986 research outputs found
A Light Dilaton in Walking Gauge Theories
We analyze the existence of a dilaton in gauge theories with approximate
infrared conformal symmetry. To the extent that these theories are governed in
the infrared by an approximate fixed point (walking), the explicit breaking of
the conformal symmetry at these scales is vanishingly small. If confinement and
spontaneous chiral-symmetry breaking set in at some infrared scale, the
resultant breaking of the approximate conformal symmetry can lead to the
existence of a dilaton with mass parametrically small compared to the
confinement scale, and potentially observable at the LHC.Comment: 5 pages, references added, final version in PR
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 is varied. We argue that there exists a
critical number of fermions , above which there is no chiral symmetry
breaking or confinement, and below which both chiral symmetry breaking and
confinement set in. We estimate and discuss the nature of the phase
transition.Comment: 13 pages, LaTeX, version published in PR
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
One Loop Effects of Non-Standard Triple Gauge Boson Vertices
Low energy effects of generic extensions of the Standard Model can be
comprehensively parametrized in terms of higher dimensional effective
operators. After the success of all the recent precission tests on the Standard
Model, we argue that any sensible description of these extensions at the
Z-scale must be stable under higher order quantum corrections. The imposition
of gauge invariance seems to be the simplest and most
natural way to fulfill this requirement. With this assumption, all the possible
deviations from the standard triple gauge boson vertices can be consistently
parametrized in terms of a finite set of gauge invariant operators. We deal
here with those operators that do not give any tree level effect on present
experimental observables and constrain them by computing their effects at the
one-loop level. We conclude that for a light Higgs boson, the direct
measurement at LEP200 can improve present bounds on these "blind directions",
while for a heavy Higgs it is most unlikely to provide any new information.Comment: 17 pags. 2 figures not included, available on request. Latex. CERN-TH
667
Matching of U_L(3) x U_R(3) and SU_L(3) x SU_R(3) Chiral Perturbation Theories
The heavy singlet field is integrated out from the U_L(3) x U_R(3) Chiral
Perturbation Theory and it is shown how its effects on the low-energy dynamics
are reduced to effective vertices for the light mesons. The results are matched
against the standard SU_L(3) x SU_R(3) Chiral Perturbation Theory in order to
establish the relations between the coupling constants from both theories to
one-loop level accuracy.Comment: 10 pages, 1 eps figure. Some comments rephrase
A Comment on the Zero Temperature Chiral Phase Transition in Gauge Theories
Recently Appelquist, Terning, and Wijewardhana investigated the zero
temperature chiral phase transition in SU(N) gauge theory as the number of
fermions N_f is varied. They argued that there is a critical number of fermions
N^c_f, above which there is no chiral symmetry breaking and below which chiral
symmetry breaking and confinement set in. They further argued that that the
transition is not second order even though the order parameter for chiral
symmetry breaking vanishes continuously as N_f approaches N^c_f on the broken
side. In this note I propose a simple physical picture for the spectrum of
states as N_f approaches N^c_f from below (i.e. on the broken side) and argue
that this picture predicts very different and non-universal behavior than is
the case in an ordinary second order phase transition. In this way the
transition can be continuous without behaving conventionally. I further argue
that this feature results from the (presumed) existence of an infrared
Banks-Zaks fixed point of the gauge coupling in the neighborhood of the chiral
transition and therefore depends on the long-distance nature of the non-abelian
gauge force.Comment: 7 pages, 2 figure
Fermion Masses and Mixing in Extended Technicolor Models
We study fermion masses and mixing angles, including the generation of a
seesaw mechanism for the neutrinos, in extended technicolor (ETC) theories. We
formulate an approach to these problems that relies on assigning right-handed
quarks and charged leptons to ETC representations that are conjugates
of those of the corresponding left-handed fermions. This leads to a natural
suppression of these masses relative to the quarks, as well as the
generation of quark mixing angles, both long-standing challenges for ETC
theories. Standard-model-singlet neutrinos are assigned to ETC representations
that provide a similar suppression of neutrino Dirac masses, as well as the
possibility of a realistic seesaw mechanism with no mass scale above the
highest ETC scale of roughly TeV. A simple model based on the ETC group
SU(5) is constructed and analyzed. This model leads to non-trivial, but not
realistic mixing angles in the quark and lepton sectors. It can also produce
sufficiently light neutrinos, although not simultaneously with a realistic
quark spectrum. We discuss several aspects of the phenomenology of this class
of models.Comment: 74 pages, revtex with embedded figure
Beta Functions of Orbifold Theories and the Hierarchy Problem
We examine a class of gauge theories obtained by projecting out certain
fields from an N=4 supersymmetric SU(N) gauge theory. These theories are
non-supersymmetric and in the large N limit are known to be conformal. Recently
it was proposed that the hierarchy problem could be solved by embedding the
standard model in a theory of this kind with finite N. In order to check this
claim one must find the conformal points of the theory. To do this we calculate
the one-loop beta functions for the Yukawa and quartic scalar couplings. We
find that with the beta functions set to zero the one-loop quadratic
divergences are not canceled at sub-leading order in N; thus the hierarchy
between the weak scale and the Planck scale is not stabilized unless N is of
the order 10^28 or larger. We also find that at sub-leading orders in N
renormalization induces new interactions, which were not present in the
original Lagrangian.Comment: 21 pages, LaTeX, 6 figures. Minor clarifications, references adde
The static quark-antiquark potential in QCD to three loops
The static potential between an infinitely heavy quark and antiquark is
derived in the framework of perturbative QCD to three loops by performing a
full calculation of the two-loop diagrams and using the renormalization group.
The contribution of massless fermions is included.Comment: Latex, 11 pages, 3 figures included. The complete paper, including
figures, is also available via anonymous ftp at
ftp://ttpux2.physik.uni-karlsruhe.de/ , or via www at
http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints/ . Revised version,
essentially identical to the version published in Physical Review Letter
Axigluons cannot explain the observed top quark forward-backward asymmetry
We study an SU(3)^2 axigluon model introduced by Frampton, Shu, and Wang to
explain the recent Fermilab Tevatron observation of a significant positive
enhancement in the top quark forward-backward asymmetry relative to standard
model predictions. First, we demonstrate that data on neutral B_d-meson mixing
excludes the region of model parameter space where the top asymmetry is
predicted to be the largest. Keeping the gauge couplings below the critical
value that would lead to fermion condensation imposes further limits at large
axigluon mass, while precision electroweak constraints on the model are
relatively mild. Furthermore, by considering an extension to an SU(3)^3 color
group, we demonstrate that embedding the model in an extra-dimensional
framework can only dilute the axigluon effect on the forward-backward
asymmetry. We conclude that axigluon models are unlikely to be the source of
the observed top quark asymmetry.Comment: 12 pages, 7 eps figures included. Minor changes to conform with
published versio
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