Particle theory

Abstract:

We study the constraints on five-dimensional script N = 1 heterotic M-theory imposed by a consistent anomaly-free coupling of bulk and boundary theory. This requires analyzing the cancellation of triangle gauge anomalies on the four-dimensional orbifold planes due to anomaly inflow from the bulk. We find that the semi-simple part of the orbifold gauge groups and certain U(1) symmetries have to be free of quantum anomalies. In addition there can be several anomalous U(1) symmetries on each orbifold plane whose anomalies are cancelled by a non-trivial variation of the bulk vector fields. The mixed U(1) non-abelian anomaly is universal and there is at most one U(1) symmetry with such an anomaly on each plane. In an alternative approach, we also analyze the coupling of five-dimensional gauged supergravity to orbifold gauge theories. We find a somewhat generalized structure of anomaly cancellation in this case which allows, for example, non-universal mixed U(1) gauge anomalies. Anomaly cancellation from the perspective of four-dimensional script N = 1 effective actions obtained from E8 x E8 heterotic string- or M-theory by reduction on a Calabi-Yau three-fold is studied as well. The results are consistent with the ones found for five-dimensional heterotic M-theory. Finally, we consider some related issues of phenomenological interest such as model building with anomalous U(1) symmetries, Fayet-Illiopoulos terms and threshold corrections to gauge kinetic functions.

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.