1,759 research outputs found
Statistics on the Heterotic Landscape: Gauge Groups and Cosmological Constants of Four-Dimensional Heterotic Strings
Recent developments in string theory have reinforced the notion that the
space of stable supersymmetric and non-supersymmetric string vacua fills out a
``landscape'' whose features are largely unknown. It is then hoped that
progress in extracting phenomenological predictions from string theory -- such
as correlations between gauge groups, matter representations, potential values
of the cosmological constant, and so forth -- can be achieved through
statistical studies of these vacua. To date, most of the efforts in these
directions have focused on Type I vacua. In this note, we present the first
results of a statistical study of the heterotic landscape, focusing on more
than 10^5 explicit non-supersymmetric tachyon-free heterotic string vacua and
their associated gauge groups and one-loop cosmological constants. Although
this study has several important limitations, we find a number of intriguing
features which may be relevant for the heterotic landscape as a whole. These
features include different probabilities and correlations for different
possible gauge groups as functions of the number of orbifold twists. We also
find a vast degeneracy amongst non-supersymmetric string models, leading to a
severe reduction in the number of realizable values of the cosmological
constant as compared with naive expectations. Finally, we also find strong
correlations between cosmological constants and gauge groups which suggest that
heterotic string models with extremely small cosmological constants are
overwhelmingly more likely to exhibit the Standard-Model gauge group at the
string scale than any of its grand-unified extensions. In all cases, heterotic
worldsheet symmetries such as modular invariance provide important constraints
that do not appear in corresponding studies of Type I vacua.Comment: 58 pages, LaTeX, 17 figures, 3 tables; v2: one new figure and
references adde
Dynamical Dark Matter: II. An Explicit Model
In a recent paper (arXiv:1106.4546), we introduced "dynamical dark matter," a
new framework for dark-matter physics, and outlined its underlying theoretical
principles and phenomenological possibilities. Unlike most traditional
approaches to the dark-matter problem which hypothesize the existence of one or
more stable dark-matter particles, our dynamical dark-matter framework is
characterized by the fact that the requirement of stability is replaced by a
delicate balancing between cosmological abundances and lifetimes across a vast
ensemble of individual dark-matter components. This setup therefore
collectively produces a time-varying cosmological dark-matter abundance, and
the different dark-matter components can interact and decay throughout the
current epoch. While the goal of our previous paper was to introduce the broad
theoretical aspects of this framework, the purpose of the current paper is to
provide an explicit model of dynamical dark matter and demonstrate that this
model satisfies all collider, astrophysical, and cosmological constraints. The
results of this paper therefore constitute an "existence proof" of the
phenomenological viability of our overall dynamical dark-matter framework, and
demonstrate that dynamical dark matter is indeed a viable alternative to the
traditional paradigm of dark-matter physics. Dynamical dark matter must
therefore be considered alongside other approaches to the dark-matter problem,
particularly in scenarios involving large extra dimensions or string theory in
which there exist large numbers of particles which are neutral under
Standard-Model symmetries.Comment: 45 pages, LaTeX, 10 figures. Replaced to match published versio
Bulk Fermion Stars with New Dimensions
Many efforts have been devoted to the studies of the phenomenology in
particle physics with extra dimensions. We propose degenerate fermion stars
with extra dimensions and study what features characterized by the size of
extra dimensions should appear in its structure. We find that Kaluza-Klein
excited modes arise for the larger scale of extra dimensions and examine the
conditions on which different layers should be caused in the inside of the
stars. We expound how the extra dimensions affect on physical quantities.Comment: 20 pages, 14 figure
Invisible Axions and Large-Radius Compactifications
We study some of the novel effects that arise when the QCD axion is placed in
the ``bulk'' of large extra spacetime dimensions. First, we find that the mass
of the axion can become independent of the energy scale associated with the
breaking of the Peccei-Quinn symmetry. This implies that the mass of the axion
can be adjusted independently of its couplings to ordinary matter, thereby
providing a new method of rendering the axion invisible. Second, we discuss the
new phenomenon of laboratory axion oscillations (analogous to neutrino
oscillations), and show that these oscillations cause laboratory axions to
``decohere'' extremely rapidly as a result of Kaluza-Klein mixing. This
decoherence may also be a contributing factor to axion invisibility. Third, we
discuss the role of Kaluza-Klein axions in axion-mediated processes and decays,
and propose several experimental tests of the higher-dimensional nature of the
axion. Finally, we show that under certain circumstances, the presence of an
infinite tower of Kaluza-Klein axion modes can significantly accelerate the
dissipation of the energy associated with cosmological relic axion
oscillations, thereby enabling the Peccei-Quinn symmetry-breaking scale to
exceed the usual four-dimensional relic oscillation bounds. Together, these
ideas therefore provide new ways of obtaining an ``invisible'' axion within the
context of higher-dimensional theories with large-radius compactifications.Comment: 43 pages, LaTeX, 6 figure
Spatial scaling in fracture propagation in dilute systems
The geometry of fracture patterns in a dilute elastic network is explored
using molecular dynamics simulation. The network in two dimensions is subjected
to a uniform strain which drives the fracture to develop by the growth and
coalescence of the vacancy clusters in the network. For strong dilution, it has
been shown earlier that there exists a characteristic time at which a
dynamical transition occurs with a power law divergence (with the exponent )
of the average cluster size. Close to , the growth of the clusters is
scale-invariant in time and satisfies a dynamical scaling law. This paper shows
that the cluster growth near also exhibits spatial scaling in addition to
the temporal scaling. As fracture develops with time, the connectivity length
of the clusters increses and diverges at as , with . As a result of the scale-invariant
growth, the vacancy clusters attain a fractal structure at with an
effective dimensionality . These values are independent
(within the limit of statistical error) of the concentration (provided it is
sufficiently high) with which the network is diluted to begin with. Moreover,
the values are very different from the corresponding values in qualitatively
similar phenomena suggesting a different universality class of the problem. The
values of and supports the scaling relation with the
value of obtained before.Comment: A single ps file (6 figures included), 12 pages, to appear in Physica
On Effective Theory of Brane World with Small Tension
The five dimensional theory compactified on with two ``branes'' (two
domain walls) embedded in it is constructed, based on the field-theoretic
mechanism to generate the ``brane''. Some light states localized in the
``brane'' appear in the theory. One is the Nambu-Goldstone boson, which
corresponds to the breaking of the translational invariance in the transverse
direction of the ``brane''. In addition, if the tension of the ``brane'' is
smaller than the fundamental scale of the original theory, it is found that
there may exist not only massless states but also some massive states lighter
than the fundamental scale in the ``brane''. We analyze the four dimensional
effective theory by integrating out the freedom of the fifth dimension. We show
that some effective couplings can be explicitly calculated. As one of our
results, some effective couplings of the state localized in the ``brane'' to
the higher Kaluza-Klein modes in the bulk are found to be suppressed by the
width of the ``brane''. The resultant suppression factor can be quantitatively
different from the one analyzed by Bando et al. using the Nambu-Goto action,
while they are qualitatively the same.Comment: 17 pages, uses REVTEX macr
Can extra dimensions accessible to the SM explain the recent measurement of anomalous magnetic moment of the muon?
We investigate whether models with flat extra dimensions in which SM fields
propagate can give a significant contribution to the anomalous magnetic moment
of the muon (MMM). In models with only SM gauge and Higgs fields in the bulk,
the contribution to the MMM from Kaluza-Klein (KK) excitations of gauge bosons
is very small. This is due to the constraint on the size of the extra
dimensions from tree-level effects of KK excitations of gauge bosons on
precision electroweak observables such as Fermi constant. If the quarks and
leptons are also allowed to propagate in the (same) bulk (``universal'' extra
dimensions), then there are no contributions to precision electroweak
observables at tree-level. However, in this case, the constraint from one-loop
contribution of KK excitations of (mainly) the top quark to T parameter again
implies that the contribution to the MMM is small. We show that in models with
leptons, electroweak gauge and Higgs fields propagating in the (same) bulk, but
with quarks and gluon propagating in a sub-space of this bulk, both the above
constraints can be relaxed. However, with only one Higgs doublet, the
constraint from the process b -> s gamma requires the contribution to the MMM
to be smaller than the SM electroweak correction. This constraint can be
relaxed in models with more than one Higgs doublet.Comment: Latex, 11 pages, 1 ps fig. included. In the revised version, a
reference has been added. Version to be published in Phys. Lett.
Stabilization of Sub-Millimeter Dimensions: The New Guise of the Hierarchy Problem
A new framework for solving the hierarchy problem was recently proposed which
does not rely on low energy supersymmetry or technicolor. The fundamental
Planck mass is at a \tev and the observed weakness of gravity at long
distances is due the existence of new sub-millimeter spatial dimensions. In
this picture the standard model fields are localized to a -dimensional
wall or ``3-brane''. The hierarchy problem becomes isomorphic to the problem of
the largeness of the extra dimensions. This is in turn inextricably linked to
the cosmological constant problem, suggesting the possibility of a common
solution. The radii of the extra dimensions must be prevented from both
expanding to too great a size, and collapsing to the fundamental Planck length
\tev^{-1}. In this paper we propose a number of mechanisms addressing this
question. We argue that a positive bulk cosmological constant can
stabilize the internal manifold against expansion, and that the value of
is not unstable to radiative corrections provided that the
supersymmetries of string theory are broken by dynamics on our 3-brane. We
further argue that the extra dimensions can be stabilized against collapse in a
phenomenologically successful way by either of two methods: 1) Large,
topologically conserved quantum numbers associated with higher-form bulk U(1)
gauge fields, such as the naturally occurring Ramond-Ramond gauge fields, or
the winding number of bulk scalar fields. 2) The brane-lattice-crystallization
of a large number of 3-branes in the bulk. These mechanisms are consistent with
theoretical, laboratory, and cosmological considerations such as the absence of
large time variations in Newton's constant during and after primordial
nucleosynthesis, and millimeter-scale tests of gravity.Comment: Corrected referencing to important earlier work by Sundrum, errors
fixed, additional discussion on radion phenomenology, conclusions unchanged,
23 pages, LaTe
Search for solar Kaluza-Klein axions in theories of low-scale quantum gravity
We explore the physics potential of a terrestrial detector for observing
axionic Kaluza-Klein excitations coming from the Sun within the context of
higher-dimensional theories of low-scale quantum gravity. In these theories,
the heavier Kaluza-Klein axions are relatively short-lived and may be detected
by a coincidental triggering of their two-photon decay mode. Because of the
expected high multiplicity of the solar axionic excitations, we find
experimental sensitivity to a fundamental Peccei-Quinn axion mass up to
eV (corresponding to an effective axion-photon coupling GeV) in theories with 2 extra
dimensions and a fundamental quantum-gravity scale of order 100
TeV, and up to eV (corresponding to GeV) in theories with 3 extra dimensions and
TeV. For comparison, based on recent data obtained from lowest
level underground experiments, we derive the experimental limits: GeV and GeV in the
aforementioned theories with 2 and 3 large compact dimensions, respectively.Comment: 19 pages, extended version, as to appear in Physical Review
Neutrino Masses from Large Extra Dimensions
Recently it was proposed that the standard model (SM) degrees of freedom
reside on a -dimensional wall or ``3-brane'' embedded in a
higher-dimensional spacetime. Furthermore, in this picture it is possible for
the fundamental Planck mass \mst to be as small as the weak scale \mst\simeq
O(\tev) and the observed weakness of gravity at long distances is due the
existence of new sub-millimeter spatial dimensions. We show that in this
picture it is natural to expect neutrino masses to occur in the 10^{-1} -
10^{-4}\ev range, despite the lack of any fundamental scale higher than
\mst. Such suppressed neutrino masses are not the result of a see-saw, but
have intrinsically higher-dimensional explanations. We explore two
possibilities. The first mechanism identifies any massless bulk fermions as
right-handed neutrinos. These give naturally small Dirac masses for the same
reason that gravity is weak at long distances in this framework. The second
mechanism takes advantage of the large {\it infrared} desert: the space in the
extra dimensions. Here, small Majorana neutrino masses are generated by
breaking lepton number on distant branes.Comment: 17 pages, late
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