997 research outputs found
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
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 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, , is varied. We argue that the transition takes
place at a large enough value of so that it is governed by the infrared
fixed point of the function. We study the nature of the phase
transition analytically and numerically, and discuss the spectrum of the theory
as the critical value of 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
Phases of Chiral Gauge Theories
We discuss the behavior of two non-supersymmetric chiral SU(N) gauge
theories, involving fermions in the symmetric and antisymmetric two-index
tensor representations respectively. In addition to global anomaly matching, we
employ a recently proposed inequality constraint on the number of effective low
energy (massless) degrees of freedom of a theory, based on the thermodynamic
free energy. Several possible zero temperature phases are consistent with the
constraints. A simple picture for the phase structure emerges if these theories
choose the phase, consistent with global anomaly matching, that minimizes the
massless degree of freedom count defined through the free energy. This idea
suggests that confinement with the preservation of the global symmetries
through the formation of massless composite fermions is in general not
preferred. While our discussion is restricted mainly to bilinear condensate
formation, higher dimensional condensates are considered for one case. We
conclude by commenting briefly on two related supersymmetric chiral theories.Comment: 23 pages, 2 figures, ReVTeX, improved forma
QCD with Large Number of Quarks: Effects of the Instanton -- Anti-instanton Pairs
We calculate the contribution of the instanton -- anti-instanton ()
pairs to the vacuum energy of QCD-like theories with light fermions using
the saddle point method. We find a qualitative change of the behavior: for it starts to oscillate with . Similar behaviour was known for
quantum mechanical systems interacting with fermions. We discuss the possible
consequences of this phenomenon, and its relation to the mechanism of chiral
symmetry breaking in these theories. We also discuss the asymptotics of the
perturbative series associated with the contribution, comparing our
results with those in literature.Comment: 11 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 in technicolor models, because the relation
between the fermion masses and depends on the dimensionality of the
interaction responsible for generating the fermion mass. Depending on this
dimensionality it may happens that does not depend on at all. We show
that exactly in this case 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
Walking Technicolor And The Vertex
A slowly running technicolor coupling will affect the size of non-oblique
corrections to the vertex from extended technicolor dynamics. We
show that while ``walking technicolor'' reduces the magnitude of the
corrections, they generally remain large enough to be seen at LEP.Comment: LaTeX, 11 pages, 3 figure
Proton Stability in Six Dimensions
We show that Lorentz and gauge invariance explain the long proton lifetime
within the standard model in six dimensions. The baryon-number violating
operators have mass dimension 15 or higher. Upon TeV-scale compactification of
the two universal extra dimensions on a square orbifold, a discrete
subgroup of the 6-dimensional Lorentz group continues to forbid dangerous
operators.Comment: PRL accepted versio
Study of an Alternate Mechanism for the Origin of Fermion Generations
In usual extended technicolor (ETC) theories based on the group
, the quarks of charge 2/3 and -1/3 and the charged
leptons of all generations arise from ETC fermion multiplets transforming
according to the fundamental representation. Here we investigate a different
idea for the origin of SM fermion generations, in which quarks and charged
leptons of different generations arise from ETC fermions transforming according
to different representations of . Although this
mechanism would have the potential, {\it a priori}, to allow a reduction in the
value of relative to conventional ETC models, we show that, at least
in simple models, it is excluded by the fact that the technicolor sector is not
asymptotically free or by the appearance of fermions with exotic quantum
numbers which are not observed.Comment: 6 pages, late
Generalized BRST Quantization and Massive Vector Fields
A previously proposed generalized BRST quantization on inner product spaces
for second class constraints is further developed through applications. This
BRST method involves a conserved generalized BRST charge Q which is not
nilpotent but which satisfies Q=\delta+\delta^{\dagger}, \delta^2=0, and by
means of which physical states are obtained from the projection
\delta|ph>=\delta^{\dagger}|ph>=0. A simple model is analyzed in detail from
which some basic properties and necessary ingredients are extracted. The method
is then applied to a massive vector field. An effective theory is derived which
is close to the one of the Stueckelberg model. However, since the scalar field
here is introduced in order to have inner product solutions, a massive
Yang-Mills theory with polynomial interaction terms might be possible to
construct.Comment: 19 pages,Latexfil
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