1,762 research outputs found
Vanishing of Gravitational Particle Production in the Formation of Cosmic Strings
We consider the gravitationally induced particle production from the quantum
vacuum which is defined by a free, massless and minimally coupled scalar field
during the formation of a gauge cosmic string. Previous discussions of this
topic estimate the power output per unit length along the string to be of the
order of ergs/sec/cm in the s-channel. We find that this production
may be completely suppressed. A similar result is also expected to hold for the
number of produced photons.Comment: 10 pages, Plain LaTex. Minor improvements. To appear in PR
Maximum gradient embeddings and monotone clustering
Let (X,d_X) be an n-point metric space. We show that there exists a
distribution D over non-contractive embeddings into trees f:X-->T such that for
every x in X, the expectation with respect to D of the maximum over y in X of
the ratio d_T(f(x),f(y)) / d_X(x,y) is at most C (log n)^2, where C is a
universal constant. Conversely we show that the above quadratic dependence on
log n cannot be improved in general. Such embeddings, which we call maximum
gradient embeddings, yield a framework for the design of approximation
algorithms for a wide range of clustering problems with monotone costs,
including fault-tolerant versions of k-median and facility location.Comment: 25 pages, 2 figures. Final version, minor revision of the previous
one. To appear in "Combinatorica
Scalars from Top-condensation Models at Hadron Colliders
We study the production and decay of neutral scalars and pseudo-scalars at
hadron colliders, in theories where the top-quark mass is the result of a
condensate. We show that the dominant decay channel for masses below
the threshold is the flavor changing mode . This is a consequence
of the non-universal nature of the underlying interactions in all
top-condensation models and provides a model-independent signature of these
scenarios. We show that an upgraded Tevatron is sensitive to a sizeable region
of the interesting parameter space and that the LHC will highly constrain these
models through this flavor violating channel.Comment: 4 pages, 4 figures. Minor changes in figures for readibility. final
version to appear in PR
Nonlinear spectral calculus and super-expanders
Nonlinear spectral gaps with respect to uniformly convex normed spaces are
shown to satisfy a spectral calculus inequality that establishes their decay
along Cesaro averages. Nonlinear spectral gaps of graphs are also shown to
behave sub-multiplicatively under zigzag products. These results yield a
combinatorial construction of super-expanders, i.e., a sequence of 3-regular
graphs that does not admit a coarse embedding into any uniformly convex normed
space.Comment: Typos fixed based on referee comments. Some of the results of this
paper were announced in arXiv:0910.2041. The corresponding parts of
arXiv:0910.2041 are subsumed by the current pape
Testing Rotational Mixing Predictions with New Boron Abundances in Main Sequence B-type Stars
(Abridged) New boron abundances for seven main-sequence B-type stars are
determined from HST STIS spectroscopy around the BIII 2066A line. Boron
abundances provide a unique and critical test of stellar evolution models that
include rotational mixing since boron is destroyed in the surface layers of
stars through shallow mixing long before other elements are mixed from the
stellar interior through deep mixing. Boron abundances range from 12+log(B/H) =
1.0 to 2.2. The boron abundances are compared to the published values of their
stellar nitrogen abundances (all have 12+log(N/H) < 7.8, i.e., they do not show
significant CNO-mixing) and to their host cluster ages (4 to 16 Myr) to
investigate the predictions from models of massive star evolution with
rotational mixing effects (Heger & Langer 2000). Only three stars (out of 34)
deviate from the model predictions, including HD36591, HD205021, and HD30836.
These three stars suggest that rotational mixing could be more efficient than
currently modelled at the highest rotation rates.Comment: 10 figures, 7 tables; accepted for publication in the Astrophysical
Journa
The Savvidy ``ferromagnetic vacuum'' in three-dimensional lattice gauge theory
The vacuum effective potential of three-dimensional SU(2) lattice gauge
theory in an applied color-magnetic field is computed over a wide range of
field strengths. The background field is induced by an external current, as in
continuum field theory. Scaling and finite volume effects are analyzed
systematically. The first evidence from lattice simulations is obtained of the
existence of a nontrivial minimum in the effective potential. This supports a
``ferromagnetic'' picture of gluon condensation, proposed by Savvidy on the
basis of a one-loop calculation in (3+1)-dimensional QCD.Comment: 9pp (REVTEX manuscript). Postscript figures appende
Large Tree Level CP Violation in in The Two Higgs Doublet Model
We find a large CP violation effect within the Two-Higgs-Doublet-Model for
the reaction at future linear colliders. The
CP-asymmetry arises already at the tree level as a result of interference
between diagrams with emission from (and ) and its emission
from a and can be about 10--20\%. In the best case one needs a few
hundred events to observe CP violation at the 3 level.Comment: UU encoded tar compressed tex file with postscript figure
Presence of the Eucalyptus gall wasp Ophelimus maskelli and its parasitoid Closterocerus chamaeleon in Portugal: first record, geographic distribution and host preference
The Eucalyptus gall wasp Ophelimus
maskelli (Hymenoptera: Eulophidae) and its parasitoid
Closterocerus chamaeleon (Hymenoptera:
Eulophidae) were observed for the first time in
Portugal, in 2006 and 2007, respectively. Data on
the distribution of O. maskelli in Portugal, differences
in the susceptibility of two host species, Eucalyptus
globulus and Eucalyptus camaldulensis, and parasitism
by C. chamaeleon are given
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