872 research outputs found
Hitting Diamonds and Growing Cacti
We consider the following NP-hard problem: in a weighted graph, find a
minimum cost set of vertices whose removal leaves a graph in which no two
cycles share an edge. We obtain a constant-factor approximation algorithm,
based on the primal-dual method. Moreover, we show that the integrality gap of
the natural LP relaxation of the problem is \Theta(\log n), where n denotes the
number of vertices in the graph.Comment: v2: several minor changes
A Semi-automatic Search for Giant Radio Galaxy Candidates and their Radio-Optical Follow-up
We present results of a search for giant radio galaxies (GRGs) with a
projected largest linear size in excess of 1 Mpc. We designed a computational
algorithm to identify contiguous emission regions, large and elongated enough
to serve as GRG candidates, and applied it to the entire 1.4-GHz NRAO VLA Sky
survey (NVSS). In a subsequent visual inspection of 1000 such regions we
discovered 15 new GRGs, as well as many other candidate GRGs, some of them
previously reported, for which no redshift was known. Our follow-up
spectroscopy of 25 of the brighter hosts using two 2.1-m telescopes in Mexico,
and four fainter hosts with the 10.4-m Gran Telescopio Canarias (GTC), yielded
another 24 GRGs. We also obtained higher-resolution radio images with the Karl
G. Jansky Very Large Array for GRG candidates with inconclusive radio
structures in NVSS.Comment: 4 pages, 1 figure, to appear in the proceedings of The Universe of
Digital Sky Surveys, Naples, Italy, Nov 25-28, 2014; Astrophysics and Space
Science, eds. N.R. Napolitano et a
Positivity constraints for lepton polarization in neutrino deep inelastic scattering
We consider the spin polarization of leptons produced in neutrino and
antineutrino nucleon deep inelastic scattering, via charged currents, and we
study the positivity constraints on the spin components in a model independent
way. These results are very important, in particular in the case of
leptons, because the polarization information is crucial in all
future neutrino oscillation experiments.Comment: 14 pages, 4 figure
Predictive Models of Large Neutrino Mixing Angles
Several experimental results could be interpreted as evidence that certain
neutrino mixing angles are large, of order unity. However, in the context of
grand unified models the neutrino angles come out characteristically to be
small, like the KM angles. It is shown how to construct simple grand-unified
models in which neutrino angles are not only large but completely predicted
with some precision. Six models are presented for illustration.Comment: 19 pages, LaTe
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
Quark and Lepton Mass Matrices in the SO(10) Grand Unified Theory with Generation Flipping
We investigate the SO(10) grand unified model with generation flipping. The
model contains one extra matter multiplet and it mixes with the
usual matter multiplets when the SO(10) is broken down to SU(5).
We find the parameter region of the model in which the observed quark masses
and mixings are well reproduced. The resulting parameter region is consistent
with the observation that only have a source of hierarchies and
indicates that the mixing between second and third generations tends to be
large in the lepton sector, which is consistent with the observed maximal
mixing of the atmospheric neutrino oscillation. We also show that the model can
accommodate MSW and vacuum oscillation solutions to the solar neutrino deficit
depending on the form of the Majorana mass matrix for the right-handed
neutrinos.Comment: 28 pages, Late
Induction and transfer of resistance to poisoning by Amorimia pubiflora in sheep whith non-toxic dosis of the plant and ruminal content
Additional Nucleon Current Contributions to Neutrinoless Double Beta Decay
We have examined the importance of momentum dependent induced nucleon
currents such as weak-magnetism and pseudoscalar couplings to the amplitude of
neutrinoless double beta decay in the mechanisms of light and heavy Majorana
neutrino as well as in that of Majoron emission. Such effects are expected to
occur in all nuclear models in the direction of reducing the light neutrino
matrix elements by about 30%. To test this we have performed a calculation of
the nuclear matrix elements of the experimentally interesting nuclei A = 76,
82, 96, 100, 116, 128, 130, 136 and 150 within the pn-RQRPA. We have found that
indeed such corrections vary somewhat from nucleus to nucleus, but in all cases
they are greater than 25 percent. In the case of heavy neutrino the effect is
much larger (a factor of 3). Combining out results with the best presently
available experimental limits on the half-life of the neutrinoless double beta
decay we have extracted new limits on the effective neutrino mass (light and
heavy) and the effective Majoron coupling constant.Comment: 31 pages, RevTex, 3 Postscript figures, submitted to Phys. Rev.
Muon Track Reconstruction and Data Selection Techniques in AMANDA
The Antarctic Muon And Neutrino Detector Array (AMANDA) is a high-energy
neutrino telescope operating at the geographic South Pole. It is a lattice of
photo-multiplier tubes buried deep in the polar ice between 1500m and 2000m.
The primary goal of this detector is to discover astrophysical sources of high
energy neutrinos. A high-energy muon neutrino coming through the earth from the
Northern Hemisphere can be identified by the secondary muon moving upward
through the detector. The muon tracks are reconstructed with a maximum
likelihood method. It models the arrival times and amplitudes of Cherenkov
photons registered by the photo-multipliers. This paper describes the different
methods of reconstruction, which have been successfully implemented within
AMANDA. Strategies for optimizing the reconstruction performance and rejecting
background are presented. For a typical analysis procedure the direction of
tracks are reconstructed with about 2 degree accuracy.Comment: 40 pages, 16 Postscript figures, uses elsart.st
DNA Damage Mediated S and G2 Checkpoints in Human Embryonal Carcinoma Cells
For mouse embryonic stem (ES) cells, the importance of the S and G2 cell cycle checkpoints for genomic integrity is increased by the absence of the G1 checkpoint. We have investigated ionizing radiation (IR)-mediated cell cycle checkpoints in undifferentiated and retinoic acid-differentiated human embryonal carcinoma (EC) cells. Like mouse ES cells, human EC cells did not undergo G1 arrest after IR but displayed a prominent S-phase delay followed by a G2-phase delay. In contrast, although differentiated EC cells also failed to arrest at G1-phase after IR, they quickly exited S-phase and arrested in G2-phase. In differentiated EC cells, the G2-M-phase cyclin B1/CDC2 complex was upregulated after IR, but the G1-S-phase cyclin E and the cyclin E/CDK2 complex were expressed at constitutively low levels, which could be an important factor distinguishing DNA damage responses between undifferentiated and differentiated EC cells. S-phase arrest and expression of p21 could be inhibited by 7-hydroxystaurosporine, suggesting that the ataxia-telangiectasia and Rad-3-related-checkpoint kinase 1 (ATR-CHK1), and p21 pathways might play a role in the IR-mediated S-phase checkpoint in EC cells. IR-mediated phosphorylation of ataxia-telangiectasia mutated, (CHK1), and checkpoint kinase 2 were distinctly higher in undifferentiated EC cells compared with differentiated EC cells. Combined with the prominent S and G2 checkpoints and a more efficient DNA damage repair system, these mechanisms operate together in the maintenance of genome stability for EC cells. Stem Cells 2009;27:568–57
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