179,461 research outputs found
Developing Cross Section Sets for Fluorocarbon Etchants
Successful modeling of plasmas used in materials processing depends on knowledge of a variety of collision cross sections and reaction rates, both within the plasma and at the surface. Electron-molecule collision cross sections are especially important, affecting both electron transport and the generation of reactive fragments by dissociation and ionization. Because the supply of cross section data is small and measurements are difficult, computational approaches may make a valuable contribution, provided they can cope with the significant challenges posed. In particular, a computational method must deal with the full complexity of low-energy electron-molecule interactions, must treat polyatomic molecules, and must be capable of computing cross sections for electronic excitation. These requirements imply that the method will be numerically intensive and thus must exploit high-performance computers to be practical. We have developed an ab initio computational method, the Schwinger multichannel (SMC) method, that possesses the characteristics just described, and we have applied it to compute cross sections for a variety of molecules, with particular emphasis on fluorocarbon and hydrofluorocarbon etchants used in the semiconductor industry. A key aspect of this work has been an awareness that cross section sets, validated when possible against swarm data, are more useful than individual cross sections. To develop such sets, cross section calculations must be integrated within a focused collaborative effort. Here we describe electron cross section calculations carried out within the context of such a focused effort, with emphasis on fluorinated hydrocarbons including CHF3 (trifluoromethane), c-C_(4)F_(8) (octafluorocyclobutane), and C_(2)F_(4) (tetrafluoroethene)
Mining SOM expression portraits: Feature selection and integrating concepts of molecular function
Background: 
Self organizing maps (SOM) enable the straightforward portraying of high-dimensional data of large sample collections in terms of sample-specific images. The analysis of their texture provides so-called spot-clusters of co-expressed genes which require subsequent significance filtering and functional interpretation. We address feature selection in terms of the gene ranking problem and the interpretation of the obtained spot-related lists using concepts of molecular function.

Results: 
Different expression scores based either on simple fold change-measures or on regularized Students t-statistics are applied to spot-related gene lists and compared with special emphasis on the error characteristics of microarray expression data. The spot-clusters are analyzed using different methods of gene set enrichment analysis with the focus on overexpression and/or overrepresentation of predefined sets of genes. Metagene-related overrepresentation of selected gene sets was mapped into the SOM images to assign gene function to different regions. Alternatively we estimated set-related overexpression profiles over all samples studied using a gene set enrichment score. It was also applied to the spot-clusters to generate lists of enriched gene sets. We used the tissue body index data set, a collection of expression data of human tissues, as an illustrative example. We found that tissue related spots typically contain enriched populations of gene sets well corresponding to molecular processes in the respective tissues. In addition, we display special sets of housekeeping and of consistently weak and highly expressed genes using SOM data filtering. 

Conclusions:
The presented methods allow the comprehensive downstream analysis of SOM-transformed expression data in terms of cluster-related gene lists and enriched gene sets for functional interpretation. SOM clustering implies the ability to define either new gene sets using selected SOM spots or to verify and/or to amend existing ones
Longitudinally Polarized Photoproduction of Inclusive Hadrons at Fixed-Target Experiments
We present a detailed phenomenological study of spin-dependent
single-inclusive high-p_T hadron photoproduction with particular emphasis on
the kinematics relevant for the Compass and Hermes fixed-target experiments. We
carefully examine the theoretical uncertainties associated with the only
moderate transverse momenta accessible in such measurements and analyze the
sensitivity of the relevant spin asymmetries to the gluon polarization in the
nucleon as well as to the completely unknown parton content of circularly
polarized photons.Comment: 12 pages, 14 figures; final version to appear in EPJC; comparison to
E155 data and references adde
Investigation of the 6He cluster structures
The 4He+2n and t+t clustering of the 6He ground state were investigated by
means of the transfer reaction 6He(p,t)4He at 25 MeV/nucleon. The experiment
was performed in inverse kinematics at GANIL with the SPEG spectrometer coupled
to the MUST array. Experimental data for the transfer reaction were analyzed by
a DWBA calculation including the two neutrons and the triton transfer. The
couplings to the 6He --> 4He + 2n breakup channels were taken into account with
a polarization potential deduced from a coupled-discretized-continuum channels
analysis of the 6He+1H elastic scattering measured at the same time. The
influence on the calculations of the 4He+t exit potential and of the triton
sequential transfer is discussed. The final calculation gives a spectroscopic
factor close to one for the 4He+2n configuration as expected. The spectroscopic
factor obtained for the t+t configuration is much smaller than the theoretical
predictions.Comment: 10 pages, 11 figures, accepted in PR
The Structure of the Proton in the LHC Precision Era
We review recent progress in the determination of the parton distribution
functions (PDFs) of the proton, with emphasis on the applications for precision
phenomenology at the Large Hadron Collider (LHC). First of all, we introduce
the general theoretical framework underlying the global QCD analysis of the
quark and gluon internal structure of protons. We then present a detailed
overview of the hard-scattering measurements, and the corresponding theory
predictions, that are used in state-of-the-art PDF fits. We emphasize here the
role that higher-order QCD and electroweak corrections play in the description
of recent high-precision collider data. We present the methodology used to
extract PDFs in global analyses, including the PDF parametrization strategy and
the definition and propagation of PDF uncertainties. Then we review and compare
the most recent releases from the various PDF fitting collaborations,
highlighting their differences and similarities. We discuss the role that QED
corrections and photon-initiated contributions play in modern PDF analysis. We
provide representative examples of the implications of PDF fits for
high-precision LHC phenomenological applications, such as Higgs coupling
measurements and searches for high-mass New Physics resonances. We conclude
this report by discussing some selected topics relevant for the future of PDF
determinations, including the treatment of theoretical uncertainties, the
connection with lattice QCD calculations, and the role of PDFs at future
high-energy colliders beyond the LHC.Comment: 170 pages, 85 figures, version to be published in Physics Report
The PDF4LHC report on PDFs and LHC data: Results from Run I and preparation for Run II
The accurate determination of the Parton Distribution Functions (PDFs) of the
proton is an essential ingredient of the Large Hadron Collider (LHC) program.
PDF uncertainties impact a wide range of processes, from Higgs boson
characterisation and precision Standard Model measurements to New Physics
searches. A major recent development in modern PDF analyses has been to exploit
the wealth of new information contained in precision measurements from the LHC
Run I, as well as progress in tools and methods to include these data in PDF
fits. In this report we summarise the information that PDF-sensitive
measurements at the LHC have provided so far, and review the prospects for
further constraining PDFs with data from the recently started Run II. This
document aims to provide useful input to the LHC collaborations to prioritise
their PDF-sensitive measurements at Run II, as well as a comprehensive
reference for the PDF-fitting collaborations.Comment: 55 pages, 13 figure
Evidence of Odderon-exchange from scaling properties of elastic scattering at TeV energies
We study the scaling properties of the differential cross section of elastic
proton-proton () and proton-antiproton () collisions at high
energies. We introduce a new scaling function, that scales -- within the
experimental errors -- all the ISR data on elastic scattering from
to GeV to the same universal curve. We explore the
scaling properties of the differential cross-sections of the elastic and
collisions in a limited TeV energy range. Rescaling the TOTEM
data from TeV to and TeV, and comparing it to D0
data at TeV, our results provide an evidence for a -channel
Odderon exchange at TeV energies, with a significance of at least 6.26.
We complete this work with a model-dependent evaluation of the domain of
validity of the new scaling and its violations. We find that the scaling
is valid, model dependently, within GeV TeV,
with a range gradually narrowing with decreasing colliding energies.Comment: Accepted in EPJ C, with typos fixed, reorganized institutions
updated, Appendix A, B, C, D, E added, 60 pages, 29 figures, 13 tables,
Odderon significance: 6.26 sigma, conclusions unchange
Are PDFs still consistent with Tevatron data?
As active data taking has moved to the LHC at CERN, more and more LHC data
have been included into fits of parton distribution functions. An anomaly has
arisen where formerly excellent agreement between theoretical predictions and
experiment in single-top-quark production at the Tevatron is no longer quite as
good. Is this indicative of a deeper issue?Comment: 7 p., to appear in "XLVIIth International Symposium on Multiparticle
Dynamics (ISMD 2017)," EPJ Web of Conference
Lessons to be learned from the coherent photoproduction of pseudoscalar mesons
We study the coherent photoproduction of pseudoscalar mesons---particularly
of neutral pions---placing special emphasis on the various sources that put
into question earlier nonrelativistic-impulse-approximation calculations. These
include: final-state interactions, relativistic effects, off-shell ambiguities,
and violations to the impulse approximation. We establish that, while
distortions play an essential role in the modification of the coherent cross
section, the uncertainty in our results due to the various choices of
optical-potential models is relatively small (of at most 30%). By far the
largest uncertainty emerges from the ambiguity in extending the many
on-shell-equivalent representations of the elementary amplitude off the mass
shell. Indeed, relativistic impulse-approximation calculations that include the
same pionic distortions, the same nuclear-structure model, and two sets of
elementary amplitudes that are identical on-shell, lead to variations in the
magnitude of the coherent cross section by up to factors of five. Finally, we
address qualitatively the assumption of locality implicit in most
impulse-approximation treatments, and suggest that the coherent reaction
probes---in addition to the nuclear density---the polarization structure of the
nucleus.Comment: Manuscript is 27 pages long and includes 11 eps figure
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