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 $1/N_c$-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 $e^\pi$ and $e^{2\pi}$. 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 $M_W/m_t$ 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 $M_W$ in terms of integrals containing the dynamically generated mass function $\Sigma_t(p^2)$. 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 $M_W/m_t$ and $M_W/M_H$ 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 $SU(2)_W\times U(1)_Y$ 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 $R_b$ and $R_c$.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 $\chi^2 < 3.1 (3.2)$ for $m_H = 115 (300) GeV$ in comparison with $\chi^2 = 3.5 (5.0)$ 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 $SU(2)$ 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 $\rho$--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 fil