7,892 research outputs found
Gluon-induced W-boson pair production at the LHC
Pair production of W bosons constitutes an important background to Higgs
boson and new physics searches at the Large Hadron Collider LHC. We have
calculated the loop-induced gluon-fusion process gg -> W*W* -> leptons,
including intermediate light and heavy quarks and allowing for arbitrary
invariant masses of the W bosons. While formally of next-to-next-to-leading
order, the gg -> W*W* -> leptons process is enhanced by the large gluon flux at
the LHC and by experimental Higgs search cuts, and increases the
next-to-leading order WW background estimate for Higgs searches by about 30%.
We have extended our previous calculation to include the contribution from the
intermediate top-bottom massive quark loop and the Higgs signal process. We
provide updated results for cross sections and differential distributions and
study the interference between the different gluon scattering contributions. We
describe important analytical and numerical aspects of our calculation and
present the public GG2WW event generator.Comment: 20 pages, 4 figure
b-quark decay in the collinear approximation
The semileptonic decay of a b-quark, b--> c l nu, is considered in the
relativistic limit where the decay products are approximately collinear.
Analytic results for the double differential lepton energy distributions are
given for finite charm-quark mass. Their use for the fast simulation of
isolated lepton backgrounds from heavy quark decays is discussed.Comment: 7 pages, 1 figure, submitted to Phys.Rev.
NNLO Logarithmic Expansions and Precise Determinations of the Neutral Currents near the Z Resonance at the LHC: The Drell-Yan case
We present a comparative study of the invariant mass and rapidity
distributions in Drell-Yan lepton pair production, with particular emphasis on
the role played by the QCD evolution. We focus our study around the Z resonance
( GeV) and perform a general analysis of the
factorization/renormalization scale dependence of the cross sections, with the
two scales included both in the evolution and in the hard scatterings. We also
present the variations of the cross sections due to the errors on the parton
distributions (pdf's) and an analysis of the corresponding -factors.
Predictions from several sets of pdf's, evolved by MRST and Alekhin are
compared with those generated using \textsc{Candia}, a NNLO evolution program
that implements the theory of the logarithmic expansions, developed in a
previous work. These expansions allow to select truncated solutions of varying
accuracy using the method of the -space iterates. The evolved parton
distributions are in good agreement with other approaches. The study can be
generalized for high precision searches of extra neutral gauge interactions at
the LHC.Comment: 75 pages,30 figures, 30 table
Phosphate limitation triggers the dissolution of precipitated iron by the marine bacterium Pseudovibrio sp. FO-BEG1
Phosphorus is an essential nutrient for all living organisms. In bacteria, the preferential phosphorus source is phosphate, which is often a limiting macronutrient in many areas of the ocean. The geochemical cycle of phosphorus is strongly interconnected with the cycles of other elements and especially iron, because phosphate tends to adsorb onto iron minerals, such as iron oxide formed in oxic marine environments. Although the response to either iron or phosphate limitation has been investigated in several bacterial species, the metabolic interplay between these two nutrients has rarely been considered. In this study we evaluated the impact of phosphate limitation on the iron metabolism of the marine bacterium Pseudovibrio sp. FO-BEG1. We observed that phosphate limitation led to an initial decrease of soluble iron in the culture up to three times higher than under phosphate surplus conditions. Similarly, a decrease in soluble cobalt was more pronounced under phosphate limitation. These data point toward physiological changes induced by phosphate limitation that affect either the cellular surface and therefore the metal adsorption onto it or the cellular metal uptake. We discovered that under phosphate limitation strain FO-BEG1, as well as selected strains of the Roseobacter clade, secreted iron-chelating molecules. This leads to the hypothesis that these bacteria might release such molecules to dissolve iron minerals, such as iron-oxyhydroxide, in order to access the adsorbed phosphate. As the adsorption of phosphate onto iron minerals can significantly decrease phosphate concentrations in the environment, the observed release of iron-chelators might represent an as yet unrecognized link between the biogeochemical cycle of phosphorus and iron, and it suggests another biological function of iron-chelating molecules in addition to metal-scavenging
Effective K-factors for gg -> H -> WW -> lnu lnu at the LHC
A simulation of the search for the Standard Model Higgs boson at the LHC, in
the channel gg -> H -> WW -> lnu lnu, is described. Higher-order QCD
corrections are taken into account by using a reweighting procedure, which
allows us to combine event rates obtained with the PYTHIA Monte Carlo program
with the most up-to-date theoretical predictions for the transverse-momentum
spectra of the Higgs signal and its corresponding WW background. With this
method the discovery potential for Higgs masses between 140 and 180 GeV is
recalculated and the potential statistical significance of this channel is
found to increase considerably. For a Higgs mass of 165 GeV a
signal-to-background ratio of almost 2:1 can be obtained. A statistical
significance of five standard deviations might already be achieved with an
integrated luminosity close to 0.4 fb^{-1}. Using this approach, an
experimental effective K-factor of about 2.04 is obtained for the considered
Higgs signature, which is only about 15 % smaller than the theoretical
inclusive K-factor.Comment: 16 pages, 8 eps figures, comparison of jet veto efficiencies between
PYTHIA and HERWIG added, to appear on JHE
Isoflurane Induces Endothelial Apoptosis of the Post-Hypoxic Blood-Brain Barrier in a Transdifferentiated Human Umbilical Vein Edothelial Cell Model
Isoflurane is a popular volatile anesthetic agent used in humans as well as in experimental animal research. In previous animal studies of the blood-brain barrier (BBB), observations towards an increased permeability after exposure to isoflurane are reported. In this study we investigated the effect of a 2-hour isoflurane exposure on apoptosis of the cerebral endothelium following 24 hours of hypoxia in an in vitro BBB model using astrocyte-conditioned human umbilical vein endothelial cells (AC-HUVECs). Apoptosis of AC-HUVECs was investigated using light microscopy of the native culture for morphological changes, Western blot (WB) analysis of Bax and Bcl-2, and a TUNEL assay. Treatment of AC-HUVECs with isoflurane resulted in severe cellular morphological changes and a significant dose-dependent increase in DNA fragmentation, which was observed during the TUNEL assay analysis. WB analysis confirmed increases in pro-apoptotic Bax levels at 4 hours and 24 hours and decreases in anti-apoptotic Bcl-2 in a dose-dependent manner compared with the control group. These negative effects of isoflurane on the BBB after a hypoxic challenge need to be taken into account not only in experimental stroke research, but possibly also in clinical practice
The Bjorken sum rule with Monte Carlo and Neural Network techniques
Determinations of structure functions and parton distribution functions have
been recently obtained using Monte Carlo methods and neural networks as
universal, unbiased interpolants for the unknown functional dependence. In this
work the same methods are applied to obtain a parametrization of polarized Deep
Inelastic Scattering (DIS) structure functions. The Monte Carlo approach
provides a bias--free determination of the probability measure in the space of
structure functions, while retaining all the information on experimental errors
and correlations. In particular the error on the data is propagated into an
error on the structure functions that has a clear statistical meaning. We
present the application of this method to the parametrization from polarized
DIS data of the photon asymmetries and from which we determine
the structure functions and , and discuss the
possibility to extract physical parameters from these parametrizations. This
work can be used as a starting point for the determination of polarized parton
distributions.Comment: 24 pages, 6 figure
Combining Monte Carlo generators with next-to-next-to-leading order calculations: event reweighting for Higgs boson production at the LHC
We study a phenomenological ansatz for merging next-to-next-to-leading order
(NNLO) calculations with Monte Carlo event generators. We reweight them to
match bin-integrated NNLO differential distributions. To test this procedure,
we study the Higgs boson production cross-section at the LHC, for which a fully
differential partonic NNLO calculation is available. We normalize PYTHIA and
MC@NLO Monte Carlo events for Higgs production in the gluon fusion channel to
reproduce the bin integrated NNLO double differential distribution in the
transverse momentum and rapidity of the Higgs boson. These events are used to
compute differential distributions for the photons in the pp \to H \to \gamma
\gamma decay channel, and are compared to predictions from fixed-order
perturbation theory at NNLO. We find agreement between the reweighted
generators and the NNLO result in kinematic regions where we expect a good
description using fixed-order perturbation theory. Kinematic boundaries where
resummation is required are also modeled correctly using this procedure. We
then use these events to compute distributions in the pp \to H \to W^+W^- \to
l^+l^- \nu\bar{\nu} channel, for which an accurate description is needed for
measurements at the LHC. We find that the final state lepton distributions
obtained from PYTHIA are not significantly changed by the reweighting
procedure.Comment: 18 pages, 14 fig
Development of Proficiency Testing for Detection of Irradiated Food: Project E01068. Results of First Round PSL Trials, September 2005
How accurately can we measure the W cross section?
We study the QCD sources of systematic uncertainties in the experimental
extraction of the W cross section at hadron colliders. The uncertainties appear
in the evaluation of the detector acceptances used to convert the number of
observed events into a total production cross section. We consider the effect
of NLO corrections, as well as of the inclusion of parton showers, and evaluate
the impact of spin correlations and of PDF and scale uncertainties.Comment: 16 pages, 6 figure
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