424 research outputs found
Flavor Changing Neutral Currents in a Realistic Composite Technicolor Model
We consider the phenomenology of a composite technicolor model proposed
recently by Georgi. Composite technicolor interactions produce four-quark
operators in the low energy theory that contribute to flavor changing neutral
current processes. While we expect operators of this type to be induced at the
compositeness scale by the flavor-symmetry breaking effects of the preon mass
matrices, the Georgi model also includes operators from higher scales that are
not GIM-suppressed. Since these operators are potentially large, we study their
impact on flavor changing neutral currents and CP violation in the neutral ,
, and meson systems.Comment: 16 pages, LaTeX + embedded PicTeX figures requiring prepictex,
pictex, and postpictex inputs. HUTP.STY include
The structure of electroweak corrections due to extended gauge symmetries
This paper studies models with extended electroweak gauge sectors of the form
SU(2) x SU(2) x U(1) x [SU(2) or U(1)]. We establish the general behavior of
corrections to precision electroweak observables in this class of theories and
connect our results to previous work on specific models whose electroweak
sectors are special cases of our extended group.Comment: 18 pages, 2 figures; added a referenc
Effective Field Theory of Vacuum Tilting
Simple models of topcolor and topcolor-assisted technicolor rely on a
relatively strong U(1) gauge interaction to ``tilt'' the vacuum. This tilting
is necessary to produce a top-condensate, thereby naturally obtaining a heavy
top-quark, and to avoid producing a bottom-condensate. We identify some
peculiarities of the Nambu-Jona-Lasinio approximation often used to analyze the
topcolor dynamics. We resolve these puzzles by constructing the low-energy
effective field theory appropriate to a mass-independent renormalization
scheme. We construct the power-counting rules for such an effective theory. By
requiring that the Landau pole associated with the U(1) gauge theory be
sufficiently above the topcolor gauge boson scale, we derive an upper bound on
the strength of the U(1) gauge-coupling evaluated at the topcolor scale. The
upper bound on the U(1) coupling implies that these interactions can shift the
composite Higgs boson mass-squared by only a few per cent and, therefore, that
the topcolor coupling must be adjusted to equal the critical value for chiral
symmetry breaking to within a few per cent.Comment: 15 pages, LaTeX and Pictex. Minor comments and references added to
clarify relation of current work to previous work on composite Higgs model
Finding Z' bosons coupled preferentially to the third family at CERN LEP and the Fermilab Tevatron
Z' bosons that couple preferentially to the third generation fermions can
arise in models with extended weak (SU(2)xSU(2)) or hypercharge (U(1)xU(1))
gauge groups. We show that existing limits on quark-lepton compositeness set by
the LEP and Tevatron experiments translate into lower bounds of order a few
hundred GeV on the masses of these Z' bosons. Resonances of this mass can be
directly produced at the Tevatron. Accordingly, we explore in detail the limits
that can be set at Run II using the process p pbar -> Z' -> tau tau -> e mu. We
also comment on the possibility of using hadronically-decaying taus to improve
the limits.Comment: LaTeX2e, 24 pages (including title page), 13 figures; version 2:
corrected typographical errors and bad figure placement; version 3: added
references and updated introduction; version 4: changes to compensate for old
latex version on arXiv server; version 5: additional references, and embedded
fonts in eps files for PRD; version 6: corrected some minor typos to address
PRD referee's comment
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
Critical Constraints on Chiral Hierarchies
We consider the constraints that critical dynamics places on models with a
top quark condensate or strong extended technicolor (ETC). These models require
that chiral-symmetry-breaking dynamics at a high energy scale plays a
significant role in electroweak symmetry breaking. In order for there to be a
large hierarchy between the scale of the high energy dynamics and the weak
scale, the high energy theory must have a second order chiral phase transition.
If the transition is second order, then close to the transition the theory may
be described in terms of a low-energy effective Lagrangian with composite
``Higgs'' scalars. However, scalar theories in which there are more than one
coupling can have a {\it first order} phase transition instead, due to
the Coleman-Weinberg instability. Therefore, top-condensate or strong ETC
theories in which the composite scalars have more than one coupling
cannot always support a large hierarchy. In particular, if the
Nambu--Jona-Lasinio model solved in the large- limit is a good
approximation to the high-energy dynamics, then these models will not produce
acceptable electroweak symmetry breaking.Comment: 10 pages, 1 postscript figure (appended), BUHEP-92-35, HUTP-92/A05
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
Limits on a Composite Higgs Boson
Precision electroweak data are generally believed to constrain the Higgs
boson mass to lie below approximately 190 GeV at 95% confidence level. The
standard Higgs model is, however, trivial and can only be an effective field
theory valid below some high energy scale characteristic of the underlying
non-trivial physics. Corrections to the custodial isospin violating parameter T
arising from interactions at this higher energy scale dramatically enlarge the
allowed range of Higgs mass. We perform a fit to precision electroweak data and
determine the region in the (m_H, Delta T) plane that is consistent with
experimental results. Overlaying the estimated size of corrections to T arising
from the underlying dynamics, we find that a Higgs mass up to 500 GeV is
allowed. We review two composite Higgs models which can realize the possibility
of a phenomenologically acceptable heavy Higgs boson. We comment on the
potential of improvements in the measurements of m_t and M_W to improve
constraints on composite Higgs models.Comment: 9 pages, 2 eps figures. Shortened for PRL; some references elminate
Moose models with vanishing parameter
In the linear moose framework, which naturally emerges in deconstruction
models, we show that there is a unique solution for the vanishing of the
parameter at the lowest order in the weak interactions. We consider an
effective gauge theory based on SU(2) gauge groups, chiral fields and
electroweak groups and at the ends of the chain of the
moose. vanishes when a link in the moose chain is cut. As a consequence one
has to introduce a dynamical non local field connecting the two ends of the
moose. Then the model acquires an additional custodial symmetry which protects
this result. We examine also the possibility of a strong suppression of
through an exponential behavior of the link couplings as suggested by Randall
Sundrum metric.Comment: LaTex file, 27 pages, 8 figure
Coloron Phenomenology
A flavor-universal extension of the strong interactions was recently proposed
in response to the apparent excess of high- jets in the inclusive jet
spectrum measured at the Tevatron. This paper studies the color octet of
massive gauge bosons (`colorons') that is present in the low-energy spectrum of
the model's Higgs phase. Constraints from searches for new particles decaying
to dijets and from measurements of the weak-interaction parameter imply
that the colorons must have masses greater than 870-1000 GeV. The implications
of recent Tevatron data and the prospective input from future experiments are
also discussed.Comment: 13 pages, 4 embedded Postscript figures, LaTeX, full postscript
version also available at http://smyrd.bu.edu/htfigs/htfigs.html rectified
confusing phrase at end of sub-section on 'dijets
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