Particle theory

Abstract:

From an observational perspective cosmology is today in excellent shape - advances in instrumentation and data processing have enabled us to study the universe in detail back to when the first galaxies formed, map the fluctuations in the cosmic microwave background which provide a measure of the overall geometry, and reconstruct the thermal history reliably back to at least the primordial nucleosynthesis era. However recent deep studies of the Hubble expansion rate have suggested that the universe is accelerating, driven by some form of `dark' (vacuum) energy. If true, this implies a new energy scale in Nature of order 0.001 eV, well below any known scale of fundamental physics. This has refocussed attention on the notorious cosmological constant problem at the interface of general relativity and quantum field theory. It is possible that the resolution of this situation will require fundamental modifications to our ideas about gravity.

Abstract:

We present a new scenario for baryogenesis in the context of heterotic brane-world models. The baryon asymmetry of the universe is generated at a small-instanton phase transition which is initiated by a moving brane colliding with the observable boundary. We demonstrate, in the context of a simple model, that reasonable values for the baryon asymmetry can be obtained. As a byproduct we find a new class of moving-brane cosmological solutions in the presence of a perfect fluid. © 2002 The American Physical Society.

Abstract:

We present results for the static interquark potential, lightest glueballs, light hadron spectrum, and topological susceptibility using a nonperturbatively improved action on a 163×32 lattice at a set of values of the bare gauge coupling and bare dynamical quark mass chosen to keep the lattice size fixed in physical units (∼1.7 fm). By comparing these measurements with a matched quenched ensemble, we study the effects due to two degenerate flavors of dynamical quarks. With the greater control over residual lattice spacing effects which these methods afford, we find some evidence of charge screening and some minor effects on the light hadron spectrum over the range of quark masses studied (MPS/Mv ≥ 0.58, where PS denotes pseudoscalar and V denotes vector). More substantial differences between quenched and unquenched simulations are observed in measurements of topological quantities. ©2002 The American Physical Society.