103 research outputs found
To respond or not to respond - a personal perspective of intestinal tolerance
For many years, the intestine was one of the poor relations of the immunology world, being a realm inhabited mostly by specialists and those interested in unusual phenomena. However, this has changed dramatically in recent years with the realization of how important the microbiota is in shaping immune function throughout the body, and almost every major immunology institution now includes the intestine as an area of interest. One of the most important aspects of the intestinal immune system is how it discriminates carefully between harmless and harmful antigens, in particular, its ability to generate active tolerance to materials such as commensal bacteria and food proteins. This phenomenon has been recognized for more than 100 years, and it is essential for preventing inflammatory disease in the intestine, but its basis remains enigmatic. Here, I discuss the progress that has been made in understanding oral tolerance during my 40 years in the field and highlight the topics that will be the focus of future research
Search for the pentaquark in the reaction
A search for the \thp in the reaction was completed
using the CLAS detector at Jefferson Lab. A study of the same reaction,
published earlier, reported the observation of a narrow \thp resonance. The
present experiment, with more than 30 times the integrated luminosity of our
earlier measurement, does not show any evidence for a narrow pentaquark
resonance. The angle-integrated upper limit on \thp production in the mass
range of 1.52 to 1.56 GeV/c for the reaction is
0.3 nb (95% CL). This upper limit depends on assumptions made for the mass and
angular distribution of \thp production. Using \lamstar production as an
empirical measure of rescattering in the deuteron, the cross section upper
limit for the elementary reaction is estimated to be
a factor of 10 higher, {\it i.e.}, nb (95% CL).Comment: 5 figures, submitted to PRL, revised for referee comment
Photodisintegration of He into p+t
The two-body photodisintegration of He into a proton and a triton has
been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at Jefferson
Laboratory. Real photons produced with the Hall-B bremsstrahlung-tagging system
in the energy range from 0.35 to 1.55 GeV were incident on a liquid He
target. This is the first measurement of the photodisintegration of He
above 0.4 GeV. The differential cross sections for the He
reaction have been measured as a function of photon-beam energy and
proton-scattering angle, and are compared with the latest model calculations by
J.-M. Laget. At 0.6-1.2 GeV, our data are in good agreement only with the
calculations that include three-body mechanisms, thus confirming their
importance. These results reinforce the conclusion of our previous study of the
three-body breakup of He that demonstrated the great importance of
three-body mechanisms in the energy region 0.5-0.8 GeV .Comment: 13 pages submitted in one tgz file containing 2 tex file and 22
postscrip figure
Onset of asymptotic scaling in deuteron photodisintegration
We investigate the transition from the nucleon-meson to quark-gluon
description of the strong interaction using the photon energy dependence of the
differential cross section for photon energies above 0.5 GeV and
center-of-mass proton angles between and . A possible
signature for this transition is the onset of cross section scaling
with the total energy squared, , at some proton transverse momentum, .
The results show that the scaling has been reached for proton transverse
momentum above about 1.1 GeV/c. This may indicate that the quark-gluon regime
is reached above this momentum.Comment: Accepted by PRL; 5 pages, 2 figure
photoproduction on the proton for photon energies from 0.675 to 2.875 GeV
Differential cross sections for the reaction have been
measured with the CEBAF Large Acceptance Spectrometer (CLAS) and a tagged
photon beam with energies from 0.675 to 2.875 GeV. The results reported here
possess greater accuracy in the absolute normalization than previous
measurements. They disagree with recent CB-ELSA measurements for the process at
forward scattering angles. Agreement with the SAID and MAID fits is found below
1 GeV. The present set of cross sections has been incorporated into the SAID
database, and exploratory fits have been extended to 3 GeV. Resonance couplings
have been extracted and compared to previous determinations.Comment: 18 pages, 48 figure
Exclusive electroproduction on the proton at CLAS
The reaction has been measured, using the 5.754
GeV electron beam of Jefferson Lab and the CLAS detector. This represents the
largest ever set of data for this reaction in the valence region. Integrated
and differential cross sections are presented. The , and
dependences of the cross section are compared to theoretical calculations based
on -channel meson-exchange Regge theory on the one hand and on quark handbag
diagrams related to Generalized Parton Distributions (GPDs) on the other hand.
The Regge approach can describe at the 30% level most of the features
of the present data while the two GPD calculations that are presented in this
article which succesfully reproduce the high energy data strongly underestimate
the present data. The question is then raised whether this discrepancy
originates from an incomplete or inexact way of modelling the GPDs or the
associated hard scattering amplitude or whether the GPD formalism is simply
inapplicable in this region due to higher-twists contributions, incalculable at
present.Comment: 29 pages, 29 figure
Exclusive Photoproduction of the Cascade (Xi) Hyperons
We report on the first measurement of exclusive Xi-(1321) hyperon
photoproduction in gamma p --> K+ K+ Xi- for 3.2 < E(gamma) < 3.9 GeV. The
final state is identified by the missing mass in p(gamma,K+ K+)X measured with
the CLAS detector at Jefferson Laboratory. We have detected a significant
number of the ground-state Xi-(1321)1/2+, and have estimated the total cross
section for its production. We have also observed the first excited state
Xi-(1530)3/2+. Photoproduction provides a copious source of Xi's. We discuss
the possibilities of a search for the recently proposed Xi5-- and Xi5+
pentaquarks.Comment: submitted to Phys. Rev.
First Measurement of Beam-Recoil Observables Cx and Cz in Hyperon Photoproduction
Spin transfer from circularly polarized real photons to recoiling hyperons
has been measured for the reactions and
. The data were obtained using the CLAS
detector at Jefferson Lab for center-of-mass energies between 1.6 and 2.53
GeV, and for . For the , the
polarization transfer coefficient along the photon momentum axis, , was
found to be near unity for a wide range of energy and kaon production angles.
The associated transverse polarization coefficient, , is smaller than
by a roughly constant difference of unity. Most significantly, the {\it
total} polarization vector, including the induced polarization ,
has magnitude consistent with unity at all measured energies and production
angles when the beam is fully polarized. For the this simple
phenomenology does not hold. All existing hadrodynamic models are in poor
agreement with these results.Comment: 28 pages, 18 figures, Submitted to Physical Review
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Establishment of Normal Gut Microbiota Is Compromised under Excessive Hygiene Conditions
Background: Early gut colonization events are purported to have a major impact on the incidence of infectious,
inflammatory and autoimmune diseases in later life. Hence, factors which influence this process may have important
implications for both human and animal health. Previously, we demonstrated strong influences of early-life environment on
gut microbiota composition in adult pigs. Here, we sought to further investigate the impact of limiting microbial exposure
during early life on the development of the pig gut microbiota.
Methodology/Principal Findings: Outdoor- and indoor-reared animals, exposed to the microbiota in their natural rearing
environment for the first two days of life, were transferred to an isolator facility and adult gut microbial diversity was
analyzed by 16S rRNA gene sequencing. From a total of 2,196 high-quality 16S rRNA gene sequences, 440 phylotypes were
identified in the outdoor group and 431 phylotypes in the indoor group. The majority of clones were assigned to the four
phyla Firmicutes (67.5% of all sequences), Proteobacteria (17.7%), Bacteroidetes (13.5%) and to a lesser extent,
Actinobacteria (0.1%). Although the initial maternal and environmental microbial inoculum of isolator-reared animals was
identical to that of their naturally-reared littermates, the microbial succession and stabilization events reported previously in
naturally-reared outdoor animals did not occur. In contrast, the gut microbiota of isolator-reared animals remained highly
diverse containing a large number of distinct phylotypes.
Conclusions/Significance: The results documented here indicate that establishment and development of the normal gut
microbiota requires continuous microbial exposure during the early stages of life and this process is compromised under
conditions of excessive hygiene
First measurement of target and double spin asymmetries for polarized e- polarized p --> e p pi0 in the nucleon resonance region above the Delta(1232)
The exclusive channel polarized proton(polarized e,e prime p)pi0 was studied
in the first and second nucleon resonance regions in the Q2 range from 0.187 to
0.770 GeV2 at Jefferson Lab using the CEBAF Large Acceptance Spectrometer
(CLAS). Longitudinal target and beam-target asymmetries were extracted over a
large range of center-of-mass angles of the pi0 and compared to the unitary
isobar model MAID, the dynamic model by Sato and Lee, and the dynamic model
DMT. A strong sensitivity to individual models was observed, in particular for
the target asymmetry and in the higher invariant mass region. This data set,
once included in the global fits of the above models, is expected to place
strong constraints on the electrocoupling amplitudes A_{1/2} and S_{1/2} for
the Roper resonance N(1400)P11, and the N(1535)S11 and N(1520)D13 states.Comment: 13 pages, 13 figure
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