726 research outputs found
Initial Systematic Investigations of the Landscape of Low Layer NAHE Extensions
The discovery that the number of physically consistent string vacua is on the
order of 10^500 has prompted several statistical studies of string
phenomenology. Contained here is one such study that focuses on the Weakly
Coupled Free Fermionic Heterotic String (WCFFHS) formalism. Presented are
systematic extensions of the well-known NAHE (Nanopoulos, Antoniadis, Hagelin,
Ellis) set of basis vectors, which have been shown to produce
phenomenologically realistic models. Statistics related to the number of
U(1)'s, gauge group factors, non-Abelian singlets, ST SUSYs, as well as the
gauge groups themselve are discussed for the full range of models produced as
well as models containing GUT groups only. Prior results of other large-scale
investigations are compared with these regarding the aforementioned quantities.
Statistical coupling between the gauge groups and the number of ST SUSYs is
also discussed, and it was found that for order-3 extensions there are more
models with enhanced ST SUSY when there is an exceptional group present. Also
discussed are some three-generation GUT models found in the data sets. These
models are unique because they come from basis vectors which still have a
geometric interpretation -- there are no "rank-cuts" in these models.Comment: 65 Pages, 31 Tables, 31 Figure
Free Fermionic Heterotic Model Building and Root Systems
We consider an alternative derivation of the GSO Projection in the free
fermionic construction of the weakly coupled heterotic string in terms of root
systems, as well as the interpretation of the GSO Projection in this picture.
We then present an algorithm to systematically and efficiently generate input
sets (i.e. basis vectors) in order to study Landscape statistics with minimal
computational cost. For example, the improvement at order 6 is approximately
10^{-13} over a traditional brute force approach, and improvement increases
with order. We then consider an example of statistics on a relatively simple
class of models.Comment: Standard Latex, 12 page
Phenomenology of A Three-Family Standard-like String Model
We discuss the phenomenology of a three-family supersymmetric Standard-like
Model derived from the orientifold construction, in which the ordinary chiral
states are localized at the intersection of branes at angles. In addition to
the Standard Model group, there are two additional U(1)' symmetries, one of
which has family non-universal and therefore flavor changing couplings, and a
quasi-hidden non-abelian sector which becomes strongly coupled above the
electroweak scale. The perturbative spectrum contains a fourth family of exotic
(SU(2)- singlet) quarks and leptons, in which, however, the left-chiral states
have unphysical electric charges. It is argued that these decouple from the low
energy spectrum due to hidden sector charge confinement, and that anomaly
matching requires the physical left-chiral states to be composites. The model
has multiple Higgs doublets and additional exotic states. The moduli-dependent
predictions for the gauge couplings are discussed. The strong coupling agrees
with experiment for reasonable moduli, but the electroweak couplings are too
small.Comment: 22 pages, 4 figure
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
Left-Right Symmetric Heterotic-String Derived Models
Recently it was demonstrated that free fermionic heterotic-strings can
produce models with solely the Minimal Supersymmetric Standard Model states in
the low energy spectrum. This unprecedented result provides further strong
evidence for the possibility that the true string vacuum shares some of the
properties of the free fermionic models. Past free fermionic models have
focused on several possible unbroken observable SO(10) subgroups at the string
scale, which include the flipped SU(5) (FSU5), the Pati-Salam (PS) string
models, and the string Standard-like Models (SLM). We extend this study to
include the case in which the SO(10) symmetry is broken to the Left-Right
Symmetric (LRS) gauge group, SO(10) -> SU(3)_C X U(1)_{B-L} X SU(2)_L X
SU(2)_R. We present several models of this type and discuss their
phenomenological features. The most striking new outcome of the LRS string
models, in contrast to the case of the FSU5, the PS, and the SLM string models,
is that they can produce effective field theories that are free of Abelian
anomalies. We discuss the distinction between the two types of free fermionic
models which result in the presence, or absence, of an anomalous U(1). As a
counter example we also present a LRS model that does contain an anomalous
U(1). Additionally, we discuss how in string models the Standard Model spectrum
may arise from the three \mbf 16 representations of SO(10), while the
weak-hypercharge does not have the canonical SO(10) embedding.Comment: 39 pages. Standard Latex. Version to appear in PR
Stringent Phenomenological Investigation into Heterotic String Optical Unification
For the weakly coupled heterotic string (WCHS) there is a well-known factor
of twenty conflict between the minimum string coupling unification scale,
Lambda_H ~5x10^(17) GeV, and the projected MSSM unification scale, Lambda_U ~
2.5x10^(16) GeV, assuming an intermediate scale desert (ISD). Renormalization
effects of intermediate scale MSSM-charged exotics (ISME) (endemic to
quasi-realistic string models) can resolve this issue, pushing the MSSM scale
up to the string scale. However, for a generic string model, this implies that
the projected Lambda_U unification under ISD is accidental. If the true
unification scale is 5.0x10^(17) GeV, is it possible that illusionary
unification at 2.5x10^(17) GeV in the ISD scenario is not accidental? If it is
not, then under what conditions would the assumption of ISME in a WCHS model
imply apparent unification at Lambda_U when ISD is falsely assumed? Geidt's
"optical unification" suggests that Lambda_U is not accidental, by offering a
mechanism whereby a generic MSSM scale Lambda_U < Lambda_H is guaranteed. A
WCHS model was constructed that offers the possibility of optical unification,
depending on the availability of anomaly-cancelling flat directions meeting
certain requirements. This paper reports on the systematic investigation of the
optical unification properties of the set of stringent flat directions of this
model. Stringent flat directions can be guaranteed to be F-flat to all finite
order (or to at least a given finite order consistent with electroweak scale
supersymmetry breaking) and can be viewed as the likely roots of more general
flat directions. Analysis of the phenomenology of stringent flat directions
gives an indication of the remaining optical unification phenomenology that
must be garnered by flat directions developed from them.Comment: standard latex, 18 pages of tex
Massive Neutrinos and (Heterotic) String Theory
String theories in principle address the origin and values of the quark and
lepton masses. Perhaps the small values of neutrino masses could be explained
generically in string theory even if it is more difficult to calculate
individual values, or perhaps some string constructions could be favored by
generating small neutrino masses. We examine this issue in the context of the
well-known three-family standard-like Z_3 heterotic orbifolds, where the theory
is well enough known to construct the corresponding operators allowed by string
selection rules, and analyze the D- and F-flatness conditions. Surprisingly, we
find that a simple see-saw mechanism does not arise. It is not clear whether
this is a property of this construction, or of orbifolds more generally, or of
string theory itself. Extended see-saw mechanisms may be allowed; more analysis
will be needed to settle that issue. We briefly speculate on their form if
allowed and on the possibility of alternatives, such as small Dirac masses and
triplet see-saws. The smallness of neutrino masses may be a powerful probe of
string constructions in general. We also find further evidence that there are
only 20 inequivalent models in this class, which affects the counting of string
vacua.Comment: 18 pages in RevTeX format. Single-column postscript version available
at http://sage.hep.upenn.edu/~bnelson/singpre.p
- …