Universal Extra Dimensions and the Higgs Boson Mass

We study the combined constraints on the compactification scale 1/R and the Higgs mass m_H in the standard model with one or two universal extra dimensions. Focusing on precision measurements and employing the Peskin-Takeuchi S and T parameters, we analyze the allowed region in the (m_H, 1/R) parameter space consistent with current experiments. For this purpose, we calculate complete one-loop KK mode contributions to S, T, and U, and also estimate the contributions from physics above the cutoff of the higher-dimensional standard model. A compactification scale 1/R as low as 250 GeV and significantly extended regions of m_H are found to be consistent with current precision data.Comment: 21 pages, Latex, 6 eps figures, an error in calculations was corrected and results of analysis changed accordingly, references adde

Maximum Wavelength of Confined Quarks and Gluons and Properties of Quantum Chromodynamics

Because quarks and gluons are confined within hadrons, they have a maximum wavelength of order the confinement scale. Propagators, normally calculated for free quarks and gluons using Dyson-Schwinger equations, are modified by bound-state effects in close analogy to the calculation of the Lamb shift in atomic physics. Because of confinement, the effective quantum chromodynamic coupling stays finite in the infrared. The quark condensate which arises from spontaneous chiral symmetry breaking in the bound state Dyson-Schwinger equation is the expectation value of the operator $\bar q q$ evaluated in the background of the fields of the other hadronic constituents, in contrast to a true vacuum expectation value. Thus quark and gluon condensates reside within hadrons. The effects of instantons are also modified. We discuss the implications of the maximum quark and gluon wavelength for phenomena such as deep inelastic scattering and annihilation, the decay of heavy quarkonia, jets, and dimensional counting rules for exclusive reactions. We also discuss implications for the zero-temperature phase structure of a vectorial SU($N$) gauge theory with a variable number $N_f$ of massless fermions.Comment: 6 pages, late

2+1 Dimensional QED and a Novel Phase Transition

We investigate the chiral phase transition in 2+1 dimensional QED. Previous gap equation and lattice Monte-Carlo studies of symmetry breaking have found that symmetry breaking ceases to occur when the number of fermion flavors exceeds a critical value. Here we focus on the order of the transition. We find that there are no light scalar degrees of freedom present as the critical number of flavors is approached from above (in the symmetric phase). Thus the phase transition is not second order, rendering irrelevant the renormalization group arguments for a fluctuation induced transition. However, the order parameter vanishes continuously in the broken phase, so this transition is also unlike a conventional first order phase transition.Comment: 11 pages, Late

Infrared behaviour of massless QED in space-time dimensions 2 < d < 4

We show that the logarithmic infrared divergences in electron self-energy and vertex function of massless QED in 2+1 dimensions can be removed at all orders of 1/N by an appropriate choice of a non-local gauge. Thus the infrared behaviour given by the leading order in 1/N is not modified by higher order corrections. Our analysis gives a computational scheme for the Amati-Testa model, resulting in a non-trivial conformal invariant field theory for all space-time dimensions 2 < d < 4.Comment: 12 pages, uses axodraw.sty; added comments at the end, and one reference; to appear in Phys. Lett.

On the Unification of Gauge Symmetries in Theories with Dynamical Symmetry Breaking

We analyze approaches to the partial or complete unification of gauge symmetries in theories with dynamical symmetry breaking. Several types of models are considered, including those that (i) involve sufficient unification to quantize electric charge, (ii) attempt to unify the three standard-model gauge interactions in a simple Lie group that forms a direct product with an extended technicolor group, and, most ambitiously, (iii) attempt to unify the standard-model gauge interactions with (extended) technicolor in a simple Lie group.Comment: 24 pages, ReVTe

Limit on the fermion masses in technicolor models

Recently it has been pointed out that no limits can be put on the scale of fermion mass generation $(M)$ in technicolor models, because the relation between the fermion masses $(m_f)$ and $M$ depends on the dimensionality of the interaction responsible for generating the fermion mass. Depending on this dimensionality it may happens that $m_f$ does not depend on $M$ at all. We show that exactly in this case $m_f$ may reach its largest value, which is almost saturated by the top quark mass. We make few comments on the question of how large can be a dynamically generated fermion mass.Comment: 5 pages, 1 figure, RevTeX

Zero Temperature Chiral Phase Transition in (2+1)-Dimensional QED with a Chern-Simons Term

We investigate the zero temperature chiral phase transition in (2+1)-dimensional QED in the presence of a Chern-Simons term, changing the number of fermion flavors. In the symmetric phase, there are no light degrees of freedom even at the critical point. Unlike the case without a Chern-Simons term, the phase transition is first-order.Comment: 7 pages, RevTeX, no figure