14,873 research outputs found
Expansions of hadronic spectral function moments in a nonpower QCD perturbation theory with tamed large order behavior
The moments of the hadronic spectral functions are of interest for the
extraction of the strong coupling and other QCD parameters from the
hadronic decays of the lepton. Motivated by the recent analyses of a
large class of moments in the standard fixed-order and contour-improved
perturbation theories, we consider the perturbative behavior of these moments
in the framework of a QCD nonpower perturbation theory, defined by the
technique of series acceleration by conformal mappings, which simultaneously
implements renormalization-group summation and has a tame large-order behavior.
Two recently proposed models of the Adler function are employed to generate the
higher order coefficients of the perturbation series and to predict the exact
values of the moments, required for testing the properties of the perturbative
expansions. We show that the contour-improved nonpower perturbation theories
and the renormalization-group-summed nonpower perturbation theories have very
good convergence properties for a large class of moments of the so-called
"reference model", including moments that are poorly described by the standard
expansions. The results provide additional support for the plausibility of the
description of the Adler function in terms of a small number of dominant
renormalons.Comment: 15 pages, latex using revtex, 4 figures; compared to v1, slightly
improved figures and discussion, version to appear in PR
Spectral weight suppression in response functions of ultracold fermion-boson mixtures
We study the dynamical response of ultracold fermion-boson mixture in the
Bogoliubov regime, where the interactions between fermionic impurities and
bosonic excitations (phonons) are described by an effective Frohlich model
under the Bogoliubov approximation. A characteristic suppression of the
single-particle spectral weight is found in the small momentum region where the
impurity band and phonon mode intersect. Using diagrammatic technique we
compute the Bragg spectra as well as the momentum dependent force-force
correlation function. We fnd that both of them are heavily affected by the
spectral weight suppression effect at low impurity densities in both 1D and 2D
systems. We show that the the spectral weight suppression feature in Bragg
spectra, which was previously found in the quantum Monte Carlo simulations and
which cannot be recovered by the random phase approximation, can be accurately
reproduced with the help of vertex corrections.Comment: 14 pages, 10 figures. Final version with a new title, some revisions
and a new figur
Where is the fuzz? Undetected Lyman alpha nebulae around QSOs at z~2.3
We observed a small sample of 5 radio-quiet QSOs with integral field
spectroscopy to search for possible extended emission in the Ly line.
We subtracted the QSO point sources using a simple PSF self-calibration
technique that takes advantage of the simultaneous availability of spatial and
spectral information. In 4 of the 5 objects we find no significant traces of
extended Ly emission beyond the contribution of the QSO nuclei itself,
while in UM 247 there is evidence for a weak and spatially quite compact excess
in the Ly line at several kpc outside the nucleus. For all objects in
our sample we estimated detection limits for extended, smoothly distributed
Ly emission by adding fake nebulosities into the datacubes and trying
to recover them after PSF subtraction. Our observations are consistent with
other studies showing that giant Ly nebulae such as those found
recently around some quasars are very rare. Ly fuzz around typical
radio-quiet QSOs is fainter, less extended and is therefore much harder to
detect. The faintness of these structures is consistent with the idea that
radio-quiet QSOs typically reside in dark matter haloes of modest masses.Comment: 12 Pages, Accepted for publication in A&
Perturbative expansion of the QCD Adler function improved by renormalization-group summation and analytic continuation in the Borel plane
We examine the large-order behaviour of a recently proposed
renormalization-group-improved expansion of the Adler function in perturbative
QCD, which sums in an analytically closed form the leading logarithms
accessible from renormalization-group invariance. The expansion is first
written as aneffective series in powers of the one-loop coupling, and its
leading singularities in the Borel plane are shown to be identical to those of
the standard "contour-improved" expansion. Applying the technique of conformal
mappings for the analytic continuation in the Borel plane, we define a class of
improved expansions, which implement both the renormalization-group invariance
and the knowledge about the large-order behaviour of the series. Detailed
numerical studies of specific models for the Adler function indicate that the
new expansions have remarkable convergence properties up to high orders. Using
these expansions for the determination of the strong coupling from the the
hadronic width of the lepton we obtain, with a conservative estimate of
the uncertainty due to the nonperturbative corrections, , which translates to .Comment: 15 pages latex using revtex, 4 figures; v2 corresponds to PRD
version; compared to v1, power-correction estimates have been enlarged
resulting in somewhat larger errors for alpha_S, relevant discussion has been
provided, a reference has been added, minor typographical errors have been
remove
Bandwidth in bolometric interferometry
Bolometric Interferometry is a technology currently under development that
will be first dedicated to the detection of B-mode polarization fluctuations in
the Cosmic Microwave Background. A bolometric interferometer will have to take
advantage of the wide spectral detection band of its bolometers in order to be
competitive with imaging experiments. A crucial concern is that interferometers
are presumed to be importantly affected by a spoiling effect known as bandwidth
smearing. In this paper, we investigate how the bandwidth modifies the work
principle of a bolometric interferometer and how it affects its sensitivity to
the CMB angular power spectra. We obtain analytical expressions for the
broadband visibilities measured by broadband heterodyne and bolometric
interferometers. We investigate how the visibilities must be reconstructed in a
broadband bolometric interferometer and show that this critically depends on
hardware properties of the modulation phase shifters. Using an angular power
spectrum estimator accounting for the bandwidth, we finally calculate the
sensitivity of a broadband bolometric interferometer. A numerical simulation
has been performed and confirms the analytical results. We conclude (i) that
broadband bolometric interferometers allow broadband visibilities to be
reconstructed whatever the kind of phase shifters used and (ii) that for
dedicated B-mode bolometric interferometers, the sensitivity loss due to
bandwidth smearing is quite acceptable, even for wideband instruments (a factor
2 loss for a typical 20% bandwidth experiment).Comment: 13 pages, 14 figures, submitted to A&
Analytic Perturbation Theory for Practitioners and Upsilon Decay
Within the ghost-free Analytic Perturbation Theory (APT), devised in the last
decade for low energy QCD, simple approximations are proposed for 3-loop
analytic couplings and their effective powers, in both the space-like
(Euclidean) and time-like (Minkowskian) regions, accurate enough in the large
range (1--100 GeV) of current physical interest.\par Effectiveness of the new
Model is illustrated by the example of decay where the
standard analysis gives value that is
inconsistent with the bulk of data for .
Instead, we obtain that
corresponds to that is close to the world
average.\par The issue of scale uncertainty for decay is also
discussed.Comment: 12 pages, 0 figures. Model slightly modified to increase its
accuracy. Numerical results upgraded, references added. The issue of scale
uncertainty is discusse
Optimal renormalization and the extraction of the strange quark mass from moments of the -decay spectral function
We introduce an optimal renormalization group analysis pertinent to the
analysis of polarization functions associated with the -quark mass relevant
in -decay. The technique is based on the renormalization group invariance
constraints which lead to closed form summation of all the leading and
next-to-leading logarithms at each order in perturbation theory. The new
perturbation series exhibits reduced sensitivity to the renormalization scale
and improved behavior in the complex plane along the integration contour. Using
improved experimental and theory inputs, we have extracted the value of the
strange quark mass and from presently available ALEPH and OPAL data
respectively. These determinations are in agreement with the determinations in
other phenomenological methods and from the lattice.Comment: 12 pages, 4 tables, 7 figures, v2 corresponds to version to appear in
Physical Review
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