25,070 research outputs found
The BRST treatment of stretched membranes
The BRST-invariant formulation of the bosonic stretched membrane is
considered. In this formulation the stretched membrane is given as a
perturbation around zero-tension membranes, where the BRST-charge decomposes as
a sum of a string-like BRST-charge and a perturbation. It is proven, by means
of cohomology techniques, that there exists to any order in perturbation theory
a canonical transformation that reduces the full BRST-charge to the string-like
one. It is also shown that one may extend the results to the quantum level
yielding a nilpotent charge in 27 dimensions.Comment: 21 pages, typo's corrected, minor changes in the tex
Vertex Operators in Background with NS_NS Flux using Berkovits-Vafa-Witten Variables
String equations for the vertex operators of type IIB on
background with NS-NS flux are calculated using Berkovits-Vafa-Witten
formalism. With suitable field definitions, the linearized field equations for
six-dimensional supergravity and a tensor multiplet on are
recovered from these. We also discuss the massless degrees of freedom that
survive the Kaluza-Klein compactification and how our vertex operators
are related to the vertex operators introduced by Giveon, Kutasov and Seiberg.Comment: 14 pages, LaTeX, added new reference
New Coherent String States and Minimal Uncertainty in WZWN Models
We study the properties of {\bf exact} (all level ) quantum coherent
states in the context of string theory on a group manifold (WZWN models).
Coherent states of WZWN models may help to solve the unitarity problem: Having
positive norm, they consistently describe the very massive string states
(otherwise excluded by the spin-level condition). These states can be
constructed by (at least) two alternative procedures: (i) as the exponential of
the creation operator on the ground state, and (ii) as eigenstates of the
annhilation operator. In the limit, all the known properties of
ordinary coherent states are recovered. States (i) and (ii) (which are
equivalent in the context of ordinary quantum mechanics and string theory in
flat spacetime) are not equivalent in the context of WZWN models. The set (i)
was constructed by these authors in a previous article. In this paper we
provide the construction of states (ii), we compare the two sets and discuss
their properties. We analyze the uncertainty relation, and show that states
(ii) satisfy automatically the {\it minimal uncertainty} condition for any ;
they are thus {\it quasiclassical}, in some sense more classical than states
(i) which only satisfy it in the limit. Modification to the
Heisenberg relation is given by , where is connected
to the string energy.Comment: More discussion on relation to previous work. More references added.
14 pages, Latex, no figure
Yang-Lee Zeros of the Triangular Ising Antiferromagnets
Using both the exact enumeration method (microcanonical transfer matrix) for
a small system (L = 9) and the Wang-Landau Monte Carlo algorithm for large
systems to L = 30, we obtain the exact and approximate densities of states
g(M,E), as a function of magnetization M and exchange energy E, for the
triangular-lattice Ising model. Based on the density of states g(M,E), we
investigate the phase transition properties of Yang-Lee zeros for the
triangular Ising antiferromagnets and obtain the magnetic exponents at various
temperatures
Structural origins of the properties of rare earth nickelate superlattices
NiO6 octahedral tilts in the LaNiO3/SrTiO3 superlattices are quantified using
position averaged convergent beam electron diffraction in scanning transmission
electron microscopy. It is shown that maintaining oxygen octahedra connectivity
across the interface controls the octahedral tilts in the LaNiO3 layers, their
lattice parameters and their transport properties. Unlike films and layers that
are connected on one side to the substrate, subsequent LaNiO3 layers in the
superlattice exhibit a relaxation of octahedral tilts towards bulk values. This
relaxation is facilitated by correlated tilts in SrTiO3 layers and is
correlated with the conductivity enhancement of the LaNiO3 layers in the
superlattices relative to individual films.Comment: Accepted for publication in Physical Review B (Rapid Communication
Mutation supply and the repeatability of selection for antibiotic resistance
Whether evolution can be predicted is a key question in evolutionary biology.
Here we set out to better understand the repeatability of evolution. We
explored experimentally the effect of mutation supply and the strength of
selective pressure on the repeatability of selection from standing genetic
variation. Different sizes of mutant libraries of an antibiotic resistance
gene, TEM-1 -lactamase in Escherichia coli, were subjected to different
antibiotic concentrations. We determined whether populations went extinct or
survived, and sequenced the TEM gene of the surviving populations. The
distribution of mutations per allele in our mutant libraries- generated by
error-prone PCR- followed a Poisson distribution. Extinction patterns could be
explained by a simple stochastic model that assumed the sampling of beneficial
mutations was key for survival. In most surviving populations, alleles
containing at least one known large-effect beneficial mutation were present.
These genotype data also support a model which only invokes sampling effects to
describe the occurrence of alleles containing large-effect driver mutations.
Hence, evolution is largely predictable given cursory knowledge of mutational
fitness effects, the mutation rate and population size. There were no clear
trends in the repeatability of selected mutants when we considered all
mutations present. However, when only known large-effect mutations were
considered, the outcome of selection is less repeatable for large libraries, in
contrast to expectations. Furthermore, we show experimentally that alleles
carrying multiple mutations selected from large libraries confer higher
resistance levels relative to alleles with only a known large-effect mutation,
suggesting that the scarcity of high-resistance alleles carrying multiple
mutations may contribute to the decrease in repeatability at large library
sizes.Comment: 31pages, 9 figure
A covariant investigation of Neutral Vector Mesons: dynamical properties and electromagnetic decay widths
A simple, but fully-covariant model for describing neutral Vector Mesons, in
both light and heavy sectors, is briefly illustrated. The main ingredients of
our relativistic constituent model are i) an Ansatz for the Bethe-Salpeter
vertex for Vector Mesons, and ii) a Mandelstam-like formula for the
electromagnetic decay widths. The free parameters of our approach are fixed
through a comparison with the valence transverse-momentum distribution,
, obtained within phenomenological Light-Front Hamiltonian Dynamics
models reproducing the mass spectra. Preliminary results for the
transverse-momentum distributions, the parton distribution and the
electromagnetic decay constants are shown.Comment: 6 pages, 4 figures. Proceedings of LC2009, to appear in Nucl. Phys. B
supp
Hubble Space Telescope Pixel Analysis of the Interacting Face-on Spiral Galaxy NGC 5194 (M51A)
A pixel analysis is carried out on the interacting face-on spiral galaxy NGC
5194 (M51A), using the HST/ACS images in the F435W, F555W and F814W (BVI)
bands. After 4x4 binning of the HST/ACS images to secure a sufficient
signal-to-noise ratio for each pixel, we derive several quantities describing
the pixel color-magnitude diagram (pCMD) of NGC 5194: blue/red color cut, red
pixel sequence parameters, blue pixel sequence parameters and blue-to-red pixel
ratio. The red sequence pixels are mostly older than 1 Gyr, while the blue
sequence pixels are mostly younger than 1 Gyr, in their luminosity-weighted
mean stellar ages. The color variation in the red pixel sequence from V = 20
mag arcsec^(-2) to V = 17 mag arcsec^(-2) corresponds to a metallicity
variation of \Delta[Fe/H] ~ 2 or an optical depth variation of \Delta\tau_V ~ 4
by dust, but the actual sequence is thought to originate from the combination
of those two effects. At V < 20 mag arcsec^(-2), the color variation in the
blue pixel sequence corresponds to an age variation from 5 Myr to 300 Myr under
the assumption of solar metallicity and \tau_V = 1. To investigate the spatial
distributions of stellar populations, we divide pixel stellar populations using
the pixel color-color diagram and population synthesis models. As a result, we
find that the pixel population distributions across the spiral arms agree with
a compressing process by spiral density waves: dense dust \rightarrow
newly-formed stars. The tidal interaction between NGC 5194 and NGC 5195 appears
to enhance the star formation at the tidal bridge connecting the two galaxies.
We find that the pixels corresponding to the central active galactic nucleus
(AGN) area of NGC 5194 show a tight sequence at the bright-end of the pCMD,
which are in the region of R ~ 100 pc and may be a photometric indicator of AGN
properties.Comment: 27 pages, 20 figures, accepted for publication in Ap
Evidence for a Quantum Hall Insulator in an InGaAs/InP Heterostructure
We study the quantum critical behavior of the plateau-insulator (PI)
transition in a low mobility InGaAs/InP heterostructure. By reversing the
direction of the magnetic field (B) we find an averaged Hall resistance \rho_xy
which remains quantized at the plateau value h/e^2 throughout the PI
transition. We extract a critical exponent \kappa'= 0.57 +/- 0.02 for the PI
transition which is slightly different from (and possibly more accurate than)
the established value 0.42 +/- 0.04 as previously obtained from the
plateau-plateau (PP) transitions.Comment: 3pages, 2 figures; submitted to EP2DS-14 conference proceeding
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