38,497 research outputs found
Is the Redshift Clustering of Long-Duration Gamma-Ray Bursts Significant?
The 26 long-duration gamma-ray bursts (GRBs) with known redshifts form a
distinct cosmological set, selected differently than other cosmological probes
such as quasars and galaxies. Since the progenitors are now believed to be
connected with active star-formation and since burst emission penetrates dust,
one hope is that with a uniformly-selected sample, the large-scale redshift
distribution of GRBs can help constrain the star-formation history of the
Universe. However, we show that strong observational biases in ground-based
redshift discovery hamper a clean determination of the large-scale GRB rate and
hence the connection of GRBs to the star formation history. We then focus on
the properties of the small-scale (clustering) distribution of GRB redshifts.
When corrected for heliocentric motion relative to the local Hubble flow, the
observed redshifts appear to show a propensity for clustering: 8 of 26 GRBs
occurred within a recession velocity difference of 1000 km/s of another GRB.
That is, 4 pairs of GRBs occurred within 30 h_65^-1 Myr in cosmic time, despite
being causally separated on the sky. We investigate the significance of this
clustering. Comparison of the numbers of close redshift pairs expected from the
simulation with that observed shows no significant small-scale clustering
excess in the present sample; however, the four close pairs occur only in about
twenty percent of the simulated datasets (the precise significance of the
clustering is dependent upon the modeled biases). We conclude with some
impetuses and suggestions for future precise GRB redshift measurements.Comment: Published in the Astronomical Journal, June 2003: see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003AJ....125.2865
W Plus Multiple Jets at the LHC with High Energy Jets
We study the production of a W boson in association with n hard QCD jets (for
n>=2), with a particular emphasis on results relevant for the Large Hadron
Collider (7 TeV and 8 TeV). We present predictions for this process from High
Energy Jets, a framework for all-order resummation of the dominant
contributions from wide-angle QCD emissions. We first compare predictions
against recent ATLAS data and then shift focus to observables and regions of
phase space where effects beyond NLO are expected to be large.Comment: 19 pages, 9 figure
Uni-directional polymerization leading to homochirality in the RNA world
The differences between uni-directional and bi-directional polymerization are
considered. The uni-directional case is discussed in the framework of the RNA
world. Similar to earlier models of this type, where polymerization was assumed
to proceed in a bi-directional fashion (presumed to be relevant to peptide
nucleic acids), left-handed and right-handed monomers are produced via an
autocatalysis from an achiral substrate. The details of the bifurcation from a
racemic solution to a homochiral state of either handedness is shown to be
remarkably independent of whether the polymerization in uni-directional or
bi-directional. Slightly larger differences are seen when dissociation is
allowed and the dissociation fragments are being recycled into the achiral
substrate.Comment: 9 pages, 4 figures, submitted to Astrobiolog
Antiferromagnetic correlations and impurity broadening of NMR linewidths in cuprate superconductors
We study a model of a d-wave superconductor with strong potential scatterers
in the presence of antiferromagnetic correlations and apply it to experimental
nuclear magnetic resonance (NMR) results on Zn impurities in the
superconducting state of YBCO. We then focus on the contribution of
impurity-induced paramagnetic moments, with Hubbard correlations in the host
system accounted for in Hartree approximation. We show that local magnetism
around individual impurities broadens the line, but quasiparticle interference
between impurity states plays an important role in smearing out impurity
satellite peaks. The model, together with estimates of vortex lattice effects,
provides a semi-quantitative description of the impurity concentration
dependence of the NMR line shape in the superconducting state, and gives a
qualitative description of the temperature dependence of the line asymmetry. We
argue that impurity-induced paramagnetism and resonant local density of states
effects are both necessary to explain existing experiments.Comment: 15 pages, 23 figures, submitted to Phys. Rev.
Three-loop HTLpt thermodynamics at finite temperature and chemical potential
In this proceedings we present a state-of-the-art method of calculating
thermodynamic potential at finite temperature and finite chemical potential,
using Hard Thermal Loop perturbation theory (HTLpt) up to
next-to-next-leading-order (NNLO). The resulting thermodynamic potential
enables us to evaluate different thermodynamic quantities including pressure
and various quark number susceptibilities (QNS). Comparison between our
analytic results for those thermodynamic quantities with the available lattice
data shows a good agreement.Comment: 5 pages, 6 figures, conference proceedings of XXI DAE-BRNS HEP
Symposium, IIT Guwahati, December 2014; to appear in 'Springer Proceedings in
Physics Series
Probing spatial spin correlations of ultracold gases by quantum noise spectroscopy
Spin noise spectroscopy with a single laser beam is demonstrated
theoretically to provide a direct probe of the spatial correlations of cold
fermionic gases. We show how the generic many-body phenomena of anti-bunching,
pairing, antiferromagnetic, and algebraic spin liquid correlations can be
revealed by measuring the spin noise as a function of laser width, temperature,
and frequency.Comment: Revised version. 4 pages, 3 figures. Accepted for PR
Dopant-modulated pair interaction in cuprate superconductors
Comparison of recent experimental STM data with single-impurity and
many-impurity Bogoliubov-de Gennes calculations strongly suggests that random
out-of-plane dopant atoms in cuprates modulate the pair interaction locally.
This type of disorder is crucial to understanding the nanoscale electronic
structure inhomogeneity observed in BSCCO-2212, and can reproduce observed
correlations between the positions of impurity atoms and various aspects of the
local density of states such as the gap magnitude and the height of the
coherence peaks. Our results imply that each dopant atom modulates the pair
interaction on a length scale of order one lattice constant.Comment: 5 pages, 4 figure
Using superlattice potentials to probe long-range magnetic correlations in optical lattices
In Pedersen et al. (2011) we proposed a method to utilize a temporally
dependent superlattice potential to mediate spin-selective transport, and
thereby probe long and short range magnetic correlations in optical lattices.
Specifically this can be used for detecting antiferromagnetic ordering in
repulsive fermionic optical lattice systems, but more generally it can serve as
a means of directly probing correlations among the atoms by measuring the mean
value of an observable, the number of double occupied sites. Here, we provide a
detailed investigation of the physical processes which limit the effectiveness
of this "conveyer belt method". Furthermore we propose a simple ways to improve
the procedure, resulting in an essentially perfect (error-free) probing of the
magnetic correlations. These results shows that suitably constructed
superlattices constitute a promising way of manipulating atoms of different
spin species as well as probing their interactions.Comment: 12 pages, 9 figure
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