Postmodern Technicolor

Using new insights into strongly coupled gauge theories arising from analytic calculations and lattice simulations, we explore a framework for technicolor model building that relies on a non-trivial infrared fixed point, and an essential role for QCD. Interestingly, the models lead to a simple relation between the electroweak scale and the QCD confinement scale, and to the possible existence of exotic leptoquarks with masses of several hundred GeV.Comment: LaTeX, 13 pages, version published in PR

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

The Zero Temperature Chiral Phase Transition in SU(N) Gauge Theories

We investigate the zero temperature chiral phase transition in an SU(N) gauge theory as the number of fermions $N_f$ is varied. We argue that there exists a critical number of fermions $N_f^c$, above which there is no chiral symmetry breaking or confinement, and below which both chiral symmetry breaking and confinement set in. We estimate $N_f^c$ and discuss the nature of the phase transition.Comment: 13 pages, LaTeX, version published in PR

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

The Phase Structure of an SU(N) Gauge Theory with N_f Flavors

We investigate the chiral phase transition in SU(N) gauge theories as the number of quark flavors, $N_f$, is varied. We argue that the transition takes place at a large enough value of $N_f$ so that it is governed by the infrared fixed point of the $\beta$ function. We study the nature of the phase transition analytically and numerically, and discuss the spectrum of the theory as the critical value of $N_f$ is approached in both the symmetric and broken phases. Since the transition is governed by a conformal fixed point, there are no light excitations on the symmetric side. We extend previous work to include higher order effects by developing a renormalization group estimate of the critical coupling.Comment: 34 pages, 1 figure. More references adde

Curvature perturbations from dimensional decoupling

The scalar modes of the geometry induced by dimensional decoupling are investigated. In the context of the low energy string effective action, solutions can be found where the spatial part of the background geometry is the direct product of two maximally symmetric Euclidean manifolds whose related scale factors evolve at a dual rate so that the expanding dimensions first accelerate and then decelerate while the internal dimensions always contract. After introducing the perturbative treatment of the inhomogeneities, a class of five-dimensional geometries is discussed in detail. Quasi-normal modes of the system are derived and the numerical solution for the evolution of the metric inhomogeneities shows that the fluctuations of the internal dimensions provide a term that can be interpreted, in analogy with the well-known four-dimensional situation, as a non-adiabatic pressure density variation. Implications of this result are discussed with particular attention to string cosmological scenarios.Comment: 25 pages, 3 figure

The Electroweak Chiral Lagrangian and CP-Violating Effects in Technicolor Theories

We estimate the CP-violating $WW\gamma$ and $WWZ$ anomalous form factors, arising from CP-violating interactions in extended technicolor theories, and discuss their future experimental detectability. The electric dipole moment of the $W$ boson is found to be as large as {\cal O}(10^{-21}) \; \mbox{e cm}. We connect the CP-odd $WW\gamma$ and $WWZ$ couplings to the corresponding CP-violating electroweak chiral lagrangian operators. The electric dipole moments of the neutron and the electron in technicolor theories are estimated to be as large as {\cal O}(10^{-26}) \; \mbox{e cm} and {\cal O}(10^{-29}) \; \mbox{e cm} respectively. We also suggest the potential to observe large CP-violating technicolor effects in the decay $t \rightarrow b + W^+$.Comment: 34 pages, YCTP-P9-94, LaTex. (minor changes in wording and notation, the figures are appended at the end as one postscript file

Extended Technicolor Models with Two ETC Groups

We construct extended technicolor (ETC) models that can produce the large splitting between the masses of the $t$ and $b$ quarks without necessarily excessive contributions to the $\rho$ parameter or to neutral flavor-changing processes. These models make use of two different ETC gauge groups, such that left- and right-handed components of charge $Q=2/3$ quarks transform under the same ETC group, while left- and right-handed components of charge -1/3 quarks and charged leptons transform under different ETC groups. The models thereby suppress the masses $m_b$ and $m_\tau$ relative to $m_t$, and $m_s$ and $m_\mu$ relative to $m_c$ because the masses of the $Q=-1/3$ quarks and charged leptons require mixing between the two ETC groups, while the masses of the $Q=2/3$ quarks do not. A related source of the differences between these mass splittings is the effect of the two hierarchies of breaking scales of the two ETC groups. We analyze a particular model of this type in some detail. Although we find that this model tends to suppress the masses of the first two generations of down-type quarks and charged leptons too much, it gives useful insights into the properties of theories with more than one ETC group.Comment: 14 pages, 4 figure

Implications of Dynamical Generation of Standard-Model Fermion Masses

We point out that if quark and lepton masses arise dynamically, then in a wide class of theories the corresponding running masses $m_{f_j}(p)$ exhibit the power-law decay $m_{f_j}(p) \propto \Lambda_j^2/p^2$ for Euclidean momenta $p >> \Lambda_j$, where $f_j$ is a fermion of generation $j$, and $\Lambda_j$ is the maximal scale relevant for the origin of $m_{f_j}$. We estimate resultant changes in precision electroweak quantities and compare with current data. It is found that this data allows the presence of such corrections. We also note that this power-law decay renders primitively divergent fermion mass corrections finite.Comment: 4 pages, late

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