40 research outputs found
Recommended from our members
Estimating North American background ozone in U.S. surface air with two independent global models: Variability, uncertainties, and recommendations
Accurate estimates for North American background (NAB) ozone (O3) in surface air over the United States are needed for setting and implementing an attainable national O3 standard. These estimates rely on simulations with atmospheric chemistry-transport models that set North American anthropogenic emissions to zero, and to date have relied heavily on one global model. We examine NAB estimates for spring and summer 2006 with two independent global models (GEOS-Chem and GFDL AM3). We evaluate the base simulations, which include North American anthropogenic emissions, with mid-tropospheric O3 retrieved from space and ground-level O3 measurements. The models often bracket the observed values, implying value in developing a multi-model approach to estimate NAB O3. Consistent with earlier studies, the models robustly simulate the largest nation-wide NAB levels at high-altitude western U.S. sites (seasonal average maximum daily 8-h values of ∼40–50 ppb in spring and ∼25–40 ppb in summer) where it correlates with observed O3. At these sites, a 27-year GFDL AM3 simulation simulates observed O3 events above 60 ppb and indicates that year-to-year variations in NAB O3 influence their annual frequency (with NAB contributing 50–60 ppb or more during individual events). During summer over the eastern United States (EUS), when photochemical production from regional anthropogenic emissions peaks, NAB is largely uncorrelated with observed values and it is lower than at high-altitude sites (average values of ∼20–30 ppb). Four processes contribute substantially to model differences in specific regions and seasons: lightning NOx, biogenic isoprene emissions and chemistry, wildfires, and stratosphere-to-troposphere transport. Differences in the representations of these processes within the GFDL AM3 and GEOS-Chem models contribute more to uncertainty in NAB estimates, particularly in spring when NAB is highest, than the choice of horizontal resolution within a single model (GEOS-Chem). We propose that future efforts seek to constrain these processes with targeted analysis of multi-model simulations evaluated with observations of O3 and related species from multiple platforms, and thereby reduce the error on NAB estimates needed for air quality planning.Engineering and Applied Science
Toward a Quantization of Null Dust Collapse
Spherically symmetric, null dust clouds, like their time-like counterparts,
may collapse classically into black holes or naked singularities depending on
their initial conditions. We consider the Hamiltonian dynamics of the collapse
of an arbitrary distribution of null dust, expressed in terms of the physical
radius, , the null coordinates, for a collapsing cloud or for an
expanding cloud, the mass function, , of the null matter, and their
conjugate momenta. This description is obtained from the ADM description by a
Kucha\v{r}-type canonical transformation. The constraints are linear in the
canonical momenta and Dirac's constraint quantization program is implemented.
Explicit solutions the constraints are obtained for both expanding and
contracting null dust clouds with arbitrary mass functions.Comment: 10 pages, 2 figures (eps), RevTeX4. The last two sections have been
revised and corrected. To appear in Phys. Rev.
The Role of Color Neutrality in Nuclear Physics--Modifications of Nucleonic Wave Functions
The influence of the nuclear medium upon the internal structure of a
composite nucleon is examined. The interaction with the medium is assumed to
depend on the relative distances between the quarks in the nucleon consistent
with the notion of color neutrality, and to be proportional to the nucleon
density. In the resulting description the nucleon in matter is a superposition
of the ground state (free nucleon) and radial excitations. The effects of the
nuclear medium on the electromagnetic and weak nucleon form factors, and the
nucleon structure function are computed using a light-front constituent quark
model. Further experimental consequences are examined by considering the
electromagnetic nuclear response functions. The effects of color neutrality
supply small but significant corrections to predictions of observables.Comment: 37 pages, postscript figures available on request to
[email protected]
Conformal collider physics: Energy and charge correlations
We study observables in a conformal field theory which are very closely
related to the ones used to describe hadronic events at colliders. We focus on
the correlation functions of the energies deposited on calorimeters placed at a
large distance from the collision. We consider initial states produced by an
operator insertion and we study some general properties of the energy
correlation functions for conformal field theories. We argue that the small
angle singularities of energy correlation functions are controlled by the twist
of non-local light-ray operators with a definite spin. We relate the charge two
point function to a particular moment of the parton distribution functions
appearing in deep inelastic scattering. The one point energy correlation
functions are characterized by a few numbers. For superconformal
theories the one point function for states created by the R-current or the
stress tensor are determined by the two parameters and characterizing
the conformal anomaly. Demanding that the measured energies are positive we get
bounds on . We also give a prescription for computing the energy and
charge correlation functions in theories that have a gravity dual. The
prescription amounts to probing the falling string state as it crosses the
horizon with gravitational shock waves. In the leading, two derivative,
gravity approximation the energy is uniformly distributed on the sphere at
infinity, with no fluctuations. We compute the stringy corrections and we show
that they lead to small, non-gaussian, fluctuations in the energy distribution.
Corrections to the one point functions or antenna patterns are related to
higher derivative corrections in the bulk.Comment: 73 pages, 8 figures; v2: minor changes and added references; v3: more
references adde
Predictions of total and total reaction cross sections for nucleon-nucleus scattering up to 300 MeV
Total reaction cross sections are predicted for nucleons scattering from various nuclei. Projectile energies to 300 MeV are considered. So also are mass variations of those cross sections at selected energies. All predictions have been obtained from coordinate space optical potentials formed by full folding effective two-nucleon (NN) interactions with one body density matrix elements (OBDME) of the nuclear ground states. Good comparisons with data result when effective NN interactions defined by medium modification of free NN t matrices are used. Coupled with analyses of differential cross sections, these results are sensitive to details of the model ground states used to describe nuclei
Strong evidences of hadron acceleration in Tycho's Supernova Remnant
Very recent gamma-ray observations of G120.1+1.4 (Tycho's) supernova remnant
(SNR) by Fermi-LAT and VERITAS provided new fundamental pieces of information
for understanding particle acceleration and non-thermal emission in SNRs. We
want to outline a coherent description of Tycho's properties in terms of SNR
evolution, shock hydrodynamics and multi-wavelength emission by accounting for
particle acceleration at the forward shock via first order Fermi mechanism. We
adopt here a quick and reliable semi-analytical approach to non-linear
diffusive shock acceleration which includes magnetic field amplification due to
resonant streaming instability and the dynamical backreaction on the shock of
both cosmic rays (CRs) and self-generated magnetic turbulence. We find that
Tycho's forward shock is accelerating protons up to at least 500 TeV,
channelling into CRs about the 10 per cent of its kinetic energy. Moreover, the
CR-induced streaming instability is consistent with all the observational
evidences indicating a very efficient magnetic field amplification (up to ~300
micro Gauss). In such a strong magnetic field the velocity of the Alfv\'en
waves scattering CRs in the upstream is expected to be enhanced and to make
accelerated particles feel an effective compression factor lower than 4, in
turn leading to an energy spectrum steeper than the standard prediction
{\propto} E^-2. This latter effect is crucial to explain the GeV-to-TeV
gamma-ray spectrum as due to the decay of neutral pions produced in nuclear
collisions between accelerated nuclei and the background gas. The
self-consistency of such an hadronic scenario, along with the fact that the
concurrent leptonic mechanism cannot reproduce both the shape and the
normalization of the detected the gamma-ray emission, represents the first
clear and direct radiative evidence that hadron acceleration occurs efficiently
in young Galactic SNRs.Comment: Minor changes. Accepted for publication in Astronomy & Astrophysic
The Clinical Sequencing Evidence-Generating Research Consortium: Integrating Genomic Sequencing in Diverse and Medically Underserved Populations
The Clinical Sequencing Evidence-Generating Research (CSER) consortium, now in its second funding cycle, is investigating the effectiveness of integrating genomic (exome or genome) sequencing into the clinical care of diverse and medically underserved individuals in a variety of healthcare settings and disease states. The consortium comprises a coordinating center, six funded extramural clinical projects, and an ongoing National Human Genome Research Institute (NHGRI) intramural project. Collectively, these projects aim to enroll and sequence over 6,100 participants in four years. At least 60% of participants will be of non-European ancestry or from underserved settings, with the goal of diversifying the populations that are providing an evidence base for genomic medicine. Five of the six clinical projects are enrolling pediatric patients with various phenotypes. One of these five projects is also enrolling couples whose fetus has a structural anomaly, and the sixth project is enrolling adults at risk for hereditary cancer. The ongoing NHGRI intramural project has enrolled primarily healthy adults. Goals of the consortium include assessing the clinical utility of genomic sequencing, exploring medical follow up and cascade testing of relatives, and evaluating patient-provider-laboratory level interactions that influence the use of this technology. The findings from the CSER consortium will offer patients, healthcare systems, and policymakers a clearer understanding of the opportunities and challenges of providing genomic medicine in diverse populations and settings, and contribute evidence toward developing best practices for the delivery of clinically useful and cost-effective genomic sequencing in diverse healthcare settings
Gluons and the quark sea at high energies: distributions, polarization, tomography
This report is based on a ten-week program on "Gluons and the quark sea at
high-energies", which took place at the Institute for Nuclear Theory in Seattle
in Fall 2010. The principal aim of the program was to develop and sharpen the
science case for an Electron-Ion Collider (EIC), a facility that will be able
to collide electrons and positrons with polarized protons and with light to
heavy nuclei at high energies, offering unprecedented possibilities for
in-depth studies of quantum chromodynamics. This report is organized around
four major themes: i) the spin and flavor structure of the proton, ii)
three-dimensional structure of nucleons and nuclei in momentum and
configuration space, iii) QCD matter in nuclei, and iv) Electroweak physics and
the search for physics beyond the Standard Model. Beginning with an executive
summary, the report contains tables of key measurements, chapter overviews for
each of the major scientific themes, and detailed individual contributions on
various aspects of the scientific opportunities presented by an EIC.Comment: 547 pages, A report on the joint BNL/INT/Jlab program on the science
case for an Electron-Ion Collider, September 13 to November 19, 2010,
Institute for Nuclear Theory, Seattle; v2 with minor changes, matches printed
versio
Machine Learning Detects Pan-cancer Ras Pathway Activation in The Cancer Genome Atlas
Precision oncology uses genomic evidence to match patients with treatment but often fails to identify all patients who may respond. The transcriptome of these \u201chidden responders\u201d may reveal responsive molecular states. We describe and evaluate a machine-learning approach to classify aberrant pathway activity in tumors, which may aid in hidden responder identification. The algorithm integrates RNA-seq, copy number, and mutations from 33 different cancer types across The Cancer Genome Atlas (TCGA) PanCanAtlas project to predict aberrant molecular states in tumors. Applied to the Ras pathway, the method detects Ras activation across cancer types and identifies phenocopying variants. The model, trained on human tumors, can predict response to MEK inhibitors in wild-type Ras cell lines. We also present data that suggest that multiple hits in the Ras pathway confer increased Ras activity. The transcriptome is underused in precision oncology and, combined with machine learning, can aid in the identification of hidden responders. Way et al. develop a machine-learning approach using PanCanAtlas data to detect Ras activation in cancer. Integrating mutation, copy number, and expression data, the authors show that their method detects Ras-activating variants in tumors and sensitivity to MEK inhibitors in cell lines