arXiv subjects

Alon E. Faraggi

Publications and source records attributed to Alon E. Faraggi.

At least 145 records · Page 8Linked to original sources

arXiv1997

New dark matter candidates motivated from superstring derived unification

Perturbative gauge coupling unification in realistic superstring models suggests the existence of additional heavy down-type quarks, beyond the minimal supersymmetric standard model. The mass scale of the heavy down-type quarks is constrained by requiring agreement between the measured low energy gauge parameters and the string-scale gauge coupling unification. These additional quarks arise and may be stable due to the gauge symmetry breaking by ``Wilson lines'' in the superstring models. We argue that there is a window in the parameter space within which this down-type quark is a good candidate for the dark matter.

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arXiv1997

Local discrete symmetries from superstring derived models

Discrete and global symmetries play an essential role in many extensions of the Standard Model, for example, to preserve the proton lifetime, to prevent flavor changing neutral currents, etc. An important question is how can such symmetries survive in a theory of quantum gravity, like superstring theory. In a specific string model I illustrate how local discrete symmetries may arise in string models and play an important role in preventing fast proton decay and flavor changing neutral currents. The local discrete symmetry arises due to the breaking of the non--Abelian gauge symmetries by Wilson lines in the superstring models and forbids, for example dimension five operators which mediate rapid proton decay, to all orders of nonrenormalizable terms. In the context of models of unification of the gauge and gravitational interactions, it is precisely this type of local discrete symmetries that must be found in order to insure that a given model is not in conflict with experimental observations.

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arXiv1997

R-parity violation in superstring derived models

The ALEPH collaboration has recently reported a significant excess of four-jet events at the LEP 1.5 and LEP 2. While not yet confirmed by the other collaborations, it was recently proposed that this excess may be explained by certain R-parity violating operators in supersymmetric models. R-parity violating operators introduce the danger of inducing rapid proton decay. I discuss how the operators required to explain the ALEPH four jet events may arise from superstring derived models without inducing rapid proton decay.

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arXiv1996

Calculating Fermion masses in Superstring Derived Standard-like Models

I discuss the calculation of the heavy generation masses in the superstring derived standard-like models. The top quark Yukawa coupling is obtained from a cubic level mass term while the bottom quark and tau lepton mass terms are obtained from nonrenormalizable terms. The calculation of the heavy fermion Yukawa couplings is outlined in detail in a specific toy model. The dependence of the effective bottom quark and tau lepton Yukawa couplings on the flat directions at the string scale is examined. The gauge and Yukawa couplings are extrapolated from the string unification scale to low energies. Agreement with $α_{strong}$, $sin^2θ_W$ and $α_{em}$ at M_Z is imposed, which necessitates the existence of intermediate matter thresholds. The needed intermediate matter thresholds exist in the specific toy model. The effect of the intermediate matter thresholds on the extrapolated Yukawa couplings in studied. It is observed that the intermediate matter thresholds also help to maintain the correct $b/τ$ mass relation. It is found that for a large portion of the parameter space that the LEP precision data for $α_{strong}$, $sin^2θ_W$ and $α_{em}$, as well as the top quark mass and the $b/τ$ mass relation can all simultaneously be consistent with the superstring derived standard-like models. Possible corrections due to the supersymmetric mass spectrum are studied as well as the minimization of the supersymmetric Higgs potential. It is demonstrated that the calculated values of the Higgs VEV ratio, $tanβ=v_1/v_2$, can be compatible with the minimization of the one-loop effective Higgs potential.

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arXiv1996

A Statistical Interpretation of Space and Classical-Quantum duality

By defining a prepotential function for the stationary Schrödinger equation we derive an inversion formula for the space variable $x$ as a function of the wave-function $ψ$. The resulting equation is a Legendre transform that relates $x$, the prepotential ${\cal F}$, and the probability density. We invert the Schrödinger equation to a third-order differential equation for ${\cal F}$ and observe that the inversion procedure implies a $x$-$ψ$ duality. This phenomenon is related to a modular symmetry due to the superposition of the solutions of the Schrödinger equation. We propose that in quantum mechanics the space coordinate can be interpreted as a macroscopic variable of a statistical system with $\hbar$ playing the role of a scaling parameter. We show that the scaling property of the space coordinate with respect to $τ=\partial_ψ^2{\cal F}$ is determined by the ``beta-function''. We propose that the quantization of the inversion formula is a natural way to quantize geometry. The formalism is extended to higher dimensions and to the Klein-Gordon equation.

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arXiv1996

Cosmological and phenomenological implications of Wilsonian matter in realistic superstring derived models

Superstring phenomenology aims at achieving two goals. The first is to reproduce the observed physics of the Standard Model. The second is to identify experimental signatures of superstring unification which, if observed, will provide further evidence for the validity of superstring theory. I discuss such potential signatures of superstring unification. I propose that proton lifetime constraints imply that the Standard Model gauge group must be obtained directly at the string level. In this case the unifying gauge group, for example $SO(10)$, is broken to the Standard Model gauge group by ``Wilson lines''. The symmetry breaking by ``Wilson line'' has important implications. It gives rise to exotic massless states which cannot fit into multiplets of the original unifying gauge group. This is an important feature because it results in conservation laws which forbid the interaction of the exotic ``Wilsonian'' states with the Standard Model states. The ``Wilsonian'' matter states then have important phenomenological implications. I discuss two such implications: exotic ``Wilsonian'' states as dark matter candidates and ``Wilsonian'' matter as the messenger sector in gauge mediated dynamical SUSY breaking scenarios.

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arXiv1996

A low energy dynamical SUSY breaking scenario motivated from superstring derived unification

Recently there has been a resurgence of interest in gauge mediated dynamical supersymmetry breaking scenarios. I investigate how low energy dynamical SUSY breaking may arise from superstring models. In a three generation string derived model I propose that the unbroken hidden non--Abelian gauge group at the string scale is $SU(3)_H$ with matter multiplets. Due to the small gauge content of the hidden gauge group the supersymmetry breaking scale may be consistent with the dynamical SUSY breaking scenarios. The messenger states are obtained in the superstring model from sectors which arise due to the ``Wilson--line'' breaking of the unifying non--Abelian gauge symmetry. An important property of the string motivated messenger states is the absence of superpotential terms with the Standard Model states. The stringy symmetries therefore forbid the flavor changing processes which may arise due to couplings between the messenger sector states and the Standard Model states. Motivated from the problem of string gauge coupling unification I contemplate a scenario in which the messenger sector consists solely of color triplets. This hypothesis predicts a chargino mass below the $W$--boson mass. Imposing the current limits from the LEP1 and LEP1.5 experiments the lightest supersymmetric particles predicted by this hypothesis are in the mass ranges $M_{χ^\pm}\approx55-65$ GeV, $M_{χ^0}\approx35-50$ GeV and $M_{\tildeν}\approx45-60$ GeV which will be tested in the forthcoming LEP2 experiments.

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arXiv1996

Stable Superstring Relics

We investigate the cosmological constraints on exotic stable matter states which arise in realistic free fermionic superstring models. These states appear in the superstring models due to a ``Wilson--line'' breaking of the unifying non--Abelian gauge symmetry. In the models that we consider the unifying $SO(10)$ gauge symmetry is broken at the string level to $SO(6)\times SO(4)$, $SU(5)\times U(1)$ or $SU(3)\times SU(2)\times U(1)^2$. The exotic matter states are classified according to the patterns of the $SO(10)$ symmetry breaking. In $SO(6)\times SO(4)$ and $SU(5)\times U(1)$ type models one obtains fractionally charged states with $Q_{\rm e.m.}=\pm1/2$. In $SU(3)\times SU(2)\times U(1)^2$ type models one also obtains states with the regular charges under the Standard Model gauge group but with ``fractional'' charges under the $U(1)_{Z^\prime}$ symmetry. These states include down--like color triplets and electroweak doublets, as well as states which are Standard Model singlets. By analyzing the renormalizable and nonrenormalizable terms of the superpotential in a specific superstring model, we show that these exotic states can be stable. We investigate the cosmological constraints on the masses and relic density of the exotic states. We propose that, while the abundance and the masses of the fractionally charged states are highly constrained, the Standard Model -- like states, and in particular the Standard Model singlet, are good dark matter candidates.

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arXiv1996

Leptophobic Z' from Superstring Derived Models

It was recently suggested that the reported anomalies in $R_b$ and $R_c$ can be interpreted as the effect of a heavy vector boson that couples to quarks and is universally decoupled from leptons. We examine how an extra gauge boson with this property can arise from superstring derived models. In a specific three generation model we show that the $U(1)_{B-L}$ symmetry combines with the horizontal flavor symmetries to form a universal leptophobic $U(1)$ symmetry. In our model there is an enhancement of the color gauge group from twisted sectors. The enhancement occurs after the breaking of the unifying gauge symmetry by ``Wilson lines''. The leptophobic $U(1)$ symmetry then becomes a generator of the color $SU(4)$ gauge group. We examine how similar symmetries may appear in other string models without the enhancement. We propose that if the current LEP anomalies persist it may be evidence for a certain class of un--unified superstring models.

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arXiv1995

Gauge Coupling Unification in Realistic Free-Fermionic String Models

We discuss the unification of gauge couplings within the framework of a wide class of realistic free-fermionic string models which have appeared in the literature, including the flipped SU(5), SO(6)xSO(4), and various SU(3)xSU(2)xU(1) models. If the matter spectrum below the string scale is that of the Minimal Supersymmetric Standard Model (MSSM), then string unification is in disagreement with experiment. We therefore examine several effects that may modify the minimal string predictions. First, we develop a systematic procedure for evaluating the one-loop heavy string threshold corrections in free-fermionic string models, and we explicitly evaluate these corrections for each of the realistic models. We find that these string threshold corrections are small, and we provide general arguments explaining why such threshold corrections are suppressed in string theory. Thus heavy thresholds cannot resolve the disagreement with experiment. We also study the effect of non-standard hypercharge normalizations, light SUSY thresholds, and intermediate-scale gauge structure, and similarly conclude that these effects cannot resolve the disagreement with low-energy data. Finally, we examine the effects of additional color triplets and electroweak doublets beyond the MSSM. Although not required in ordinary grand unification scenarios, such states generically appear within the context of certain realistic free-fermionic string models. We show that if these states exist at the appropriate thresholds, then the gauge couplings will indeed unify at the string scale. Thus, within these string models, string unification can be in agreement with low-energy data.

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arXiv1995

String Unification, Higher-Level Gauge Symmetries, and Exotic Hypercharge Normalizations

We explore the extent to which string theories with higher-level gauge symmetries and non-standard hypercharge normalizations can reconcile the discrepancy between the string unification scale and the GUT scale extrapolated from the Minimal Supersymmetric Standard Model (MSSM). We determine the phenomenologically allowed regions of (k_Y,k_2,k_3) parameter space, and investigate the proposal that there might exist string models with exotic hypercharge normalizations k_Y which are less than their usual value k_Y=5/3. For a broad class of heterotic string models (encompassing most realistic string models which have been constructed), we prove that k_Y >= 5/3. Beyond this class, however, we show that there exist consistent MSSM embeddings which lead to k_Y < 5/3. We also consider the constraints imposed on k_Y by demanding charge integrality of all unconfined string states, and show that only a limited set of hypercolor confining groups and corresponding values of k_Y are possible.

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arXiv1995

On the origin of three generation free fermionic superstring models

The three generation superstring models in the free fermionic models have had remarkable success in describing the real--world. The most explored models use the NAHE set to obtain three generations and to separate the hidden and observable sectors. It is of course well known that the full NAHE set is not required in order to construct three generation free fermionic models. I argue that all the semi--realistic free fermionic models that have been constructed to date correspond to $Z_2\times Z_2$ orbifolds. Thus, the successes of the semi--realistic free fermionic models, if taken seriously, suggest that the true string vacuum is a $Z_2\times Z_2$ orbifold with nontrivial background fields and quantized Wilson lines.

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arXiv1995

Making Ends Meet: String Unification and Low-Energy Data

A long-standing problem in string phenomenology has been the fact that the string unification scale disagrees with the GUT scale obtained by extrapolating low-energy data within the framework of the minimal supersymmetric standard model (MSSM). In this paper we examine several effects that may modify the minimal string predictions and thereby bring string-scale unification into agreement with low-energy data. These include heavy string threshold corrections, non-standard hypercharge normalizations, light SUSY thresholds, intermediate gauge structure, and thresholds arising from extra matter beyond the MSSM. We explicitly evaluate these contributions within a variety of realistic free-fermionic string models, including the flipped SU(5), SO(6) x SO(4), and various SU(3) x SU(2) x U(1) models, and find that most of these sources do not substantially alter the minimal string predictions. Indeed, we find that the only way to reconcile string unification with low-energy data is through certain types of extra matter. Remarkably, however, many of the realistic string models contain precisely this required matter in their low-energy spectra.

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arXiv1995

Curves of Marginal Stability in N=2 Super-QCD

We present a simple argument determining the shape of the curves of marginal stability in the N=2 supersymmetric SU(2) QCD with less than 4 massless flavors. The argument relies only on the modular properties of $a_D/a$ and its weak-coupling behavior.

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arXiv1994

The Vacuum Structure and Spectrum of N=2 Supersymmetric SU(N) Gauge Theory

We present an exact description of the metric on the moduli space of vacua and the spectrum of massive states for four dimensional N=2 supersymmetric SU(n) gauge theories. The moduli space of quantum vacua is identified with the moduli space of a special set of genus n-1 hyperelliptic Riemann surfaces.

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arXiv1994

Custodial nonabelian gauge symmetries in realistic superstring derived models

A well known problem in supersymmetric models is the presence of, lepton and baryon number violating, dimension four operators. The traditional R parity solution may not be suitable if one tries to incorporate supersymmetry into a Planck scale theory. I propose a different solution in the context of realistic string models. I show that realistic string models can give rise to custodial nonabelian gauge symmetries under which only the leptons or quarks transform. I explain how such symmetries arise in a class of free fermionic models that are based on $Z_2\times Z_2$ orbifold with standard embedding. The custodial symmetries forbid proton decay from dimension four operators while allowing R parity violation.

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arXiv1994

Realistic Superstring Models

I discuss the construction of realistic superstring standard--like models in the four dimensional free fermionic formulation. I discuss the massless spectrum of the superstring standard--like models and the texture of fermion mass matrices. These models suggest an explanation for the top quark mass hierarchy. At the cubic level of the superpotential only the top quark get a mass term. The lighter quarks and leptons obtain their mass terms from nonrenormalizable terms that are suppressed relative to the cubic order term. A numerical estimate yielded $m_t\sim175-180~GeV$. The suppression of the lightest generation masses results from the horizontal symmetries in the superstring models. The problems of neutrino masses, gauge coupling unification and hierarchical SUSY breaking are discussed. I argue that the realistic features of these models are due to the underlying $Z_2\times Z_2$ orbifold, with standard embedding, at the free fermionic point in toroidal compactification space.

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arXiv1994

Hierarchical Supersymmetry Breaking in Superstring Derived Standard--like Models

We examine the problem of supersymmetry breaking in realistic superstring standard--like models which are constructed in the free fermionic formulation. We impose a supersymmetric vacuum at the Planck scale by requiring vanishing F and D constraints at the cubic level of the superpotential. We then study possible scenarios for supersymmetry breaking by examining the role of nonrenormalizable terms and hidden sector gaugino and matter condensates. We argue that in some scenarios hierarchical supersymmetry breaking in the observable sector is possible.

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