Particle theory

Abstract:

Steven Abel and John March-Russell discuss whether three dimensions exist in the universe and how they can be detected. Explaining why the cosmological constant is so small has occupied cosmologists and particle physicists ever since Einstein first introduced it. Many proponents of the brane-world picture are tackling this problem again. A typical process might involve a proton and antiproton colliding to produce a single spray or jet of particles plus a graviton, which is emitted into the bulk. The particles that are confined to the brane also have Kaluza - Klein or higher string-excitation states, but for them the relevant scale is either the brane thickness or the new fundamental string scale. Both of these scales should correspond in energy to the new gravity scale of 1000 GeV or higher.

Abstract:

We report first calculations of the topological susceptibility measured using the field theoretic method on SU(3) gauge configurations produced by the UKQCD collaboration with two flavours of dynamical, improved, Wilson fermions. Using three ensembles with matched lattice spacing but differing sea quark mass we find that hybrid Monte Carlo simulation appears to explore the topological sectors efficiently, and a topological susceptibility consistent with increasing linearly with the quark mass.

Abstract:

We calculate the variation with temperature of the vortex free energy in D = 2 + 1 SU(2) lattice gauge theories. We do so both above and below the deconfining transition at T = Tc. We find that this quantity is zero at all T for large enough volumes. For T < Tc this observation is consistent with the fact that the phase is linearly confining; while for T > Tc it is consistent with the conventional expectation of "spatial" linear confinement. In small spatial volumes this quantity is shown to be non zero. The way it decreases to zero with increasing volume is shown to be controlled by the (spatial) string tension and it has the functional form one would expect if the vortices being studied were responsible for the confinement at low T, and for the "spatial" confinement at large T. We also discuss in detail some of the direct numerical evidence for a non-zero spatial string tension at high T, and we show that the observed linearity of the (spatial) potential extends over distances that are large compared to typical high-T length scales.