16,873 research outputs found
Corrections to the Gell-Mann-Oakes-Renner relation and chiral couplings and
Next to leading order corrections to the
Gell-Mann-Oakes-Renner relation (GMOR) are obtained using weighted QCD Finite
Energy Sum Rules (FESR) involving the pseudoscalar current correlator. Two
types of integration kernels in the FESR are used to suppress the contribution
of the kaon radial excitations to the hadronic spectral function, one with
local and the other with global constraints. The result for the pseudoscalar
current correlator at zero momentum is , leading to the chiral corrections to GMOR: . The resulting uncertainties are mostly due to variations in the upper
limit of integration in the FESR, within the stability regions, and to a much
lesser extent due to the uncertainties in the strong coupling and the strange
quark mass. Higher order quark mass corrections, vacuum condensates, and the
hadronic resonance sector play a negligible role in this determination. These
results confirm an independent determination from chiral perturbation theory
giving also very large corrections, i.e. roughly an order of magnitude larger
than the corresponding corrections in chiral . Combining
these results with our previous determination of the corrections to GMOR in
chiral , , we are able to determine two low
energy constants of chiral perturbation theory, i.e. , and , both at the
scale of the -meson mass.Comment: Revised version with minor correction
Chiral symmetry restoration and deconfinement in QCD at finite temperature
The light-quark correlator in the axial-vector channel is used, in
conjunction with finite energy QCD sum rules at finite temperature, in order to
(a) establish a relation between chiral-symmetry restoration and deconfinement,
and (b) determine the temperature behavior of the width and
coupling. Results indicate that deconfinement takes place at a slightly lower
temperature than chiral-symmetry restoration, although this difference is not
significant given the accuracy of the method. The behaviour of the
parameters is consistent with quark-gluon deconfinement, as the width grows and
the coupling decreases with increasing temperature
Transport in random quantum dot superlattices
We present a novel model to calculate single-electron states in random
quantum dot superlattices made of wide-gap semiconductors. The source of
disorder comes from the random arrangement of the quantum dots (configurational
disorder) as well as spatial inhomogeneities of their shape (morphological
disorder). Both types of disorder break translational symmetry and prevent the
formation of minibands, as occurs in regimented arrays of quantum dots. The
model correctly describes channel mixing and broadening of allowed energy bands
due to elastic scattering by disorder
A Correlation Between Hard Gamma-ray Sources and Cosmic Voids Along the Line of Sight
We estimate the galaxy density along lines of sight to hard extragalactic
gamma-ray sources by correlating source positions on the sky with a void
catalog based on the Sloan Digital Sky Survey (SDSS). Extragalactic gamma-ray
sources that are detected at very high energy (VHE; E>100 GeV) or have been
highlighted as VHE-emitting candidates in the Fermi Large Area Telescope hard
source catalog (together referred to as "VHE-like" sources) are distributed
along underdense lines of sight at the 2.4 sigma level. There is also a less
suggestive correlation for the Fermi hard source population (1.7 sigma). A
correlation between 10-500 GeV flux and underdense fraction along the line of
sight for VHE-like and Fermi hard sources is found at 2.4 sigma and 2.6 sigma,
respectively. The preference for underdense sight lines is not displayed by
gamma-ray emitting galaxies within the second Fermi catalog, containing sources
detected above 100 MeV, or the SDSS DR7 quasar catalog. We investigate whether
this marginal correlation might be a result of lower extragalactic background
light (EBL) photon density within the underdense regions and find that, even in
the most extreme case of a entirely underdense sight line, the EBL photon
density is only 2% less than the nominal EBL density. Translating this into
gamma-ray attenuation along the line of sight for a highly attenuated source
with opacity tau(E,z) ~5, we estimate that the attentuation of gamma-rays
decreases no more than 10%. This decrease, although non-neglible, is unable to
account for the apparent hard source correlation with underdense lines of
sight.Comment: Accepted by MNRA
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