929 research outputs found
A Classical Solution in Six-dimensional Gauge Theory with Higher Derivative Coupling
We show that the spin connection of the standard metric on a six-dimensional
sphere gives an exact solution to the generalized self-dual equations suggested
by Tchrakian some years ago. We work on an SO(6) gauge theory with a
higher-derivative coupling term. The model consists of vector fields only. The
pseudo-energy is bound from below by a topological charge which is proportional
to the winding number of spatial S^5 around the internal space SO(6). The fifth
homotopy group of SO(6) is, indeed, Z. The coupling constant of higher
derivative term is quadratic in the radius of the underlying space S^6.Comment: 7 pages, comments and a reference added, typos correcte
Boosted Top Quark Signals for Heavy Vector Boson Excitations in a Universal Extra Dimension Model
In view of the fact that the Kaluza-Klein (KK) modes in a model with
a Universal Extra Dimension (UED), could mimic supersymmetry signatures at the
LHC, it is necessary to look for the KK modes, which have no analogues
in supersymmetry. We discuss the possibility of searching for heavy
vector boson resonances -- especially the -- through their decays to a
highly-boosted top quark-antiquark pair using recently-developed top-jet
tagging techniques in the hadronic channel. It is shown that signals
from the gluon resonance are as efficient a discovery mode at the LHC
as dilepton channels from the and resonances.Comment: 22 pages, 8 embedded figure
Particle Physics Probes Of Extra Spacetime Dimensions
The possibility that spacetime is extended beyond the familiar 3+1-dimensions
has intrigued physicists for a century. Indeed, the consequences of a
dimensionally richer spacetime would be profound. Recently, new theories with
higher dimensional spacetimes have been developed to resolve the hierarchy
problem in particle physics. These scenarios make distinct predictions which
allow for experiment to probe the existence of extra dimensions in new ways. We
review the conceptual framework of these scenarios, their implications in
collider and short-range gravity experiments, their astrophysical and
cosmological effects, as well as the constraints placed on these models from
present data.Comment: Submitted to Annual Review of Nuclear and Particle Science, 29 page
Asymptotically cylindrical 7-manifolds of holonomy G_2 with applications to compact irreducible G_2-manifolds
We construct examples of exponentially asymptotically cylindrical Riemannian
7-manifolds with holonomy group equal to G_2. To our knowledge, these are the
first such examples. We also obtain exponentially asymptotically cylindrical
coassociative calibrated submanifolds. Finally, we apply our results to show
that one of the compact G_2-manifolds constructed by Joyce by desingularisation
of a flat orbifold T^7/\Gamma can be deformed to one of the compact
G_2-manifolds obtainable as a generalized connected sum of two exponentially
asymptotically cylindrical SU(3)-manifolds via the method given by the first
author (math.DG/0012189).Comment: 36 pages; v2: corrected trivial typos; v3: some arguments corrected
and improved; v4: a number of improvements on presentation, paritularly in
sections 4 and 6, including an added picture
Target detection in insects: optical, neural and behavioral optimizations.
This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.conb.2016.09.001Motion vision provides important cues for many tasks. Flying insects, for example, may pursue small, fast moving targets for mating or feeding purposes, even when these are detected against self-generated optic flow. Since insects are small, with size-constrained eyes and brains, they have evolved to optimize their optical, neural and behavioral target visualization solutions. Indeed, even if evolutionarily distant insects display different pursuit strategies, target neuron physiology is strikingly similar. Furthermore, the coarse spatial resolution of the insect compound eye might actually be beneficial when it comes to detection of moving targets. In conclusion, tiny insects show higher than expected performance in target visualization tasks.Air Force Office of Scientific Research (Grant ID: FA9550-15-1-0188
Current driven magnetization dynamics in helical spin density waves
A mechanism is proposed for manipulating the magnetic state of a helical spin
density wave using a current. In this paper, we show that a current through a
bulk system with a helical spin density wave induces a spin transfer torque,
giving rise to a rotation of the order parameter.The use of spin transfer
torque to manipulate the magnetization in bulk systems does not suffer from the
obstacles seen for magnetization reversal using interface spin transfer torque
in multilayered systems. We demonstrate the effect by a quantitative
calculation of the current induced magnetization dynamics of Erbium. Finally we
propose a setup for experimental verification.Comment: In the previous version of this paper was a small numerical mistake
made when evaluating equation 3 and 9. The number of digits given in the
calculation of the torque current tensor is reduced to better represent the
accuracy of the calculation. A slightly modified paper have been published in
Phys. Rev. Lett. 96, 256601 (2006) 4 pages 3 figure
Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo*; A new classification scheme based on Sr and Ba
Carbon-enhanced metal-poor (CEMP) stars span a wide range of stellar
populations, from bona fide second-generation stars to later forming stars that
provide excellent probes of, e.g., binary mass transfer. Here we analyse 11
metal-poor stars of which 10 are CEMP stars. Based on high signal-to-noise
(SNR) X-Shooter spectra, we derive abundances of 20 elements (C, N, O, Na, Mg,
Ca, Sc, Ti, Cr, Mn, Fe, Ni, Sr, Y, Ba, La, Ce, Pr, Nd, Eu). From the high SNR
spectra, we trace the chemical contribution of the rare earth elements (REE)
from various production sites, finding a preference for metal-poor low-mass AGB
stars of 1.5Mo in CEMP-s stars, while CEMP-r/s stars may indicate a more
massive AGB contribution (2-5Mo). A contribution from the r-process - possibly
from neutron star mergers (NSM), is also detectable in the REE abundances,
especially in the CEMP-r/s. Combining spectra with Gaia DR2 astrometric data
indicates that all but one star in our sample (and most literature stars)
belong to the Galactic halo. They exhibit a median orbital eccentricity of 0.7,
and are found on both pro- and retrograde orbits. The orbital parameters of
CEMP-no and CEMP4s stars are remarkably similar in the 98 stars we study. A
special CEMP-no star, with very low Sr and Ba content, possesses the most
eccentric orbit among the stars in our sample, passing close to the Galactic
centre. Finally, we propose an improved scheme to sub-classify the CEMP stars,
making use of the SrBa ratio, which can also be used to separate very
metal-poor stars from CEMP stars in 93 stars in the metallicity range
[Fe/H]. The Sr/Ba ratio can also be used for distinguishing
CEMP-s,-r/s and -no stars. The Sr/Ba ratio is also a powerful astro-nuclear
indicator, as AGB stars exhibit very different Sr/Ba ratios, compared to fast
rotating massive stars and NSM, and it is fairly unbiased by NLTE and 3D
corrections.(abridged)Comment: 15 pages, 4 pages appendix, 11 figures, accepted for publication in
A&
Universal properties of Fermi gases in arbitrary dimensions
We consider spin-1/2 Fermi gases in arbitrary, integer or non-integer spatial
dimensions, interacting via a Dirac delta potential. We first generalize the
method of Tan's distributions and implement short-range boundary conditions to
arbitrary dimension and we obtain a set of universal relations for the Fermi
gas. Three-dimensional scattering under very general conditions of transversal
confinement is described by an effectively reduced-dimensional scattering
length, which we show depends on the three-dimensional scattering length in a
universal way. Our formula for non-integer dimensions interpolates between the
known results in integer dimensions 1, 2 and 3. Without any need to solve the
associated multichannel scattering problem, we find that confinement-induced
resonances occur in all dimensions different from D=2, while
reduced-dimensional contacts, related to the tails of the momentum
distributions, are connected to the three-dimensional contact by a correction
factor of purely geometric origin.Comment: 6 pages, 0 figure
Inducing charges and currents from extra dimensions
In a particular variant of Kaluza-Klein theory, the so-called induced-matter
theory (IMT), it is shown that any configuration of matter may be geometrically
induced from a five-dimensional vacuum space. By using a similar approach we
show that any distribution of charges and currents may also be induced from a
five-dimensional vacuum space. Whereas in the case of IMT the geometry is
Riemannian and the fundamental equations are the five-dimensional Einstein
equations in vacuum, here we consider a Minkowskian geometry and the
five-dimensional Maxwell equations in vacuum.Comment: 8 pages. Accepted for publication in Modern Physics Letters
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