8,524 research outputs found
Closed N=2 Strings: Picture-Changing, Hidden Symmetries and SDG Hierarchy
We study the action of picture-changing and spectral flow operators on a
ground ring of ghost number zero operators in the chiral BRST cohomology of the
closed N=2 string and describe an infinite set of symmetry charges acting on
physical states. The transformations of physical string states are compared
with symmetries of self-dual gravity which is the effective field theory of the
closed N=2 string. We derive all infinitesimal symmetries of the self-dual
gravity equations in 2+2 dimensional spacetime and introduce an infinite
hierarchy of commuting flows on the moduli space of self-dual metrics. The
dependence on moduli parameters can be recovered by solving the equations of
the SDG hierarchy associated with an infinite set of abelian symmetries
generated recursively from translations. These non-local abelian symmetries are
shown to coincide with the hidden abelian string symmetries responsible for the
vanishing of most scattering amplitudes. Therefore, N=2 string theory
"predicts" not only self-dual gravity but also the SDG hierarchy.Comment: 41 pages, no figure
Nonlinear-optical frequency-doubling meta-reflector: pulsed regime
The properties of backward-wave second harmonic meta-reflector operating in
pulse regime are investigated. We show that ratio of the income pulse length to
the thickness of the metaslab determines its important operational properties
which are in contrast with those manifested at common second harmonic
generation settings.Comment: 6 pages, 3 figure
Second harmonic generation and pulse shaping in positively and negatively spatially dispersive nanowaveguides: comparative analysis
Comparative analysis of second harmonic generation in ordinary and
backward-wave settings is presented. Extraordinary properties of frequency
doubling nonlinear optical reflectivity and pulse shaping through phase
matching of ordinary and backward electromagnetic waves in the nanowaveguides
with mixed negative/positive spatial dispersion is demonstrated with numerical
simulations.Comment: 10 pages, 5 figure
Spin-axion coupling
We establish a new covariant phenomenological model, which describes an
influence of pseudoscalar (axion) field on spins of test massive particles. The
model includes general relativistic equations of particle motion and spin
evolution in background pseudoscalar (axion), electromagnetic and gravitational
fields. It describes both the direct spin-axion coupling of the gradient type
and indirect spin-axion interaction mediated by electromagnetic fields. Special
attention is paid to the direct spin-axion coupling caused by the gradient of
the pseudoscalar (axion) field. We show that it describes a spin precession,
when the pseudoscalar (axion) field is inhomogeneous and/or non-stationary.
Applications of the model, which correspond to the three types of four-vectors
attributed to the gradient of the pseudoscalar (axion) field (time-like,
space-like, and null), are considered in detail. These are the spin precessions
induced by relic cosmological axions, axions distributed around spherically
symmetric static objects, and axions in a gravitational wave field,
respectively. We discuss features of the obtained exact solutions and some
general properties of the axionically induced spin rotation.Comment: 15 pages; replaced with the version accepted for publication in Phys.
Rev. D; new Subsection IVB and 13 references are adde
Chern-Simons flows on Aloff-Wallach spaces and Spin(7)-instantons
Due to their explicit construction, Aloff-Wallach spaces are prominent in
flux compactifications. They carry G_2-structures and admit the G_2-instanton
equations, which are natural BPS equations for Yang-Mills instantons on
seven-manifolds and extremize a Chern-Simons-type functional. We consider the
Chern-Simons flow between different G_2-instantons on Aloff-Wallach spaces,
which is equivalent to Spin(7)-instantons on a cylinder over them. For a
general SU(3)-equivariant gauge connection, the generalized instanton equations
turn into gradient-flow equations on C^3 x R^2, with a particular cubic
superpotential. For the simplest member of the Aloff-Wallach family (with
3-Sasakian structure) we present an explicit instanton solution of tanh-like
shape.Comment: 1+17 pages, 1 figur
Solutions to Yang-Mills equations on four-dimensional de Sitter space
We consider pure SU(2) Yang-Mills theory on four-dimensional de Sitter space
dS and construct a smooth and spatially homogeneous magnetic solution to
the Yang-Mills equations. Slicing dS as , via an
SU(2)-equivariant ansatz we reduce the Yang-Mills equations to ordinary matrix
differential equations and further to Newtonian dynamics in a double-well
potential. Its local maximum yields a Yang-Mills solution whose color-magnetic
field at time is given by , where for are the SU(2) generators and
is the de Sitter radius. At any moment, this spatially homogeneous
configuration has finite energy, but its action is also finite and of the value
in a spin- representation. Similarly, the
double-well bounce produces a family of homogeneous finite-action
electric-magnetic solutions with the same energy. There is a continuum of other
solutions whose energy and action extend down to zero.Comment: 1+7 pages; v2: introduction extended, gauge group representation
dependence added, minor clarifications, 3 more references; v3: title change,
published versio
Astrokit -- an Efficient Program for High-Precision Differential CCD Photometry and Search for Variable Stars
Having a need to perform differential photometry for tens of thousands stars
in a several square degrees field, we developed Astrokit program. The software
corrects the star brightness variations caused by variations of atmospheric
transparency: to this end, the program selects for each star an individual
ensemble of reference stars having similar magnitudes and positions in the
frame. With ten or more reference stars in the ensemble, the differences
between their spectral types and the spectral type of the object studied become
unimportant. Astrokit searches for variable stars using Robust Median
Statistics criterion, which allows candidate variables to be selected more
efficiently than by analyzing the standard deviation of star magnitudes. The
software allows very precise automatic analysis of long inhomogeneous sets of
photometric observations of a large number of objects to be performed, making
it possible to find "hot Jupiter" type exoplanet transits and low-amplitude
variables. We describe the algorithm of the program and the results of its
application to reduce the data of the photometric sky survey in Cygnus as well
as observations of the open cluster NGC188 and the transit of the exoplanet
WASP-11 b / HAT-P-10 b, performed with the MASTER-II-URAL telescope of the
Kourovka Astronomical Observatory of the Ural Federal University.Comment: to be published in Astrophysical Bulletin, Vol. 69, No.
Mirrorless Negative-index Parametric Micro-oscillator
The feasibility and extraordinary properties of mirrorless parametric
oscillations in strongly absorbing negative-index metamaterials are shown. They
stem from the backwardness of electromagnetic waves inherent to this type of
metamaterials.Comment: 4 pages, 2 figure
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