11,576 research outputs found
The Phase Diagram and Spectrum of Gauge-Fixed Abelian Lattice Gauge Theory
We consider a lattice discretization of a covariantly gauge-fixed abelian
gauge theory. The gauge fixing is part of the action defining the theory, and
we study the phase diagram in detail. As there is no BRST symmetry on the
lattice, counterterms are needed, and we construct those explicitly. We show
that the proper adjustment of these counterterms drives the theory to a new
type of phase transition, at which we recover a continuum theory of (free)
photons. We present both numerical and (one-loop) perturbative results, and
show that they are in good agreement near this phase transition. Since
perturbation theory plays an important role, it is important to choose a
discretization of the gauge-fixing action such that lattice perturbation theory
is valid. Indeed, we find numerical evidence that lattice actions not
satisfying this requirement do not lead to the desired continuum limit. While
we do not consider fermions here, we argue that our results, in combination
with previous work, provide very strong evidence that this new phase transition
can be used to define abelian lattice chiral gauge theories.Comment: 42 pages, 30 figure
Millimetre/submillimetre-wave emission line searches for high-redshift galaxies
The redshifted spectral line radiation emitted from both atomic
fine-structure and molecular rotational transitions in the interstellar medium
(ISM) of high-redshift galaxies can be detected in the centimetre, millimetre
and submillimetre wavebands. Here we predict the counts of galaxies detectable
in an array of molecular and atomic lines. This calculation requires a
reasonable knowledge of both the surface density of these galaxies on the sky,
and the physical conditions in their ISM. The surface density is constrained
using the results of submillimetre-wave continuum surveys. Follow-up OVRO
Millimeter Array observations of two of the galaxies detected in the dust
continuum have provided direct measurements of CO rotational line emission at
redshifts of 2.56 and 2.81. Based on these direct high-redshift observations
and on models of the ISM that are constrained by observations of low-redshift
ultraluminous infrared galaxies, we predict the surface density of
line-emitting galaxies as a function of line flux density and observing
frequency. We incorporate the sensitivities and mapping speeds of existing and
future millimetre/submillimetre-wave telescopes and spectrographs, and so
assess the prospects for blank-field surveys to detect this line emission from
gas-rich high-redshift galaxies.Comment: 13 pages, 12 figures, to appear in MNRAS. Final proof versio
Importance of adenosine-to-inosine editing adjacent to the anticodon in an Arabidopsis alanine tRNA under environmental stress
In all organisms, transfer RNAs (tRNAs) undergo extensive post-transcriptional modifications. Although base modifications in the anticodon are known to alter decoding specificity or improve decoding accuracy, much less is known about the functional relevance of modifications in other positions of tRNAs. Here, we report the identification of an A-to-I tRNA editing enzyme that modifies the tRNA-Ala(AGC) in the model plant Arabidopsis thaliana. The enzyme is homologous to Tad1p, a yeast tRNA-specific adenosine deaminase, and it selectively deaminates the adenosine in the position 3'-adjacent to the anticodon (A(37)) to inosine. We show that the AtTAD1 protein is exclusively localized in the nucleus. The tad1 loss-of-function mutants isolated in Arabidopsis show normal accumulation of the tRNA-Ala(AGC), suggesting that the loss of the I(37) modification does not affect tRNA stability. The tad1 knockout mutants display no discernible phenotype under standard growth conditions, but produce less biomass under environmental stress conditions. Our results provide the first evidence in support of a physiological relevance of the A(37)-to-I modification in eukaryotes
Composite infrared bolometers with Si_3N_4 micromesh absorbers
We report the design and performance of 300-mK composite bolometers that use micromesh absorbers and support structures patterned from thin films of low-stress silicon nitride. The small geometrical filling factor of the micromesh absorber provides 20Ă reduction in heat capacity and cosmic ray cross section relative to a solid absorber with no loss in IR-absorption efficiency. The support structure is mechanically robust and has a thermal conductance, G < 2 Ă 10^(â11) W/K, which is four times smaller than previously achieved at 300 mK. The temperature rise of the bolometer is measured with a neutron transmutation doped germanium thermistor attached to the absorbing mesh. The dispersion in electrical and thermal parameters of a sample of 12 bolometers optimized for the SunyaevâZelâdovich Infrared Experiment is ±7% in R (T), ±5% in optical efficiency, and ±4% in G
The far-infrared/submillimeter properties of galaxies located behind the Bullet cluster
The Herschel Lensing Survey (HLS) takes advantage of gravitational lensing by massive galaxy clusters to sample a population of high-redshift galaxies which are too faint to be detected above the confusion limit of current far-infrared/submillimeter telescopes. Measurements from 100â500 ÎŒm bracket the peaks of the far-infrared spectral energy distributions of these galaxies, characterizing their infrared luminosities and star formation rates. We introduce initial results from our science demonstration phase observations, directed toward the Bullet cluster (1E0657-56). By combining our observations with LABOCA 870 ÎŒm and AzTEC 1.1 mm data we fully constrain the spectral energy distributions of 19 MIPS 24 ÎŒm-selected galaxies which are located behind the cluster. We find that their colors are best fit using templates based on local galaxies with systematically lower infrared luminosities. This suggests that our sources are not like local ultra-luminous infrared galaxies in which vigorous star formation is contained in a compact highly dust-obscured region. Instead, they appear to be scaled up versions of lower luminosity local galaxies with star formation occurring on larger physical scales
The Herschel Lensing Survey (HLS): Overview
The Herschel Lensing Survey (HLS) will conduct deep PACS and SPIRE imaging of âŒ40 massive clusters of galaxies. The strong gravitational lensing power of these clusters will enable us to penetrate through the confusion noise, which sets the ultimate limit on our ability to probe the
Universe with Herschel. Here we present an overview of our survey and a summary of the major results from our science demonstration phase (SDP) observations of the Bullet cluster (z = 0.297). The SDP data are rich and allow us to study not only the background high-redshift galaxies
(e.g., strongly lensed and distorted galaxies at z = 2.8 and 3.2) but also the properties of cluster-member galaxies. Our preliminary analysis shows a great diversity of far-infrared/submillimeter spectral energy distributions (SEDs), indicating that we have much to learn with Herschel about the properties of galaxy SEDs. We have also detected the Sunyaev-Zelâdovich (SZ) effect increment with the SPIRE data. The success of this SDP program demonstrates the great potential of the Herschel Lensing Survey to produce exciting results in a variety of science areas
Deep Herschel view of obscured star formation in the Bullet cluster
We use deep, five band (100â500 ÎŒm) data from the Herschel Lensing Survey (HLS) to fully constrain the obscured star formation rate, SFR_(FIR), of galaxies in the Bullet cluster (z = 0.296), and a smaller background system (z = 0.35) in the same field. Herschel detects 23 Bullet cluster members with a total SFRFIR = 144±14 M_â yr^(-1). On average, the background system contains brighter far-infrared (FIR) galaxies, with ~50% higher SFRFIR (21 galaxies; 207 ± 9 M_â yr^(-1)). SFRs extrapolated from 24 ÎŒm flux via recent templates (SFR_(24 ”m)) agree well with SFRFIR for ~60% of the cluster galaxies. In the remaining ~40%, SFR24 ”m underestimates SFR_(FIR) due to a significant excess in observed S_(100)/S_(24) (rest frame S_(75)/S_(18)) compared to templates of the same FIR luminosity
Strongly coupled U(1) lattice gauge theory as a microscopic model of Yukawa theory
Dynamical chiral symmetry breaking in a strongly coupled U(1) lattice gauge
model with charged fermions and scalar is investigated by numerical simulation.
Several composite neutral states are observed, in particular a massive fermion.
In the vicinity of the tricritical point of this model we study the effective
Yukawa coupling between this fermion and the Goldstone boson. The perturbative
triviality bound of Yukawa models is nearly saturated. The theory is quite
similar to strongly coupled Yukawa models for sufficiently large coupling
except the occurrence of an additional state -- a gauge ball of mass about half
the mass of the fermion.Comment: 4 page
Optical Conductivity in a Two-Band Superconductor: Pb
We demonstrate the effect of bandstructure on the superconducting properties
of Pb by calculating the strong-coupling features in the optical conductivity,
, due to the electron-phonon interaction. The importance of
momentum dependence in the calculation of the properties of superconductors has
previously been raised for MgB. Pb resembles MgB in that it is a two
band superconductor in which the bands' contributions to the Fermi surface have
very different topologies. We calculate by calculating a
memory function which has been recently used to analyze of
BiSrCaCuO. In our calculations the two components of
the Fermi surface are described by parameterizations of de Haas--van Alphen
data. We use a phonon spectrum which is a fit to neutron scattering data. By
including the momentum dependence of the Fermi surface good agreement is found
with the experimentally determined strong-coupling features which can be
described by a broad peak at around 4.5 meV and a narrower higher peak around 8
meV of equal height. The calculated features are found to be dominated by
scattering between states within the third band. By contrast scattering between
states in the second band leads to strong-coupling features in which the height
of the high energy peak is reduced by compared to that of the low
energy peak. This result is similar to that in the conventional isotropic
(momentum independent) treatment of superconductivity. Our results show that it
is important to use realistic models of the bandstructure and phonons, and to
avoid using momentum averaged quantities, in calculations in order to get
quantitatively accurate results
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