244 research outputs found
Contribution of Surfactin and SwrA to Flagellin Expression, Swimming, and Surface Motility in Bacillus subtilis.
Multicellular communities produced by Bacillus subtilis can adopt sliding or swarming to translocate over surfaces. While sliding is a flagellum-independent motility produced by the expansive forces in a growing colony, swarming requires flagellar functionality and is characterized by the appearance of hyperflagellated swarm cells that associate in bundles or rafts during movement. Previous work has shown that swarming by undomesticated B. subtilis strains requires swrA, a gene that upregulates the expression of flagellar genes and increases swimming motility, and surfactin, a lipopeptide biosurfactant that also facilitates sliding. Through an analysis of swrA(+) and swrA mutant laboratory strains with or without a mutation in sfp (a gene involved in surfactin production), we show that both swrA and surfactin upregulate the transcription of the flagellin gene and increase bacterial swimming. Surfactin also allows the nonswarming swrA mutant strain to efficiently colonize moist surfaces by sliding. Finally, we reconfirm the essential role of swrA in swarming and show that surfactin, which increases surface wettability, allows swrA(+) strains to produce swarm cells on media at low humidity
Correlated Strength in Nuclear Spectral Function
We have carried out an (e,e'p) experiment at high momentum transfer and in
parallel kinematics to measure the strength of the nuclear spectral function
S(k,E) at high nucleon momenta k and large removal energies E. This strength is
related to the presence of short-range and tensor correlations, and was known
hitherto only indirectly and with considerable uncertainty from the lack of
strength in the independent-particle region. This experiment confirms by direct
measurement the correlated strength predicted by theory.Comment: 4 pages, 2 figures, accepted by Phys. Rev. Let
Exclusive Neutral Pion Electroproduction in the Deeply Virtual Regime
We present measurements of the ep->ep pi^0 cross section extracted at two
values of four-momentum transfer Q^2=1.9 GeV^2 and Q^2=2.3 GeV^2 at Jefferson
Lab Hall A. The kinematic range allows to study the evolution of the extracted
hadronic tensor as a function of Q^2 and W. Results will be confronted with
Regge inspired calculations and GPD predictions. An intepretation of our data
within the framework of semi-inclusive deep inelastic scattering has also been
attempted
The E00-110 experiment in Jefferson Lab's Hall A: Deeply Virtual Compton Scattering off the Proton at 6 GeV
We present final results on the photon electroproduction
() cross section in the deeply virtual Compton
scattering (DVCS) regime and the valence quark region from Jefferson Lab
experiment E00-110. Results from an analysis of a subset of these data were
published before, but the analysis has been improved which is described here at
length, together with details on the experimental setup. Furthermore,
additional data have been analyzed resulting in photon electroproduction cross
sections at new kinematic settings, for a total of 588 experimental bins.
Results of the - and -dependences of both the helicity-dependent and
helicity-independent cross sections are discussed. The -dependence
illustrates the dominance of the twist-2 handbag amplitude in the kinematics of
the experiment, as previously noted. Thanks to the excellent accuracy of this
high luminosity experiment, it becomes clear that the unpolarized cross section
shows a significant deviation from the Bethe-Heitler process in our kinematics,
compatible with a large contribution from the leading twist-2 DVCS term to
the photon electroproduction cross section. The necessity to include
higher-twist corrections in order to fully reproduce the shape of the data is
also discussed. The DVCS cross sections in this paper represent the final set
of experimental results from E00-110, superseding the previous publication.Comment: 48 pages, 32 figure
Deeply Virtual Compton Scattering off the neutron
The present experiment exploits the interference between the Deeply Virtual
Compton Scattering (DVCS) and the Bethe-Heitler processes to extract the
imaginary part of DVCS amplitudes on the neutron and on the deuteron from the
helicity-dependent D cross section measured at =1.9
GeV and =0.36. We extract a linear combination of generalized parton
distributions (GPDs) particularly sensitive to , the least constrained
GPD. A model dependent constraint on the contribution of the up and down quarks
to the nucleon spin is deduced.Comment: Published in Phys. Rev. Let
Scaling Tests of the Cross Section for Deeply Virtual Compton Scattering
We present the first measurements of the \vec{e}p->epg cross section in the
deeply virtual Compton scattering (DVCS) regime and the valence quark region.
The Q^2 dependence (from 1.5 to 2.3 GeV^2) of the helicity-dependent cross
section indicates the twist-2 dominance of DVCS, proving that generalized
parton distributions (GPDs) are accessible to experiment at moderate Q^2. The
helicity-independent cross section is also measured at Q^2=2.3 GeV^2. We
present the first model-independent measurement of linear combinations of GPDs
and GPD integrals up to the twist-3 approximation.Comment: 5 pages, 4 figures, 2 tables. Text shortened for publication.
References added. One figure remove
Phenomenology of the Deuteron Electromagnetic Form Factors
A rigorous extraction of the deuteron charge form factors from tensor
polarization data in elastic electron-deuteron scattering, at given values of
the 4-momentum transfer, is presented. Then the world data for elastic
electron-deuteron scattering is used to parameterize, in three different ways,
the three electromagnetic form factors of the deuteron in the 4-momentum
transfer range 0-7 fm^-1. This procedure is made possible with the advent of
recent polarization measurements. The parameterizations allow a
phenomenological characterization of the deuteron electromagnetic structure.
They can be used to remove ambiguities in the form factors extraction from
future polarization data.Comment: 18 pages (LaTeX), 2 figures Feb. 25: minor changes of content and in
Table
Measurements of Deuteron Photodisintegration up to 4.0 GeV
The first measurements of the differential cross section for the d(gamma,p)n
reaction up to 4.0 GeV were performed at Continuous Electron Beam Accelerator
Facility (CEBAF) at Jefferson Lab. We report the cross sections at the proton
center-of-mass angles of 36, 52, 69 and 89 degrees. These results are in
reasonable agreement with previous measurements at lower energy. The 89 and 69
degree data show constituent-counting-rule behavior up to 4.0 GeV photon
energy. The 36 and 52 degree data disagree with the counting rule behavior. The
quantum chromodynamics (QCD) model of nuclear reactions involving reduced
amplitudes disagrees with the present data.Comment: 5 pages (REVTeX), 1 figure (postscript
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