104 research outputs found
A low-energy compatible SU(4)-type Model for Vector Leptoquarks of Mass <= 1 TeV
The Standard Model is extended by a SU(2)L singlet of vector leptoquarks. An
additional SU(4) gauge symmetry between right-handed up quarks and right-handed
leptons is introduced to render the model renormalizable. The arrangement is
made in such a way that no conflict with low energy restrictions is
encountered. The SU(2)L singlet mediates interactions between the right-handed
leptons and up type quarks for which only moderate low energy restrictions
M(LQ)/g(LQ) > few hundred GeV exist. We cancel the anomalies of the model and
furthermore argue that the inequality g(LQ) > g(QCD) is a general feature of
consistent vector leptoquark models. Although our model is not relevant for
HERA, it is interesting per se as a description of leptoquarks of mass < 1 TeV
consistent with all low-energy requirements.Comment: 20 pages, now including anomaly cancellatio
Top Condensation without Fine-Tuning
Quadratic divergencies which lead to the usual fine-tuning or hierarchy
problem are discussed in top condensation models. As in the Standard Model a
cancellation of quadratic divergencies is not possible without the boson
contributions in the radiative corrections which are absent in lowest order of
an -expansion. To deal with the cancellation of quadratic divergencies
we propose therefore an expansion in the flavor degrees of freedom. In leading
order we find the remarkable result that quadratic divergencies automatically
disappear.Comment: LMU - 17/93, in LATEX, 12 pages and 3 pages of figures appended in
Postscrip
Family Structure from Periodic Solutions of an Improved Gap Equation
Fermion mass models usually contain a horizontal symmetry and therefore fail
to predict the exponential mass spectrum of the Standard Model in a natural
way. In dynamical symmetry breaking there are different concepts to introduce a
fermion mass spectrum, which automatically has the desired hierarchy. In
constructing a specific model we show that in some modified gap equations
periodic solutions with several fermion poles appear. The stability of these
excitations and the application of this toy model are discussed. The mass
ratios turn out to be approximately and . Thus the model
explains the large ratios of fermion masses between successive generations in
the Standard Model without introducing large or small numbers by hand.Comment: 19 pages, 8 figure
Connections between Dynamical and Renormalization Group Techniques in Top Condensation Models
Predictions for the ratio arise in top condensation models from
different methods. One type of prediction stems from Pagels--Stokar relations
based on the use of Ward Identities in the calculation of the \GB decay
constants and expresses in terms of integrals containing the dynamically
generated mass function . Another type of prediction emerges
from the renormalization group equations via infrared quasi--fixed--points of
the running top quark Yukawa coupling. We demonstrate in this paper that in the
limit of a high cutoff these two methods lead to the same predictions for
and in lowest loop order.Comment: Slightly revised version. LaTeX and uuencoded, packed Postscript
figures. The complete paper, including figures, is also available via WWW at
the following http URL
http://www.physik.tu-muenchen.de/tumphy/d/T30d/PAPERS/TUM-HEP-216-95.ps.g
Bounds on scalar leptoquark masses from S, T, U parameters in the minimal four-color quark-lepton symmetry model
The contributions into radiative correction parameters S, T, U from the
scalar leptoquarks are calculated in the minimal gauge model with the
four-color quark-lepton symmetry. It is shown that the contributions into T and
U from the scalar leptoquark doublets are not positive definite. Using the
current experimental data on S, T, U the bounds on the scalar leptoquark masses
are obtained. In particular, the existence of the relatively light scalar
leptoquark with electric charge 2/3 is shown to be compatible with the current
experimental data on S, T, U.Comment: 14 pages, 3 ps figures, LaTeX, uses epsf.sty, accepted for
publication in Physics Letters
A Renormalization Group Analysis of the Higgs Boson with Heavy Fermions and Compositeness
We study the properties of heavy fermions in the vector-like representation
of the electro-weak gauge group with Yukawa couplings to
the standard model (SM) Higgs boson. Using the renormalization group analysis,
we discuss their effects on the vacuum stability and the triviality bound on
the Higgs self-coupling, within the context of the standard model (i.e., the
Higgs particle is elementary). Contrary to the low energy case where the
decoupling theorem dictates their behavior, the inclusion of heavy fermions
drastically change the SM structure at high scale. We also discuss the
interesting possibility of compositeness, i.e., the Higgs particle is composed
of the heavy fermions using the method of Bardeen, Hill and
Lindner~\cite{BHL91}. Finally we briefly comment on their possible role in
explaining and .Comment: Some typographic errors are corrected and title is changed. Version
to appear in Physics Letter B. 9 pages 6 Postscript figures, use epsf.st
Bounds on scalar leptoquark and scalar gluon masses from S, T, U in the minimal four color symmetry model
The contributions into radiative correction parameters S, T, U from scalar
leptoquark and scalar gluon doublets are investigated in the minimal four color
symmetry model. It is shown that the current experimental data on S, T, U allow
the scalar leptoquarks and the scalar gluons to be relatively light (with
masses of order of 1 TeV or less), the lightest particles are preferred to lie
below 400 GeV. In particular, the lightest scalar leptoquarks with masses below
300 GeV are shown to be compatible with the current data on S, T, U at for in comparison with
in the Standard Model. The lightest scalar gluon in this case is expected to
lie below 850 (720) GeV. The possible significance of such particles in the
t-quark physics at LHC is emphasized.Comment: 14 pages, 2 figures, to appear in Physics Letters
Custodial SU(2) Violation and the Origin of Fermion Masses
Custodial breaking due to dynamical fermion masses is studied in a
rather general context and it is shown how some well known limiting cases are
correctly described. The type of ``gap equation'' which can systematically lead
to extra negative contributions to the so--called --parameter is
emphasized. Furthermore general model independent features are discussed and it
is shown how electro--weak precision measurements can be sensitive to the
fermion content and/or dynamical features of a given theory.Comment: HD-THEP-92-55, 18 pages and 2 pages of figures appended as Postscript
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