quasiparticle excitations in the dual finite-temperature gauge theory. Correlation functions of operators dual to massive scalar, vector and gravitational perturbations in a five-dimensional AdS-Schwarzschild background are considered. We find asymptotic formulas for quasinormal frequencies in the massive scalar and tensor cases, and an exact expression for vector perturbations. In the long-distance, low-frequency limit we recover results of the hydrodynamic approximation to thermal Yang-Mills theory. © 2003 The American Physical Society.

Abstract:

The production of black holes in extra-dimensional brane-world theories can lead to detectable signals via the Hawking evaporation of the black hole to brane-localized standard model modes. We calculate, as a function of partial wave number and number of toroidally compactified extra dimensions, the leading correction to the energy spectrum of such Hawking radiation (the greybody factors) for decay into spin-1/2 fermions and spin-1 gauge fields localized on the standard model brane. We derive the associated improved differential emission rates for both types of fields. We provide both simple expressions for the leading behavior of the greybody factors in the low-energy limit [Formula Presented] and numerical evaluation of our full analytic expressions for the emission rates, valid for [Formula Presented] The full analytic expressions demonstrate that both the greybody factors and emission rates are enhanced as the number of extra dimensions increases. © 2003 The American Physical Society.