9,418 research outputs found
O(N) models within the local potential approximation
Using Wegner-Houghton equation, within the Local Potential Approximation, we
study critical properties of O(N) vector models. Fixed Points, together with
their critical exponents and eigenoperators, are obtained for a large set of
values of N, including N=0 and N\to\infty. Polchinski equation is also treated.
The peculiarities of the large N limit, where a line of Fixed Points at d=2+2/n
is present, are studied in detail. A derivation of the equation is presented
together with its projection to zero modes.Comment: 27 pages, LaTeX with psfig, 7 PostScript figures. One reference
corrected and one added with respect to the journal versio
Photometric Redshift Estimation Using Spectral Connectivity Analysis
The development of fast and accurate methods of photometric redshift
estimation is a vital step towards being able to fully utilize the data of
next-generation surveys within precision cosmology. In this paper we apply a
specific approach to spectral connectivity analysis (SCA; Lee & Wasserman 2009)
called diffusion map. SCA is a class of non-linear techniques for transforming
observed data (e.g., photometric colours for each galaxy, where the data lie on
a complex subset of p-dimensional space) to a simpler, more natural coordinate
system wherein we apply regression to make redshift predictions. As SCA relies
upon eigen-decomposition, our training set size is limited to ~ 10,000
galaxies; we use the Nystrom extension to quickly estimate diffusion
coordinates for objects not in the training set. We apply our method to 350,738
SDSS main sample galaxies, 29,816 SDSS luminous red galaxies, and 5,223
galaxies from DEEP2 with CFHTLS ugriz photometry. For all three datasets, we
achieve prediction accuracies on par with previous analyses, and find that use
of the Nystrom extension leads to a negligible loss of prediction accuracy
relative to that achieved with the training sets. As in some previous analyses
(e.g., Collister & Lahav 2004, Ball et al. 2008), we observe that our
predictions are generally too high (low) in the low (high) redshift regimes. We
demonstrate that this is a manifestation of attenuation bias, wherein
measurement error (i.e., uncertainty in diffusion coordinates due to
uncertainty in the measured fluxes/magnitudes) reduces the slope of the
best-fit regression line. Mitigation of this bias is necessary if we are to use
photometric redshift estimates produced by computationally efficient empirical
methods in precision cosmology.Comment: Resubmitted to MNRAS (11 pages, 8 figures
de Branges-Rovnyak spaces: basics and theory
For a contractive analytic operator-valued function on the unit disk
, de Branges and Rovnyak associate a Hilbert space of analytic
functions and related extension space
consisting of pairs of analytic functions on the unit disk . This
survey describes three equivalent formulations (the original geometric de
Branges-Rovnyak definition, the Toeplitz operator characterization, and the
characterization as a reproducing kernel Hilbert space) of the de
Branges-Rovnyak space , as well as its role as the underlying
Hilbert space for the modeling of completely non-isometric Hilbert-space
contraction operators. Also examined is the extension of these ideas to handle
the modeling of the more general class of completely nonunitary contraction
operators, where the more general two-component de Branges-Rovnyak model space
and associated overlapping spaces play key roles. Connections
with other function theory problems and applications are also discussed. More
recent applications to a variety of subsequent applications are given in a
companion survey article
On Two-Body Decays of A Scalar Glueball
We study two body decays of a scalar glueball. We show that in QCD a spin-0
pure glueball (a state only with gluons) cannot decay into a pair of light
quarks if chiral symmetry holds exactly, i.e., the decay amplitude is chirally
suppressed. However, this chiral suppression does not materialize itself at the
hadron level such as in decays into and , because in
perturbative QCD the glueball couples to two (but not one) light quark pairs
that hadronize to two mesons. Using QCD factorization based on an effective
Lagrangian, we show that the difference of hadronization into and
already leads to a large difference between and , even the decay amplitude is not chirally suppressed. Moreover,
the small ratio of of
measured in experiment does not imply to be a pure glueball. With
our results it is helpful to understand the partonic contents if or is measured reliably.Comment: revised versio
Space- and time-like electromagnetic pion form factors in light-cone pQCD
We present a combined analysis of the space- and time-like electromagnetic
pion form factors in light-cone perturbative QCD with transverse momentum
dependence and Sudakov suppression. Including the non-perturbative ``soft' QCD
and power suppressed twist-3 corrections to the standard twist-2 perturbative
QCD result, the experimental pion data available at moderate energies/momentum
transfers can be explained reasonably well. This may help towards resolving the
bulk of the existing discrepancy between the space- and time-like experimental
data.Comment: 14 pages, 3 figure
Two-loop Corrections to the B to pi Form Factor from QCD Sum Rules on the Light-Cone and |V(ub)|
We calculate the leading-twist O(alphas^2 beta0) corrections to the B to pi
transition form factor f+(0) in light-cone sum rules. We find that, as
expected, there is a cancellation between the O(alphas^2 beta0) corrections to
fB f+(0) and the large corresponding corrections to fB, calculated in QCD sum
rules. This suggests the insensitivity of the form factors calculated in the
light-cone sum rules approach to this source of radiative corrections. We
further obtain an improved determination of the CKM matrix element |V(ub)|,
using latest results from BaBar and Belle for f+(0)|V(ub)|.Comment: 18 pages, 3 figure
Predictions for with Light-Cone Wave-Functions
Predictions for from previous studies are made by
taking charmonia as a nonrelativistic bound state and by using nonrelativistic
QCD(NRQCD) approach. The predicted cross-section is smaller by an order of
magnitude than the experimentally observed. We study the process by taking
charm quark as a light quark and use light-cone wave-functions to parameterize
nonperturbative effects related to charmonia. The total cross section of
can be predicted, if these wave-functions are known.
Motivated by studies of light-cone wave-functions of light hadrons, we make a
reasonable assumption of the forms of light-cone wave-functions. With these
light-cone wave-functions we can obtain the cross section which is more closer
to the experimentally observed than that from NRQCD approach. We also discuss
in detail the difference between two approaches.Comment: extended discussio
Hemodynamic optimization in severe trauma: a systematic review and meta-analysis.
OBJECTIVE: Severe trauma can be associated with significant hemorrhagic shock and impaired organ perfusion. We hypothesized that goal-directed therapy would confer morbidity and mortality benefits in major trauma. METHODS: The MedLine, Embase and Cochrane Controlled Clinical Trials Register databases were systematically searched for randomized, controlled trials of goal-directed therapy in severe trauma patients. Mortality was the primary outcome of this review. Secondary outcomes included complication rates, length of hospital and intensive care unit stay, and the volume of fluid and blood administered. Meta-analysis was performed using RevMan software, and the data presented are as odds ratios for dichotomous outcomes and as mean differences (MDs) and standard MDs for continuous outcomes. RESULTS: Four randomized, controlled trials including 419 patients were analyzed. Mortality risk was significantly reduced in goal-directed therapy-treated patients, compared to the control group(OR=0.56, 95%CI: 0.34-0.92). Intensive care (MD: 3.7 days 95%CI: 1.06-6.5)and hospital length of stay (MD: 3.5 days,95%CI: 2.75-4.25) were significantly shorter in the protocol group patients.There were no differences in reported total fluid volume or blood transfusions administered. Heterogeneity in reporting among the studies prevented quantitative analysis of complications. CONCLUSION: Following severe trauma, early goal-directed therapy was associated with lower mortality and shorter durations of intensive care unit and hospital stays. The findings of this analysis should be interpreted with caution due to the presence of significant heterogeneity and the small number of th
P-wave Quarkonium Decays to Meson Pairs
The processes of P-wave Quarkonium exclusive decays to two mesons are
investigated, in which the final state vector mesons with various polarizations
are considered separately. In the calculation, the initial heavy quarkonia are
treated in the framework of non-relativistic quantum chromodynamics, whereas
for light mesons, the light cone distribution amplitudes up to twist-3 are
employed. It turns out that the higher twist contribution is significant and
provides a possible explanation for the observation of the hadron helicity
selection rule violated processes
by the BESIII collaboration in recently. We also evaluate the process and find that its branching ratio is big enough to be
measured at the B-factories.Comment: more results and discussions adde
Testing the meson cloud in the nucleon in Drell-Yan processes
We discuss the present status of the \bar u-\bar d asymmetry in the nucleon
and analize the quantities which are best suited to verify the asymmetry. We
find that the Drell-Yan asymmetry is the quantity insensitive to the valence
quark distributions and very sensitive to the flavour asymmetry of the sea. We
compare the prediction of the meson cloud model with different experimental
data including the Fermilab E772 data and recent data of the NA51 Collaboration
at CERN and make predictions for the planned Drell-Yan experiments.Comment: written in ReVTeX, 26 pages + 10 PS-figure
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