964 research outputs found
Line Defects in Molybdenum Disulfide Layers
Layered molecular materials and especially MoS2 are already accepted as
promising candidates for nanoelectronics. In contrast to the bulk material, the
observed electron mobility in single-layer MoS2 is unexpectedly low. Here we
reveal the occurrence of intrinsic defects in MoS2 layers, known as inversion
domains, where the layer changes its direction through a line defect. The line
defects are observed experimentally by atomic resolution TEM. The structures
were modeled and the stability and electronic properties of the defects were
calculated using quantum-mechanical calculations based on the
Density-Functional Tight-Binding method. The results of these calculations
indicate the occurrence of new states within the band gap of the semiconducting
MoS2. The most stable non-stoichiometric defect structures are observed
experimentally, one of which contains metallic Mo-Mo bonds and another one
bridging S atoms
PhylomeDB v3.0: an expanding repository of genome-wide collections of trees, alignments and phylogeny-based orthology and paralogy predictions
The growing availability of complete genomic sequences from diverse species has brought about the need to scale up phylogenomic analyses, including the reconstruction of large collections of phylogenetic trees. Here, we present the third version of PhylomeDB (http://phylomeDB.org), a public database for genome-wide collections of gene phylogenies (phylomes). Currently, PhylomeDB is the largest phylogenetic repository and hosts 17 phylomes, comprising 416 093 trees and 165 840 alignments. It is also a major source for phylogeny-based orthology and paralogy predictions, covering about 5 million proteins in 717 fully-sequenced genomes. For each protein-coding gene in a seed genome, the database provides original and processed alignments, phylogenetic trees derived from various methods and phylogeny-based predictions of orthology and paralogy relationships. The new version of phylomeDB has been extended with novel data access and visualization features, including the possibility of programmatic access. Available seed species include model organisms such as human, yeast, Escherichia coli or Arabidopsis thaliana, but also alternative model species such as the human pathogen Candida albicans, or the pea aphid Acyrtosiphon pisum. Finally, PhylomeDB is currently being used by several genome sequencing projects that couple the genome annotation process with the reconstruction of the corresponding phylome, a strategy that provides relevant evolutionary insights
Measurement of the t-channel single top quark production cross section
The D0 collaboration reports direct evidence for electroweak production of
single top quarks through the t-channel exchange of a virtual W boson. This is
the first analysis to isolate an individual single top quark production
channel. We select events containing an isolated electron or muon, missing
transverse energy, and two, three or four jets from 2.3 fb^-1 of ppbar
collisions at the Fermilab Tevatron Collider. One or two of the jets are
identified as containing a b hadron. We combine three multivariate techniques
optimized for the t-channel process to measure the t- and s-channel cross
sections simultaneously. We measure cross sections of 3.14 +0.94 -0.80 pb for
the t-channel and 1.05 +-0.81 pb for the s-channel. The measured t-channel
result is found to have a significance of 4.8 standard deviations and is
consistent with the standard model prediction.Comment: 7 pages, 6 figure
Measurements of differential cross sections of Z/gamma*+jets+X events in proton anti-proton collisions at sqrt{s}=1.96 TeV
We present cross section measurements for Z/gamma*+jets+X production,
differential in the transverse momenta of the three leading jets. The data
sample was collected with the D0 detector at the Fermilab Tevatron proton
anti-proton collider at a center-of-mass energy of 1.96 TeV and corresponds to
an integrated luminosity of 1 fb-1. Leading and next-to-leading order
perturbative QCD predictions are compared with the measurements, and agreement
is found within the theoretical and experimental uncertainties. We also make
comparisons with the predictions of four event generators. Two
parton-shower-based generators show significant shape and normalization
differences with respect to the data. In contrast, two generators combining
tree-level matrix elements with a parton shower give a reasonable description
of the the shapes observed in data, but the predicted normalizations show
significant differences with respect to the data, reflecting large scale
uncertainties. For specific choices of scales, the normalizations for either
generator can be made to agree with the measurements.Comment: Published in PLB. 11 pages, 3 figure
Measurement of trilinear gauge boson couplings from WW + WZ to lnu jj events in pp-bar collisions at sqrt{s}=1.96 TeV
We present a direct measurement of trilinear gauge boson couplings at gammaWW
and ZWW vertices in WW and WZ events produced in pp-bar collisions at
sqrt{s}=1.96 TeV. We consider events with one electron or muon, missing
transverse energy, and at least two jets. The data were collected using the D0
detector and correspond to 1.1/fb of integrated luminosity. Considering two
different relations between the couplings at the gammaWW and ZWW vertices, we
measure these couplings at 68% C.L. to be kappa_{gamma}=1.07^{+0.26}_{-0.29},
lambda =0.00^{+0.06}_{-0.06} and g_{1}^{Z}=1.04^{+0.09}_{-0.09} in a scenario
respecting SU(2)_L x U(1)_Y gauge symmetry and kappa =1.04^{+0.11}_{-0.11} and
lambda=0.00^{+0.06}_{-0.06} in an "equal couplings" scenario.Comment: 14 pages, 7 figures, published in Phys. Rev. D, updated to published
versio
Search for dark photons from supersymmetric hidden valleys
We search for a new light gauge boson, a dark photon, with the D0 experiment.
In the model we consider, supersymmetric partners are pair produced and cascade
to lightest neutralinos that can decay into the hidden sector state plus either
a photon or a dark photon. The dark photon decays through its mixing with a
photon into fermion pairs. We therefore investigate a previously unexplored
final state that contains a photon, two spatially close leptons, and large
missing transverse energy. We do not observe any evidence for dark photons and
set a limit on their production.Comment: submitted to PR
Measurement of the W boson mass
We present a measurement of the W boson mass in W -> ev decays using 1 fb^-1
of data collected with the D0 detector during Run II of the Fermilab Tevatron
collider. With a sample of 499830 W -> ev candidate events, we measure M_W =
80.401 +- 0.043 GeV. This is the most precise measurement from a single
experiment.Comment: As published in PR
Measurement of the Lifetime Difference in the B_s^0 System
We present a study of the decay B_s^0 -> J/psi phi We obtain the CP-odd
fraction in the final state at time zero, R_perp = 0.16 +/- 0.10 (stat) +/-
0.02 (syst), the average lifetime of the (B_s, B_sbar) system, tau (B_s^0)
=1.39^{+0.13}_{-0.16} (stat) ^{+0.01}_{-0.02} (syst) ps, and the relative width
difference between the heavy and light mass eigenstates, Delta Gamma/Gamma =
(Gamma_L - Gamma_H)/Gamma =0.24^{+0.28}_{-0.38} (stat) ^{+0.03}_{-0.04} (syst).
With the additional constraint from the world average of the B_s^0$lifetime
measurements using semileptonic decays, we find tau (B_s^0)= 1.39 +/- 0.06 ~ps
and Delta Gamma/\Gamma = 0.25^{+0.14}_{-0.15}. For the ratio of the B_s^0 and
B^0 lifetimes we obtain tau(B_s^0)/tau(B^0)} = 0.91 +/- 0.09 (stat) +/- 0.003
(syst).Comment: submitted to Phys. Rev. Lett. FERMILAB-PUB-05-324-
Search for CP violation in semileptonic Bs decays
We have performed a search for CP violation in a sample of Bs -> Ds mu X
decays corresponding to ~fb of proton-antiproton collisions collected
by the D0 detector in Run~II at the Fermilab Tevatron Collider. New physics in
mixing could contribute a significant CP violating weak phase, which
would be observed as a difference in the decay-time distribution for oscillated states versus that for . A fit
to the decay-time distributions of the candidates yields
the flavor-specific asymmetry , which
excludes CP violation due to new physics within the experimental sensitivity.Comment: published in Phys. Rev.
Simultaneous measurement of the ratio B(t->Wb)/B(t->Wq) and the top quark pair production cross section with the D0 detector at sqrt(s)=1.96 TeV
We present the first simultaneous measurement of the ratio of branching
fractions, R=B(t->Wb)/B(t->Wq), with q being a d, s, or b quark, and the top
quark pair production cross section sigma_ttbar in the lepton plus jets channel
using 0.9 fb-1 of ppbar collision data at sqrt(s)=1.96 TeV collected with the
D0 detector. We extract R and sigma_ttbar by analyzing samples of events with
0, 1 and >= 2 identified b jets. We measure R = 0.97 +0.09-0.08 (stat+syst) and
sigma_ttbar = 8.18 +0.90-0.84 (stat+syst)} +/-0.50 (lumi) pb, in agreement with
the standard model prediction.Comment: submitted to Phys.Rev.Letter
- …