668 research outputs found
Toward the classification of the realistic free fermionic models
The realistic free fermionic models have had remarkable success in providing
plausible explanations for various properties of the Standard Model which
include the natural appearance of three generations, the explanation of the
heavy top quark mass and the qualitative structure of the fermion mass spectrum
in general, the stability of the proton and more. These intriguing achievements
makes evident the need to understand the general space of these models. While
the number of possibilities is large, general patterns can be extracted. In
this paper I present a detailed discussion on the construction of the realistic
free fermionic models with the aim of providing some insight into the basic
structures and building blocks that enter the construction. The role of free
phases in the determination of the phenomenology of the models is discussed in
detail. I discuss the connection between the free phases and mirror symmetry in
(2,2) models and the corresponding symmetries in the case of the (2,0) models.
The importance of the free phases in determining the effective low energy
phenomenology is illustrated in several examples. The classification of the
models in terms of boundary condition selection rules, real world-sheet fermion
pairings, exotic matter states and the hidden sector is discussed.Comment: 43 pages. Standard Late
Cabibbo--Kobayashi--Maskawa Mixing in Superstring Derived Standard--like Models
We examine the problem of three generation quark flavor mixing in realistic,
superstring derived standard--like models, constructed in the free fermionic
formulation. We study the sources of family mixing in these models and discuss
the necessary conditions to obtain a realistic Cabibbo--Kobayashi--Maskawa
(CKM) mixing matrix. In a specific model, we estimate the mixing angles and
discuss the weak CP violating phase. We argue that the superstring
standard--like models can produce a realistic CKM mixing matrix. We discuss the
possible textures of quark mass matrices that may be obtained in these models.Comment: WIS--93/35/APR--PH, 29 page
NAHE-based string models with SU(4) X SU(2) X U(1) SO(10) Subgroup
The orbifold GUT doublet-triplet splitting mechanism was discussed in 1994 in
the framework of the NAHE-based free fermionic models in which the SO(10) GUT
symmetry is broken to SO(6) X SO(4), SU(3) X SU(2) X U(1)^2, or SU(3) X U(1) X
SU(2)^2. In this paper we study NAHE-based free fermionic models in which the
SO(10) symmetry is broken at the string level to SU(4) X SU(2) X U(1). In
addition to the doublet-triplet splitting this case also has the advantage of
inducing the doublet-doublet splitting already at the string level. We
demonstrate, however, that NAHE-based models with SU(4) X SU(2) X U(1) SO(10)
subgroup are not viable. We show that, similarly to the LRS models, and in
contrast to the FSU5, PS and SLM models, the SU421 case gives rise to models
without an anomalous U(1) symmetry, and discuss the different cases in terms of
their N=4 origins.Comment: 25 pages. Standard Latex. Revised version to appear in NP
String Inspired Z' Model With Stable Proton and Light Neutrino Masses
Grand unification, and its incarnation in the form of heterotic-string
unification, are the only extensions of the Standard Model that are rooted in
the structure of the Standard Model itself. In this context, it was proposed
that the exclusiveness of proton stability and suppression of neutrino masses
necessitates the existence of an additional U(1)_Z' symmetry, which is of
non-GUT origin and remains unbroken down to intermediate, or low, energies.
Realistic string models frequently give rise to non-GUT U(1) symmetries, which
arise from the flavor symmetries in the models. In this paper we demonstrate in
a string-inspired toy model that such a stringy Z' can indeed guarantee proton
longevity and viable phenomenology in the neutrino sector, as well as in the
quark and charged lepton sectors.Comment: 20 pages. Standard LaTex. Version to appear in NP
The Orbifold and the SUSY Flavor Problem
The characteristic property of the orbifold is its cyclic
permutation symmetry. It is argued that this property may be instrumental in
explaining simultaneously the fermion mass hierarchy and the squark mass
degeneracy. Detailed studies in free fermionic models that preserve the cyclic
permutation symmetry of anomalous U(1) charges of the three generations are
discussed.Comment: 9 pages. Standard Late
Moduli Fixing in Realistic String Vacua
I illustrate the existence of quasi-realistic heterotic-string models in
which all the untwisted Kaehler and complex structure moduli, as well as all of
the twisted sectors moduli, are projected out by the generalized GSO
projections. I discuss the conditions and characteristics of the models that
produce this result. The existence of such models offers a novel perspective on
the realization of extra dimensions in string theory. In this view, while the
geometrical picture provides a useful mean to classify string vacua, in the
phenomenologically viable cases there is no physical realization of extra
dimensions. The models under consideration correspond to Z2 X Z2 orbifolds of
six dimensional tori, plus additional identifications by internal shifts and
twists. The special property of the Z2 X Z2 orbifold is that it may act on the
compactified dimensions as real, rather than complex, dimensions. This property
enables an asymmetric projection on all six internal coordinates, which enables
the projection of all the untwisted Kaehler and complex structure moduli.Comment: 31 pages. Standard LaTeX. Additional section on twisted moduli. typos
corrected. Revised version to appear in NP
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
, 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.Comment: 22 pages. Standard Latex. 4 figures. epsf.te
Phenomenological aspects of M-theory
The Standard Model data suggests the realization of grand unification
structures in nature, in particular that of SO(10). A class of string vacua
that preserve the SO(10) embedding, are the three generation free fermion
heterotic-string models, that are related to Z2xZ2 orbifold compactification.
Attempts to use the M-theory framework to explore further this class of models
are discussed. Wilson line breaking of the SO(10) GUT symmetry results in
super-heavy meta-stable states, which produce several exotic dark matter and
UHECR candidates, with differing phenomenological characteristics. Attempts to
develop the tools to study the properties of these states in forthcoming UHECR
experiments are discussed. It is proposed that quantum mechanics arises from an
equivalence postulate, that may lay the foundations for the rigorous
formulation of quantum gravity.Comment: 25 pages. 6 figures. Standard Latex & iopart.cls. To appear in the
proceedings of Beyond the Desert 02 - Accelerator and non Accelerator
Approaches, Oulu, Finland, 2-7 June 200
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 orbifolds. Thus, the successes of the
semi--realistic free fermionic models, if taken seriously, suggest that the
true string vacuum is a orbifold with nontrivial background
fields and quantized Wilson lines.Comment: 12 pages, phyzzx.tex, tables.te
Gauge Coupling Unification in Superstring Derived Standard--like Models
I discuss gauge coupling unification in a class of superstring standard--like
models, which are derived in the free fermionic formulation. Recent
calculations indicate that the superstring unification scale is at
while the minimal supersymmetric standard model is consistent
with LEP data if the unification scale is at . A generic feature
of the superstring standard--like models is the appearance of extra color
triplets , and electroweak doublets , in
vector--like representations, beyond the minimal supersymmetric standard model.
I show that gauge coupling unification at in the superstring
standard--like models can be consistent with LEP data. I present an explicit
standard--like model that can realize superstring gauge coupling unification.Comment: WIS--92/17/FEB--PH, 12 page
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