1,514 research outputs found
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
Are There Oscillations in the Baryon/Meson Ratio?
All available data indicate a surplus of baryon states over meson states for
energies greater than about 1.5 GeV. Since hadron-scale string theory suggests
that their numbers should become equal with increasing energy, it has recently
been proposed that there must exist exotic mesons with masses just above 1.7
GeV in order to fill the deficit. We demonstrate that a string-like picture is
actually consistent with the present numbers of baryon and meson states, and in
fact predicts regular oscillations in their ratio. This suggests a different
role for new hadronic states.Comment: 14 pages (RevTeX), McGill/92-0
Ultraviolet dependence of Kaluza-Klein effects on electroweak observables
In extensions of the standard model (SM) with d extra dimensions at the TeV
scale the virtual exchange of Kaluza-Klein (KK) excitations of the gauge bosons
gives contributions that change the SM relations between electroweak
observables. These corrections are finite only for d=1; for d\ge 2 the infinite
tower of KK modes gives a divergent contribution that has to be regularized
introducing a cutoff (the string scale). However, the ultraviolet dependence of
the KK effects is completely different if the running of the couplings with the
scale is taken into account. We find that for larger d the number of
excitations at each KK level increases, but their larger number is compensated
by the smaller value of the gauge coupling at that scale. As a result, for any
number of extra dimensions the exchange of the complete KK tower always gives a
finite contribution. We show that (i) for d=1 the running of the gauge coupling
decreases an 14% the effect of the KK modes on electroweak observables; (ii) in
all cases more than 90% of the total effect comes from the excitations in the
seven lowest KK levels and is then independent of ultraviolet physics.Comment: 8 pages, to appear in Phys. Rev.
Fermion masses and quantum numbers from extra dimensions
We study the localization of fermions on a brane embedded in a space-time
with geometry. Quantum numbers of localized fermions are
associated with their rotation momenta around the brane. Fermions with
different quantum numbers have different higher-dimensional profiles. Fermion
masses and mixings, which are proportional to the overlap of higher-dimensional
profiles of the fermions, depend on the fermion quantum numbers.Comment: 14 page
Phenomenology of Noncommutative Field Theories
Experimental limits on the violation of four-dimensional Lorentz invariance
imply that noncommutativity among ordinary spacetime dimensions must be small.
In this talk, I review the most stringent bounds on noncommutative field
theories and suggest a possible means of evading them: noncommutativity may be
restricted to extra, compactified spatial dimensions. Such theories have a
number of interesting features, including Abelian gauge fields whose
Kaluza-Klein excitations have self couplings. We consider six-dimensional QED
in a noncommutative bulk, and discuss the collider signatures of the model.Comment: 7 pages RevTeX, 4 eps figures, Invited plenary talk, IX Mexican
Workshop on Particles and Fields, November 17-22, 2003, Universidad de
Colima, Mexic
Relaxing Cosmological Constraints on Large Extra Dimensions
We reconsider cosmological constraints on extra dimension theories from the
excess production of Kaluza-Klein gravitons. We point out that, if the normalcy
temperature is above 1 GeV, then graviton states produced at this temperature
will decay early enough that they do not affect the present day dark matter
density, or the diffuse gamma ray background. We rederive the relevant
cosmological constraints for this scenario.Comment: 17 pages, latex, revtex4; added a short discussion of other
constraints, reference
Central Charge Reduction and Spacetime Statistics in the Fractional Superstring
Fractional superstrings in the tensor-product formulation experience
``internal projections'' which reduce their effective central charges. Simple
expressions for the characters of the resulting effective worldsheet theory are
found. All states in the effective theory can be consistently assigned definite
spacetime statistics. The projection to the effective theory is shown to be
described by the action of a dimension-three current in the original
tensor-product theory.Comment: 11 pages (LaTeX), CLNS 92/1168, McGill/92-41 (minor typos corrected
Homogeneity, Flatness and "Large" Extra Dimensions
We consider a model in which the universe is the direct product of a
(3+1)-dimensional Friedmann, Robertson-Walker (FRW) space and a compact
hyperbolic manifold (CHM). Standard Model fields are confined to a point in the
CHM (i.e. to a brane). In such a space, the decay of massive Kaluza-Klein modes
leads to the injection of any initial bulk entropy into the observable (FRW)
universe. Both Kolmogoro-Sinai mixing due to the non-integrability of flows on
CHMs and the large statistical averaging inherent in the collapse of the
initial entropy onto the brane smooth out any initial inhomogeneities in the
distribution of matter and of 3-curvature on any slice of constant 3-position.
If, as we assume, the initial densities and curvatures in each fundamental
correlation volume are drawn from some universal underlying distributions
independent of location within the space, then these smoothing mechanisms
effectively reduce the density and curvature inhomogeneities projected onto the
FRW. This smoothing is sufficient to account for the current homogeneity and
flatness of the universe. The fundamental scale of physics can be \gsim 1TeV.
All relevant mass and length scales can have natural values in fundamental
units. All large dimensionless numbers, such as the entropy of the universe,
are understood as consequences of the topology of spacetime which is not
explained. No model for the origin of structure is proffered.Comment: minor changes, matches version published in Phys. Rev. Let
Compact Hyperbolic Extra Dimensions: Branes, Kaluza-Klein Modes and Cosmology
We reconsider theories with low gravitational (or string) scale M_* where
Newton's constant is generated via new large-volume spatial dimensions, while
Standard Model states are localized to a 3-brane. Utilizing compact hyperbolic
manifolds (CHM's) we show that the spectrum of Kaluza-Klein (KK) modes is
radically altered. This allows an early universe cosmology with normal
evolution up to substantial temperatures, and completely negates the
constraints on M_* arising from astrophysics. Furthermore, an exponential
hierarchy between the usual Planck scale and the true fundamental scale of
physics can emerge with only order unity coefficients. The linear size of the
internal space remains small. The proposal has striking testable signatures.Comment: 4 pages, no figure
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