426 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
Spinor-Vector Duality in Heterotic String Orbifolds
The three generation heterotic-string models in the free fermionic
formulation are among the most realistic string vacua constructed to date,
which motivated their detailed investigation. The classification of free
fermion heterotic string vacua has revealed a duality under the exchange of
spinor and vector representations of the SO(10) GUT symmetry over the space of
models. We demonstrate the existence of the spinor-vector duality using
orbifold techniques, and elaborate on the relation of these vacua to free
fermionic models.Comment: 20 pages. v2 minor corrections. Version to appear on JHEP. v3
misprints correcte
Investigation of Quasi--Realistic Heterotic String Models with Reduced Higgs Spectrum
Quasi--realistic heterotic-string models in the free fermionic formulation
typically contain an anomalous U(1), which gives rise to a Fayet-Iliopolous
term that breaks supersymmetry at the one--loop level in string perturbation
theory. Supersymmetry is restored by imposing F- and D-flatness on the vacuum.
In Phys. Rev. D 78 (2008) 046009, we presented a three generation free
fermionic standard-like model which did not admit stringent F- and D-flat
directions, and argued that the all the moduli in the model are fixed. The
particular property of the model was the reduction of the untwisted Higgs
spectrum by a combination of symmetric and asymmetric boundary conditions with
respect to the internal fermions associated with the compactified dimensions.
In this paper we extend the analysis of free fermionic models with reduced
Higgs spectrum to the cases in which the SO(10) symmetry is left unbroken, or
is reduced to the flipped SU(5) subgroup. We show that all the models that we
study in this paper do admit stringent flat directions. The only examples of
models that do not admit stringent flat directions remain the strandard-like
models of reference Phys. Rev. D 78 (2008) 046009.Comment: 38 pages, 1 figur
Spectral flow as a map between N=(2,0)-models
The space of models is of particular interest among all
heterotic-string models because it includes the models with the minimal
unification structure, which is well motivated by the Standard Model
of particle physics data. The fermionic
heterotic-string models revealed the existence of a new symmetry in the space
of string configurations under the exchange of spinors and vectors of the
GUT group, dubbed spinor-vector duality. Such symmetries are important
for the understanding of the landscape of string vacua and ultimately for the
possible operation of a dynamical vacuum selection mechanism in string theory.
In this paper we generalize this idea to arbitrary internal rational Conformal
Field Theories (RCFTs). We explain how the spectral flow operator normally
acting within a general theory can be used as a map between
models. We describe the details, give an example and propose more simple
currents that can be used in a similar way.Comment: 14 pages, v2: minor changes, added one referenc
M-Theory Model-Building and Proton Stability
We study the problem of baryon stability in M theory, starting from realistic
four-dimensional string models constructed using the free-fermion formulation
of the weakly-coupled heterotic string. Suitable variants of these models
manifest an enhanced custodial gauge symmetry that forbids to all orders the
appearance of dangerous dimension-five baryon-decay operators. We exhibit the
underlying geometric (bosonic) interpretation of these models, which have a
orbifold structure similar, but not identical, to the class of
Calabi-Yau threefold compactifications of M and F theory investigated by Voisin
and Borcea. A related generalization of their work may provide a solution to
the problem of proton stability in M theory.Comment: 14 pages. Standard Late
On the Possibility of Optical Unification in Heterotic Strings
Recently J. Giedt discussed a mechanism, entitled optical unification,
whereby string scale unification is facilitated via exotic matter with
intermediate scale mass. This mechanism guarantees that a virtual MSSM
unification below the string scale is extrapolated from the running of gauge
couplings upward from M_Z^o when an intermediate scale desert is assumed. In
this letter we explore the possibility of optical unification within the
context of weakly coupled heterotic strings. In particular, we investigate this
for models of free fermionic construction containing the NAHE set of basis
vectors. This class is of particular interest for optical unification, because
it provides a standard hypercharge embedding within SO(10), giving the standard
k_Y = 5/3 hypercharge level, which was shown necessary for optical unification.
We present a NAHE model for which the set of exotic SU(3)_C
triplet/anti-triplet pairs, SU(2)_L doublets, and non-Abelian singlets with
hypercharge offers the possibility of optical unification. Whether this model
can realize optical unification is conditional upon these exotics not receiving
Fayet-Iliopoulos (FI) scale masses when a flat direction of scalar vacuum
expectation values is non-perturbatively chosen to cancel the FI D-term, xi,
generated by the anomalous U(1)-breaking Green-Schwarz-Dine-Seiberg-Wittten
mechanism. A study of perturbative flat directions and their phenomenological
implications for this model is underway.
This paper is a product of the NFS Research Experiences for Undergraduates
and the NSF High School Summer Science Research programs at Baylor University.Comment: 16 pages. Standard Late
A Minimal Superstring Standard Model I: Flat Directions
Three family SU(3)_C x SU(2)_L x U(1)_Y string models in several
constructions generically possess two features: (i) an extra local anomalous
U(1)_A and (ii) numerous (often fractionally charged) exotic particles beyond
those in the minimal supersymmetric model (MSSM). Recently, we demonstrated
that the observable sector effective field theory of such a free fermionic
string model can reduce to that of the MSSM, with the standard observable gauge
group being just SU(3)_C x SU(2)_L x U(1)_Y and the SU(3)_C x SU(2)_L x
U(1)_Y-charged spectrum of the observable sector consisting solely of the MSSM
spectrum. An example of a model with this property was shown. We continue our
investigation of this model by presenting a large set of different flat
directions of the same model that all produce the MSSM spectrum. Our results
suggest that even after imposing the conditions for the decoupling of exotic
states, there may remain sufficient freedom to satisfy the remaining
phenomenological constraints imposed by the observed data.Comment: 64 pages. Latex. Revisions to match version in Int. J. Mod. Physics
Non-Abelian Flat Directions in a Minimal Superstring Standard Model
Recently, by studying exact flat directions of non-Abelian singlet fields, we
demonstrated the existence of free fermionic heterotic-string models in which
the SU(3)_C x SU(2)_L x U(1)_Y-charged matter spectrum, just below the string
scale, consists solely of the MSSM spectrum. In this paper we generalize the
analysis to include VEVs of non-Abelian fields. We find several,
MSSM-producing, exact non-Abelian flat directions, which are the first such
examples in the literature. We examine the possibility that hidden sector
condensates lift the flat directions.Comment: 14 pages. Standard Late
Towards String Predictions
The aim of superstring phenomenology is to develop the tools and methodology
needed to confront string theory with experimental data. The first mandatory
task is to find string solutions which reproduce the observable data. The
subsequent goal is to extract potential signatures beyond the observable data.
Recently, by studying exact flat directions of non-Abelian singlet fields, we
demonstrated the existence of free fermionic heterotic-string models in which
the -charged matter spectrum, just below the
string scale, consists solely of the MSSM spectrum. In this paper we study the
possibility that the exact flat directions leave a symmetry
unbroken at the Planck scale. We demonstrate in a specific example that such
unbroken is in general expected to be not of the GUT type but
of intrinsic stringy origin. We study its phenomenological characteristics and
the consequences in the case that remains unbroken down to
low energies. We suggest that observation in forthcoming colliders of a
, with universal couplings for the two light generations but
different couplings for the heavy generation may provide evidence for the
orbifold which underlies the free fermionic models.Comment: 18 pages. Standard Latex. References adde
Physics Implications of Flat Directions in Free Fermionic Superstring Models II: Renormalization Group Analysis
We continue the investigation of the physics implications of a class of flat
directions for a prototype quasi-realistic free fermionic string model (CHL5),
building upon the results of the previous paper in which the complete mass
spectrum and effective trilinear couplings of the observable sector were
calculated to all orders in the superpotential. We introduce soft supersymmetry
breaking mass parameters into the model, and investigate the gauge symmetry
breaking patterns and the renormalization group analysis for two representative
flat directions, which leave an additional as well as the SM gauge
group unbroken at the string scale. We study symmetry breaking patterns that
lead to a phenomenologically acceptable hierarchy, and for electroweak and intermediate
scale symmetry breaking, respectively, and the associated mass
spectra after electroweak symmetry breaking. The fermion mass spectrum exhibits
unrealistic features, including massless exotic fermions, but has an
interesting -quark hierarchy and associated CKM matrix in one case. There
are (some) non-canonical effective terms, which lead to a non-minimal
Higgs sector with more than two Higgs doublets involved in the symmetry
breaking, and a rich structure of Higgs particles, charginos, and neutralinos,
some of which, however, are massless or ultralight. In the electroweak scale
cases the scale of supersymmetry breaking is set by the mass, with the
sparticle masses in the several TeV range.Comment: 38 pages, 5 figures, LaTex. Minor correction
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