924 research outputs found
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
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
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
Heavy quarks and long-lived hadrons
In a recent Letter we reported on some work which led us to suggest the possibility of narrow spikes in the e+e- annihilation cross section into hadrons. In this paper, we discuss the theoretical infrastructure of this work more thoroughly, and improve and extend the calculations and experimental predictions. We examine a colored quark-gluon model of hadronic matter, with color an exact SU(3) gauge symmetry. In addition to the light quarks that make up ordinary hadrons, a heavy quark, such as the charmed c, is included. The narrow resonances recently discovered by the MIT-BNL and SLAC-LBL groups are interpreted as cc̅ bound states (orthocharmonium). In this energy range, the effective coupling has become small according to asymptotic freedom, and many aspects of the bound-state structure can be calculated. The existence of 0-(paracharmonium) states is predicted, and decay widths and mass splittings are estimated. The total e+e- cross section into hadrons is predicted to scale asymptotically, with an approach to scaling from above that can be calculated over a large energy range
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
The Electroweak Chiral Lagrangian and CP-Violating Effects in Technicolor Theories
We estimate the CP-violating and anomalous form factors,
arising from CP-violating interactions in extended technicolor theories, and
discuss their future experimental detectability. The electric dipole moment of
the boson is found to be as large as {\cal O}(10^{-21}) \; \mbox{e cm}.
We connect the CP-odd and 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 .Comment: 34 pages, YCTP-P9-94, LaTex. (minor changes in wording and notation,
the figures are appended at the end as one postscript file
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
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
Chiral phase structure of QCD with many flavors
We investigate QCD with a large number of massless flavors with the aid of
renormalization group flow equations. We determine the critical number of
flavors separating the phases with and without chiral symmetry breaking in
SU(Nc) gauge theory with many fermion flavors. Our analysis includes all
possible fermionic interaction channels in the pointlike four-fermion limit.
Constraints from gauge invariance are resolved explicitly and regulator-scheme
dependencies are studied. Our findings confirm the existence of an Nf window
where the system is asymptotically free in the ultraviolet, but remains
massless and chirally invariant on all scales, approaching a conformal fixed
point in the infrared. Our prediction for the critical number of flavors of the
zero-temperature chiral phase transition in SU(3) is Nf^{cr}=10.0\pm
0.29(fermion)[+1.55;-0.63](gluon), with the errors arising from
approximations in the fermionic and gluonic sectors, respectively.Comment: 7 pages, 3 figures, updated discussion of the uncertainties in the
gauge secto
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
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