arXiv subjects

G. K. Leontaris

Publications and source records attributed to G. K. Leontaris.

At least 73 records · Page 4Linked to original sources

arXiv1997

Uncertainty relation and non-dispersive states in Finite Quantum Mechanics

In this letter, we provide evidence for a classical sector of states in the Hilbert space of Finite Quantum Mechanics (FQM). We construct a subset of states whose the minimum bound of position -momentum uncertainty (equivalent to an effective $\hbar$) vanishes. The classical regime, contrary to standard Quantum Mechanical Systems of particles and fields, but also of strings and branes appears in short distances of the order of the lattice spacing. {}For linear quantum maps of long periods, we observe that time evolution leads to fast decorrelation of the wave packets, phenomenon similar to the behavior of wave packets in t' Hooft and Susskind holographic picture. Moreoever, we construct explicitly a non - dispersive basis of states in accordance with t' Hooft's arguments about the deterministic behavior of FQM.

hep-th↗

arXiv1997

Leptoquarks in SUSY Unified Models and the HERA Events

Motivated by the recent HERA events which are consistent with a possible leptoquark interpretation, we discuss the prospects for including additional light colour triplets and anti-triplets in the spectrum of supersymmetric unified theories. We focus on a particular string-inspired Pati-Salam model, and propose a simple mechanism by which a light colour triplet of charge -1/3 plus anti-triplet of charge 1/3, may have a mass of order 200 GeV, with one of the new states having leptoquark couplings and with proton decay suppressed. We also discuss possible scenarios for gauge unification in such a model.

hep-ph↗

arXiv1997

Constraints on Leptoquark Masses and Couplings from Rare Processes and Unification

Motivated by the recent experimental data at HERA and ZEUS, which have reported evidence for leptoqark production at Sqrt{s}=314 GeV with a mass at M_D=200 GeV we consider its implications in unified supersymmetric theories. We also present calculations for leptoquark production incorporating the existing limits from other exotic reactions on its couplings and other relevant parameters.

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arXiv1997

Yukawa Textures from Family Symmetry and Unification

In this letter, we show how the combination of both a gauged U(1)_X family symmetry and an extended vertical gauge symmetry in a single model, allows for the presence of additional Clebsch texture zeroes in the fermion mass matrices. This leads to new structures for the textures, with increased predictivity, as compared to schemes with enhanced family symmetries only. We illustrate these ideas in the context of the Pati-Salam gauge group SU(4)xSU(2)_LxSU(2)_R$ supplemented by a U(1)_X gauged family symmetry. In the case of symmetric mass matrices, two of the solutions of Ramond, Roberts and Ross that may not be obtained by family symmetries only, are accurately reproduced. For non-symmetric textures, new structures arise in models of this type. To distinguish between the solutions in this latter case, we performed a numerical fit to the charged fermion mass and mixing data. The best solution we found allows a fit with a total chi^2 of 0.39, for three degrees of freedom.

hep-ph↗

arXiv1997

Yukawa Textures in String Unified Models With SU(4)xO(4) Symmetry

We discuss the origin of Yukawa textures in the string-inspired and string derived models based on the gauge group SU(4)xSU(2)_LxSU(2)_R supplemented by a U(1)_X gauged family symmetry. The gauge symmetries are broken down to those of the minimal supersymmetric standard model which is the effective theory below 10^16 GeV. The combination of the U(1)_X family symmetry and the Pati-Salam gauge group leads to a successful and predictive set of Yukawa textures involving two kinds of texture zeroes: horizontal and vertical texture zeroes. We discuss both symmetric and non-symmetric textures in models of this kind, and in the second case perform a detailed numerical fit to the charged fermion mass and mixing data. Two of the Yukawa textures allow a low energy fit to the data with a total chi^2 of 0.39 and 1.02 respectively, for three degrees of freedom. We also make a first attempt at deriving the non - renormalisable operators required for the Yukawa textures from string theory.

hep-ph↗

arXiv1997

Supercompositeness Inspired From Superstrings

String Unified Models based on the $k=1$ level of the Kac-Moody Algebra, predict the existence of ``exotic'' new states which carry fractional electric charges. We analyse the possibility of considering these ``exotics'' as preonic matter which can be used to form the families and the gauge group breaking higgs scalars. It is proposed that such a formation may occur provided that these states transform non-trivially under a non-Abelian gauge group with a relatively large rank in order to confine them at a sufficiently large scale. Such a situation is natural in string derived unified models, since the role of the confining group can be played by (part of) the Hidden symmetry. As an example, we present a string derived toy model based on the $SU(4)\times SU(2)_L \times SU(2)_R$ Pati-Salam gauge group.

hep-ph↗

arXiv1997

Unification Bounds on the Possible N=2 Supersymmetry Breaking Scale

In this letter, the possible appearance of N=2 supersymmetry at a low energy scale is investigated in the context of unified theories. Introducing mirror particles for all the gauge and matter multiplets of the Minimal Supersymmetric extension of the Standard Model (MSSM), the measured values of sin^2 θ_W and α_3(M_Z) indicate that the N=2 threshold scale M_{S_2} cannot be lower than \sim 10^{14}GeV. If the U(1) normalization coefficient k is treated as a free parameter, M_{S_2} can be as low as 10^9 GeV. On the other hand, if mirror quarks and leptons are absent and a non-standard value for k is used, N=2 supersymmetry breaking could in principle occur at the electroweak scale.

hep-ph↗

arXiv1997

The μ- term in Effective Supergravity Theories

The Higgs mixing term coefficient $μ_{eff}$ is calculated in the scalar potential in supergravity theories with string origin, in a model independent approach. A general low energy effective expression is derived, where new contributions are included which depend on the modular weights $q_{1,2}$ of the Higgs superfields, the moduli and derivative terms. We find that in a class of models obtained in the case of compactifications of the heterotic superstring, the derivative terms are identically zero. Further, the total $μ_{eff}$-term vanishes identically if the sum of the two modular weights $q_1+q_2$ is equal to two. Subleading $μ$- corrections, in the presence of intermediate gauge symmetries predicted in viable string scenarios, are also discussed.

hep-th↗

arXiv1996

Duality Constraints on Supersymmetric Unified Models and Radiative Symmetry Breaking

Motivated from unified models with string origin, we analyse the constraints from duality invariance on effective supergravity models with an intermediate gauge symmetry. Requiring vanishing vacuum energy and invariance of the superpotential couplings, we find that modular weights of the matter and higgs fields are subject to various constraints. In addition, the vacuum expectation values of the higgs fields breaking the intermediate gauge group, are determined in terms of their modular weights and the moduli. We also examine the possibility of breaking the intermediate gauge symmetry radiatively.

hep-ph↗

arXiv1996

A String Model with $SU(4)\times O(4)\times [Sp(4)]_{Hidden}$ Gauge Symmetry

In the four dimensional free fermionic formulation of the heterotic string, a semi-realistic $SU(4)\times SU(2)_L \times SU(2)_R$ model is proposed with three fermion generations in $(4,2,1)+(\bar 4,1,2)$ representations. The gauge symmetry of the model breaks to the standard gauge group using a higgs pair in the $(4,1,2)+(\bar 4,1,2)$ representations. The massless spectrum includes exotic fractionally charged states with non -- trivial transformation properties under part ($Sp(4)$) of the non -- abelian `hidden' symmetry. Finally there is a mirror pair in $(4,2,1) , (\bar 4,2,1)$ allowing the possibility for an identical running of $g_{4,L,R}$ couplings between the string and $SU(4)$ breaking scales. This is of crucial importance for a successful prediction of the weak mixing angle. Potential shortcomings and problems of the construction are analysed and possible solutions are discussed.

hep-ph↗

arXiv1995

Gauge Coupling Unification and the Top Mass in String Models with $SU(4)\times O(4)$ Symmetry

We discuss the low energy implications of gauge coupling unification at the string scale, taking into account string threshold corrections in the $SU(4) {\times} O(4)$ model. We express $sin^2θ_W$ and $a_3$ as functions of the calculable string threshold differences and discuss simple examples of spectra which retain the successful predictions of the supersymmetric unification. Using further the low energy data and reasonable values of the common gauge coupling at the string scale, we obtain the range of the threshold corrections. Finally, we study the top Yukawa coupling ($h_t$) evolution whose initial value is determined in terms of the common gauge coupling at the string scale. We find that $h_t$ reaches its (quasi) infra-red fixed point at the weak scale and discuss the implications on the top mass.

hep-ph↗

arXiv1995

Neutrino masses in supersymmetric theories

Extensions of the Standard Model with additional U(1) symmetries can describe the hierarchy of fermion masses and mixing angles, including neutrinos. The neutrino masses and mixings are determined up to a discrete ambiguity corresponding to the representation content of the Higgs sector responsible for the Majorana mass matrix. The solar and the atmospheric neutrino deficits as well as the COBE data, may be explained simultaneously in specific schemes motivated by symmetries. Using simple, analytic expressions, it is possible to demonstrate the known effect that for small $\tanβ$, phenomenologically interesting neutrino masses would disturb the bottom-tau unification. This however can be avoided in schemes with a large $μ-τ$ mixing in the charged leptonic sector. On the other hand, for the large $\tanβ$ regime, due to the fixed point properties of the top as well as the bottom coupling (which are described by analytic expressions, for sufficiently large couplings) no modification to the bottom-tau unification would occur. Still, large mixing in the $μ$-$τ$ sector is desirable in this case as well, in order to have a solution to the atmospheric neutrino problem. In the same schemes, a relatively heavy strange quark $\approx 200$ MeV is also predicted.

hep-ph↗

arXiv1995

MSSM phenomenology in the large tanb regime

We discuss aspects of the low energy phenomenology of the MSSM, in the large $\tan β $ regime. We explore the regions of the parameter space where the $h_t$ and $h_b$ Yukawa couplings exhibit a fixed point structure, using previous analytic solutions for these couplings. Expressions for the parameters $A_{t}$ and $A_{b}$ and the renormalised soft mass terms are also derived, making it possible to estimate analytically the sparticle loop -- corrections to the bottom mass, which are important in this limit.

hep-ph↗

arXiv1995

Textures for Neutrino Mass Matrices

We give a classification of heavy Majorana neutrino mass matrices with up to three texture zeroes, assuming the Dirac masses of the neutrinos to be of the same form as the ones of the up-quarks in the five texture zero solutions for the quark matrices. This is the case for many unified and partially unified models. We find that it is possible to have solutions which account for the solar and atmospheric neutrino problems as well as the COBE observations simultaneously, and we motivate the existence of such solutions from symmetries.

hep-ph↗

arXiv1995

Heavy neutrino threshold effects in low energy phenomenology

Right handed neutrinos with mass of ${\cal O} \; (10^{12}-10^{13})$ GeV are required to implement the see-saw mechanism and generate neutrino masses capable of playing a role in structure formation. Moreover models of fermion masses often relate the Yukawa couplings involving these neutrinos to the up-quark Yukawa couplings. Here we study the effects of such couplings on the radiative corrections to quark masses. We find that $b-τ$ equality at $M_{GUT}$ may still give the correct $m_b/m_τ$-- ratio at low energies, but only if there is large $μ-τ$ mixing in the charged leptonic sector. We propose specific mass matrix ``textures'' dictated by a $U(1)$ family symmetry whose structure preserves $m_b=m_τ$ at $M_{GUT}$. In these schemes, due to the large $ν_μ - ν_τ$ mixing, it is possible to give a simultaneous solution to the solar neutrino deficit and the atmospheric neutrino problem.

hep-ph↗

arXiv1995

Analytic Approach to the MSSM Mass Spectrum in the Large Tan$β$ Regime

In various unified extensions of the Minimal Supersymmetric Standard Model, the Yukawa couplings of the third generation are predicted to be of the same order. As a result, low energy measured mass ratios, require large ratios of the standard model Higgs vacuum expectation values, corresponding to a large value of the parameter $tanβ$. We present analytic solutions for the Yukawa couplings and the Higgs and third generation squark masses, in the case of large top and bottom Yukawa couplings. We examine regions of these Yukawas which give predictions for the top mass compatible with the present experimentally determined top mass and provide useful approximate formulae for the scalars. We discuss the implications on the Radiative Symmetry Breaking mechanism and derive constraints on the undetermined initial conditions of the scalars.

hep-ph↗

arXiv1995

Soft SUSY Masses and the Dynamical Determination of the Gravitino Mass

We discuss in detail the possibility of determining dynamicaly the gravitino mass $m_{3/2}$, which is related to the supersymmetry breaking scale, within the minimal supersymmetric standard model (MSSM). Using the complete MSSM spectrum, we minimize the vacuum energy including one-loop corrections and a cosmological term of ${\cal O}(m_{3/2}^4)$ induced by the underlying fundamental theory. We find that both terms are necessary to determine dynamically the gravitino mass. Other useful constraints for the low energy phenomenology are also obtained.

hep-ph↗

arXiv1994

Low Energy Thresholds and the Scalar Mass Spectrum in Minimal Supersymmetry

We discuss low energy threshold effects and calculate the sparticle masses in the context of the Minimal Supersymmetric Standard Model. We pay particular attention to the top squark and the Higgs mass parameters, and calculate the top Yukawa corrections, taking into account the successive decoupling of each particle at its threshold. We discuss the phenomenological implications in the context of the radiative symmetry breaking scenario.

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