11,366 research outputs found
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
Heavy Domain Wall Fermions: The RBC and UKQCD charm physics program
We review the domain wall charm physics program of the RBC and UKQCD
collaborations based on simulations including ensembles with physical pion
mass. We summarise our current set-up and present a status update on the decay
constants , , the charm quark mass, heavy-light and heavy-strange
bag parameters and the ratio .Comment: 8 pagers, 4 figures, conference proceedings for Lattice2017 submitted
to EPJ Web of Conference
Hardware and software status of QCDOC
QCDOC is a massively parallel supercomputer whose processing nodes are based
on an application-specific integrated circuit (ASIC). This ASIC was
custom-designed so that crucial lattice QCD kernels achieve an overall
sustained performance of 50% on machines with several 10,000 nodes. This strong
scalability, together with low power consumption and a price/performance ratio
of $1 per sustained MFlops, enable QCDOC to attack the most demanding lattice
QCD problems. The first ASICs became available in June of 2003, and the testing
performed so far has shown all systems functioning according to specification.
We review the hardware and software status of QCDOC and present performance
figures obtained in real hardware as well as in simulation.Comment: Lattice2003(machine), 6 pages, 5 figure
The Distribution of Metallicity in the IGM at z~2.5: OVI and CIV Absorption in the Spectra of 7 QSOs
We present a direct measurement of the metallicity distribution function for
the high redshift intergalactic medium. We determine the shape of this function
using survival statistics, which account for both detections and non-detections
of OVI and CIV associated with HI absorption in quasar spectra. Our OVI sample
probes the metal content of ~50% of all baryons at z~2.5. We find a median
intergalactic abundance of [O,C/H]=-2.82; the differential abundance
distribution is approximately lognormal with mean ~-2.85 and
\sigma=0.75 dex. Some 60-70% the Lya forest lines are enriched to observable
levels ([O,C/H]>-3.5) while the remaining ~30% of the lines have even lower
abundances. Thus we have not detected a universal metallicity floor as has been
suggested for some Population III enrichment scenaria. In fact, we argue that
the bulk of the intergalactic metals formed later than the first stars that are
thought to have triggered reionization. We do not observe a significant trend
of decreasing metallicity toward the lower density IGM, at least within regions
that would be characterized as filaments in numerical simulations. However, an
[O/H] enhancement may be present at somewhat high densities. We estimate that
roughly half of all baryons at these redshifts have been enriched to
[O/H]>=-3.5. We develop a simple model for the metallicity evolution of the
IGM, to estimate the chemical yield of galaxies formed prior to z~2.5. We find
that the typical galaxy recycled 0.1-0.4% of its mass back into the IGM as
heavy elements in the first 3 Gyr after the Big Bang.Comment: 23 pages in emulateapj, 19 figures. Accepted to ApJ, pending review
of new changes. Revised comparison between our results and Schaye et al
(2003
Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion
Pulsar timing arrays (PTAs) will be sensitive to a finite number of
gravitational wave (GW) "point" sources (e.g. supermassive black hole
binaries). N quiet pulsars with accurately known distances d_{pulsar} can
characterize up to 2N/7 distant chirping sources per frequency bin \Delta
f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta
\gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are
poorly known, a PTA with F frequency bins can still characterize up to
(2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing
residuals in the vicinity of a GW source still allows the source to be
localized quasi-topologically within roughly the smallest quadrilateral of
quiet pulsars that encircles it on the sky, down to a limiting resolution
\delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be
unconfused, even at the lowest frequencies, with matched filtering always
appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio
The decay constants and in the continuum limit of domain wall lattice QCD
We present results for the decay constants of the and mesons
computed in lattice QCD with dynamical flavours. The simulations are
based on RBC/UKQCD's domain wall ensembles with both physical and unphysical
light-quark masses and lattice spacings in the range 0.11--0.07fm. We
employ the domain wall discretisation for all valence quarks.
The results in the continuum limit are
and
and
.
Using these results in a Standard Model analysis we compute the predictions
and
for the
CKM matrix elements
Proton lifetime bounds from chirally symmetric lattice QCD
We present results for the matrix elements relevant for proton decay in Grand
Unified Theories (GUTs). The calculation is performed at a fixed lattice
spacing a^{-1}=1.73(3) GeV using 2+1 flavors of domain wall fermions on
lattices of size 16^3\times32 and 24^3\times64 with a fifth dimension of length
16. We use the indirect method which relies on an effective field theory
description of proton decay, where we need to estimate the low energy
constants, \alpha = -0.0112(25) GeV^3 and \beta = 0.0120(26) GeV^3. We relate
these low energy constants to the proton decay matrix elements using leading
order chiral perturbation theory. These can then be combined with experimental
bounds on the proton lifetime to bound parameters of individual GUTs.Comment: 17 pages, 9 Figure
A one-sided Prime Ideal Principle for noncommutative rings
Completely prime right ideals are introduced as a one-sided generalization of
the concept of a prime ideal in a commutative ring. Some of their basic
properties are investigated, pointing out both similarities and differences
between these right ideals and their commutative counterparts. We prove the
Completely Prime Ideal Principle, a theorem stating that right ideals that are
maximal in a specific sense must be completely prime. We offer a number of
applications of the Completely Prime Ideal Principle arising from many diverse
concepts in rings and modules. These applications show how completely prime
right ideals control the one-sided structure of a ring, and they recover
earlier theorems stating that certain noncommutative rings are domains (namely,
proper right PCI rings and rings with the right restricted minimum condition
that are not right artinian). In order to provide a deeper understanding of the
set of completely prime right ideals in a general ring, we study the special
subset of comonoform right ideals.Comment: 38 page
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