496 research outputs found
Photoproduction of and in the reaction \gamma \lowercase{p} \to K^+ \Lambda \pi^0 at Jefferson Lab
The search for missing nucleon resonances using coupled channel analysis has
mostly been concentrated on and channels, while the contributions
of and channels have not been investigated thoroughly mostly due
to the lack of data. With an integrated luminosity of about 75 , the
photoproduction data using a proton target recently collected by the CLAS
Collaboration at Jefferson Lab with a photon energy range of 1.5-3.8 GeV
provided large statistics for the study of light hyperon photoproduction
through exclusive reactions. The reaction has
been investigated. Preliminary results of the and
cross sections are not negligible compared with the
channels. The invariant mass spectrum is dominated by the
signal and no significant structure was found around the
region.Comment: 4 pages, 3 figures, to be publised on the NSTAR05 proceeding
Repulsion leads to coupled dislocation motion and extended work hardening in bcc metals
Work hardening in bcc single crystals at low homologous temperature shows a strong orientation-dependent hardening for high symmetry loading, which is not captured by classical dislocation density based models. We demonstrate here that the high activation barrier for screw dislocation glide motion in tungsten results in repulsive interactions between screw dislocations, and triggers dislocation motion at applied loading conditions where it is not expected. In situ transmission electron microscopy and atomistically informed discrete dislocation dynamics simulations confirm coupled dislocation motion and vanishing obstacle strength for repulsive screw dislocations, compatible with the kink pair mechanism of dislocation motion in the thermally activated (low temperature) regime. We implement this additional contribution to plastic strain in a modified crystal plasticity framework and show that it can explain the extended work hardening regime observed for [100] oriented tungsten single crystal. This may contribute to better understanding the increase in ductility of highly deformed bcc metals
Quenching of the Deuteron in Flight
We investigate the Lorentz contraction of a deuteron in flight. Our starting
point is the Blankenbecler-Sugar projection of the Bethe-Salpeter equation to a
3-dimensional quasi potential equation, wqhich we apply for the deuteron bound
in an harmonic oscillator potential (for an analytical result) and by the Bonn
NN potential for a more realistic estimate. We find substantial quenching with
increasing external momenta and a significant modification of the high momentum
spectrum of the deuteron.Comment: 11 pages, 4 figure
First-principles prediction of a decagonal quasicrystal containing boron
We interpret experimentally known B-Mg-Ru crystals as quasicrystal
approximants. These approximant structures imply a deterministic decoration of
tiles by atoms that can be extended quasiperiodically. Experimentally observed
structural disorder corresponds to phason (tile flip) fluctuations.
First-principles total energy calculations reveal that many distinct tilings
lie close to stability at low temperatures. Transfer matrix calculations based
on these energies suggest a phase transition from a crystalline state at low
temperatures to a high temperature state characterized by tile fluctuations. We
predict BMgRu forms a decagonal quasicrystal that is
metastable at low temperatures and may be thermodynamically stable at high
temperatures.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Micro-plasticity and intermittent dislocation activity in a simplified micro structural model
Here we present a model to study the micro-plastic regime of a stress-strain
curve. In this model an explicit dislocation population represents the mobile
dislocation content and an internal shear-stress field represents a mean-field
description of the immobile dislocation content. The mobile dislocations are
constrained to a simple dipolar mat geometry and modelled via a dislocation
dynamics algorithm, whilst the shear-stress field is chosen to be a sinusoidal
function of distance along the mat direction. The latter, defined by a periodic
length and a shear-stress amplitude, represents a pre-existing micro-structure.
These model parameters, along with the mobile dislocation density, are found to
admit a diversity of micro-plastic behaviour involving intermittent plasticity
in the form of a scale-free avalanche phenomenon, with an exponent for the
strain burst magnitude distribution similar to those seen in experiment and
more complex dislocation dynamics simulations.Comment: 30 pages, 12 figures, to appear in "Modelling and Simulation in
Materials Science and Engineering
Double K-S(0) Photoproduction off the Proton at CLAS
The f0 (1500) meson resonance is one of several contenders to have significant mixing with the lightest glueball. This resonance is well established from several previous experiments. Here we present the first photoproduction data for the f0 (1500) via decay into the K-S(0) K-S(0) channel using the CLAS detector. The reaction γp -\u3e fJp -\u3e K-S(0) K-S(0) p, where J = 0,2, was measured with photon energies from 2.7-5.1 GeV. A clear peak is seen at 1500 MeV in the background subtracted invariant mass spectra of the two kaons. This is enhanced if the measured four-momentum transfer to the proton target is restricted to be less than 1.0 GeV2. By comparing data with simulations, it can be concluded that the peak at 1500 MeV is produced primarily at low t, which is consistent with a t-channel production mechanism
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Hadron spectroscopy and B physics at RHIC
A description is given of the physics opportunities at RHIC regarding quark-gluon spectroscopy. The basic idea is to isolate with appropriate triggers the sub-processes pomeron + pomeron {yields} hadrons and {gamma}{sup *} + {gamma}{sup *} {yields} hadrons with the net effective mass of hadrons in the range of 1.0 to 10.0 GeV, in order to study the hadronic states composed of quarks and gluons. The double-pomeron interactions are expected to produce glueballs and hybrids preferentially, while the two-offshell-photon initial states should couple predominantly to quarkonia and multiquark states. Of particular interest is the possibility of carrying out a CP-violation study in the self-tagging B decays, B{sub d}{sup 0} {yields} K{sup +}{pi}{sup {minus}} and {bar B}{sub d}{sup 0} {yields} K{sup {minus}}{pi}{sup +}. 20 refs., 4 figs
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Double-Pomeron and two-photon processes at RHIC
Double-Pomeron processes have been shown to be an important and novel source of hadron production at moderate energies at the ISR. These processes are expected to provide glue-rich hadrons from 1 GeV to 10 GeV or more, encompassing the states consisting of u, d, s and b quarks. The double-pomeron cross sections for central hadroproduction are calculated for p {times} p and Au {times} Au at RHIC. Two-photon production of hadrons in the central region begins to dominate or at least become comparable to the double-Pomeron processes as the Z of the beams increases from p to Au. Since photons couple to charge, these hadroproductions involve mainly quarkonia and multiquark states. Therefore, a comparative study of these processes is expected to provide new insights into the constituents of hadronic matter. The two-photon processes are calculated following the recipe given by Cahn and Jackson. The paper starts out with a thorough discussion of the relevant kinematics, phase space and Regge amplitudes
A new highly segmented start counter for the CLAS detector
The design, construction and performance of a highly segmented Start Counter are described. The Start Counter is an integral part of the trigger used in photon beam running with CLAS in Hall B at the Thomas Jefferson National Accelerator Facility (TJNAF). The Start Counter is constructed of 24 2.2-mm-thick single-ended scintillation paddles, forming a hermetic hexagon around the target region. This device measures the interaction time of the incoming photon in the target by detecting the outgoing particles. The counter provides complex trigger topologies, shows good efficiency and achieved a time resolution of 350 ps
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