1,362 research outputs found
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
Entropy-driven formation of the gyroid cubic phase
We show, by computer simulation, that tapered or pear-shaped particles, interacting through purely repulsive interactions, can freely self-assemble to form the three-dimensionally periodic, gyroid cubic phase. The Ia3d gyroid cubic phase is formed by these particles both on compression of an isotropic configuration and on expansion of a smectic A bilayer arrangement. For the latter case, it is possible identify the steps by which the topological transformation from non-intersecting planes to fully interpenetrating, periodic networks takes place</p
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
Sneutrino Dark Matter: Symmetry Protection and Cosmic Ray Anomalies
We present an R-parity conserving model of sneutrino dark matter within a
Higgs-philic U(1)' extension of the minimal supersymmetric standard model. In
this theory, the mu parameter and light Dirac neutrino masses are generated
naturally upon the breaking of the U(1)' gauge symmetry. The leptonic and
hadronic decays of sneutrinos in this model, taken to be the lightest and
next-to-lightest superpartners, allow for a natural fit to the recent results
reported by the PAMELA experiment.Comment: Revised to match the published version; 11 pages (2 column format), 1
table, 6 figures, to appear in PR
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
Nematic-Isotropic Transition with Quenched Disorder
Nematic elastomers do not show the discontinuous, first-order, phase
transition that the Landau-De Gennes mean field theory predicts for a
quadrupolar ordering in 3D. We attribute this behavior to the presence of
network crosslinks, which act as sources of quenched orientational disorder. We
show that the addition of weak random anisotropy results in a singular
renormalization of the Landau-De Gennes expression, adding an energy term
proportional to the inverse quartic power of order parameter Q. This reduces
the first-order discontinuity in Q. For sufficiently high disorder strength the
jump disappears altogether and the phase transition becomes continuous, in some
ways resembling the supercritical transitions in external field.Comment: 12 pages, 4 figures, to be published on PR
Understanding process, power, and meaning in adaptive governance: A critical institutional reading
Adaptive governance continues to attract considerable interest in academic and policy circles. This is with good reason, given its increasing relevance in a globalized and changing world. At the same time, adaptive governance is the subject of a growing body of critical literature concerned with the ways in which it theorizes the social world. In this paper, we respond to these critiques, which we see as broadly concerning the process, power, and meaning dimensions of environmental and natural resource governance. We argue that adaptive governance theory would benefit from engaging constructively with critical institutionalism, a school of thought that, like adaptive governance, has one foot in commons scholarship. Critical institutionalism conceives of institutional change as a process of bricolage, where those involved piece together new arrangements from the resources to hand. This approach highlights the interplay of structure and agency, and illuminates how new governance arrangements form and come to be seen as natural in dynamic relation to the wider social and cultural landscape. We consider how these arrangements tend to reflect dominant power relations, whilst the plural nature of social life also provides scope for adaptation and transformative change
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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