3,308,741 research outputs found
Non-abelian Eikonals
A functional formulation and partial solution is given of the non-abelian
eikonal problem associated with the exchange of non-interacting, charged or
colored bosons between a pair of fermions, in the large /small limit. A
simple, functional ``contiguity" prescription is devised for extracting those
terms which exponentiate, and appear to generate the leading, high-energy
behavior of each perturbative order of this simplest non-abelian eikonal
function; the lowest non-trivial order agrees with the corresponding SU(N)
perturbative amplitude, while higher-order contributions to this eikonal
generate an ``effective Reggeization" of the exchanged bosons, resembling
previous results for the perturbative amplitude. One exact and several
approximate examples are given, including an application to self-energy
radiative corrections. In particular, for this class of graphs and to all
orders in the coupling, we calculate the leading-log eikonal for SU(2). Based
on this result, we conjecture the form of the eikonal scattering amplitude for
SU(N).Comment: 19 pages, late
Singular del Pezzo fibrations and birational rigidity
A known conjecture of Grinenko in birational geometry asserts that a Mori
fibre space with the structure of del Pezzo fibration of low degree is
birationally rigid if and only if its anticanonical class is an interior point
in the cone of mobile divisors. The conjecture is proved to be true for smooth
models (with a generality assumption for degree 3). It is speculated that the
conjecture holds for, at least, Gorenstein models in degree 1 and 2. In this
article, I present a (Gorenstein) counterexample in degree 2 to this
conjecture.Comment: This is essentially a more detailed version of the second section of
arXiv:1310.5548. To appear in the proceedings of the conference 'Groups of
Automorphisms in Birational and Affine Geometry', held in Trento, Italy, 201
Exact time-reversal focusing of acoustic and quantum excitations in open cavities: The perfect inverse filter
The time-reversal mirror (TRM) prescribes the reverse playback of a signal to
focalize an acoustic excitation as a Loschmidt echo. In the quantum domain, the
perfect inverse filter (PIF) processes this signal to ensure an exact reversion
provided that the excitation originated outside the cavity delimited by the
transducers. We show that PIF takes a simple form when the initial excitation
is created inside this cavity. This also applies to the acoustical case, where
it corrects the TRM and improves the design of an acoustic bazooka. We solve an
open chaotic cavity modeling a quantum bazooka and a simple model for a
Helmholtz resonator, showing that the PIF becomes decisive to compensate the
group velocities involved in a highly localized excitation and to achieve
subwavelength resolution.Comment: 6 pages, 2 figure
Thermodynamics of Rotating Black Branes in Gauss-Bonnet-Born-Infeld Gravity
Considering both the Gauss-Bonnet and the Born-Infeld terms, which are on
similar footing with regard to string corrections on the gravity side and
electrodynamic side, we present a new class of rotating solutions in
Gauss-Bonnet gravity with rotation parameters in the presence of a
nonlinear electromagnetic field. These solutions, which are asymptotically
anti-de Sitter in the presence of cosmological constant, may be interpreted as
black brane solutions with inner and outer event horizons, an extreme black
brane or naked singularity provided the metric parameters are chosen suitably.
We calculate the finite action and conserved quantities of the solutions by
using the counterterm method, and find that these quantities do not depend on
the Gauss-Bonnet parameter. We also compute the temperature, the angular
velocities, the electric charge and the electric potential. Then, we calculate
the entropy of the black brane through the use of Gibbs-Duhem relation and show
that it obeys the area law of entropy. We obtain a Smarr-type formula for the
mass as a function of the entropy, the angular momenta and the charge, and show
that the conserved and thermodynamic quantities satisfy the first law of
thermodynamics. Finally, we perform a stability analysis in both the canonical
and grand-canonical ensemble and show that the presence of a nonlinear
electromagnetic field has no effect on the stability of the black branes, and
they are stable in the whole phase space.Comment: 17 pages, one figur
A magnetic cycle of tau Bootis? The coronal and chromospheric view
Tau Bootis is a late F-type main sequence star orbited by a Hot Jupiter.
During the last years spectropolarimetric observations led to the hypothesis
that this star may host a global magnetic field that switches its polarity once
per year, indicating a very short activity cycle of only one year duration. In
our ongoing observational campaign, we have collected several X-ray
observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to
characterize tau Boo's corona and chromosphere over the course of the supposed
one-year cycle. Contrary to the spectropolarimetric reconstructions, our
observations do not show indications for a short activity cycle.Comment: 4 pages, 2 figures, appeared in Astronomical Notes 333, 1, 26-29
(2012
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