1,554 research outputs found
Polyhedral Cosmic Strings
Quantum field theory is discussed in M\"obius corner kaleidoscopes using the
method of images. The vacuum average of the stress-energy tensor of a free
field is derived and is shown to be a simple sum of straight cosmic string
expressions, the strings running along the edges of the corners. It does not
seem possible to set up a spin-half theory easily.Comment: 15 pages, 4 text figures not include
Search for magnetic monopoles and stable particles with high electric charges in √s = 13 TeV pp collisions with the ATLAS detector
We present a search for magnetic monopoles and high-electric-charge objects using LHC Run 2 √s = 13 TeV proton-proton collisions recorded by the ATLAS detector. A total integrated luminosity of 138 fb−1 was collected by a specialized trigger. No highly ionizing particle candidate was observed. Considering the Drell-Yan and photon-fusion pair production mechanisms as benchmark models, cross-section upper limits are presented for spin-0 and spin-1/2 magnetic monopoles of magnetic charge 1gD and 2gD and for high-electric-charge objects of electric charge 20 ≤ |z| ≤ 100, for masses between 200 GeV and 4000 GeV. The search improves by approximately a factor of three the previous cross-section limits on the Drell-Yan production of magnetic monopoles and high-electric charge objects. Also, the first ATLAS limits on the photon-fusion pair production mechanism of magnetic monopoles and high-electric-charge objects are obtained
Search for vector-boson resonances decaying into a top quark and a bottom quark using pp collisions at √s = 13 TeV with the ATLAS detector
A search for a new massive charged gauge boson, W′, is performed with the ATLAS detector at the LHC. The dataset used in this analysis was collected from proton-proton collisions at a centre-of-mass energy of √s = 13 TeV, and corresponds to an integrated luminosity of 139 fb−1. The reconstructed tb invariant mass is used to search for a W′ boson decaying into a top quark and a bottom quark. The result is interpreted in terms of a W′ boson with purely right-handed or left-handed chirality in a mass range of 0.5–6 TeV. Different values for the coupling of the W′ boson to the top and bottom quarks are considered, taking into account interference with single-top-quark production in the s-channel. No significant deviation from the background prediction is observed. The results are expressed as upper limits on the W′ → tb production cross-section times branching ratio as a function of the W′-boson mass and in the plane of the coupling vs the W′-boson mass
Scaling of the Conductivity with Temperature and Uniaxial Stress in Si:B at the Metal-Insulator Transition
Using uniaxial stress to tune Si:B through the metal-insulator transition we
find the conductivity at low temperatures shows an excellent fit to scaling
with temperature and stress on both sides of the transition. The scaling
functions yield the conductivity in the metallic and insulating phases, and
allow a reliable determination of the temperature dependence in the critical
regions on both sides of the transition
Antibody mediated neutralization of myelin associated EphrinB3 accelerates CNS remyelination
This is the final version of the article. It was first available from Springer via
http://dx.doi.org/10.1007/s00401-015-1521-1Remyelination in multiple sclerosis (MS) lesions often remains incomplete despite the presence of oligodendrocyte progenitor cells (OPCs). Amongst other factors, successful remyelination depends on the phagocytic clearance of myelin debris. However, the proteins in myelin debris that act as potent and selective inhibitors on OPC differentiation and inhibit CNS remyelination remain unknown. Here, we identify the transmembrane signalling protein EphrinB3 as important mediator of this inhibition, using a protein analytical approach in combination with a primary rodent OPC assay. In the presence of EphrinB3, OPCs fail to differentiate. In a rat model of remyelination, infusion of EphrinB3 inhibits remyelination. In contrast, masking EphrinB3 epitopes using antibodies promotes remyelination. Finally, we identify EphrinB3 in MS lesions and demonstrate that MS lesion extracts inhibit OPC differentiation while antibody-mediated masking of EphrinB3 epitopes promotes it. Our findings suggest that EphrinB3 could be a target for therapies aiming at promoting remyelination in demyelinating disease.This work was supported by the UK MS Society Grant ref: 941/11. MRNK held a NIHR
Clinical Lectureship. KAN was supported by an ERC advanced award
Acceleration of Solar Wind Ions by Nearby Interplanetary Shocks: Comparison of Monte Carlo Simulations with Ulysses Observations
The most stringent test of theoretical models of the first-order Fermi
mechanism at collisionless astrophysical shocks is a comparison of the
theoretical predictions with observational data on particle populations. Such
comparisons have yielded good agreement between observations at the
quasi-parallel portion of the Earth's bow shock and three theoretical
approaches, including Monte Carlo kinetic simulations. This paper extends such
model testing to the realm of oblique interplanetary shocks: here observations
of proton and alpha particle distributions made by the SWICS ion mass
spectrometer on Ulysses at nearby interplanetary shocks are compared with test
particle Monte Carlo simulation predictions of accelerated populations. The
plasma parameters used in the simulation are obtained from measurements of
solar wind particles and the magnetic field upstream of individual shocks. Good
agreement between downstream spectral measurements and the simulation
predictions are obtained for two shocks by allowing the the ratio of the
mean-free scattering length to the ionic gyroradius, to vary in an optimization
of the fit to the data. Generally small values of this ratio are obtained,
corresponding to the case of strong scattering. The acceleration process
appears to be roughly independent of the mass or charge of the species.Comment: 26 pages, 6 figures, AASTeX format, to appear in the Astrophysical
Journal, February 20, 199
CAN IBEX IDENTIFY VARIATIONS IN THE GALACTIC ENVIRONMENT OF THE SUN USING ENERGETIC NEUTRAL ATOMS?
The Interstellar Boundary Explorer (IBEX) spacecraft is providing the first all-sky maps of the energetic neutral atoms (ENAs) produced by charge exchange between interstellar neutral Ho atoms and heliospheric solar wind and pickup ions in the heliosphere boundary regions. The "edge" of the interstellar cloud presently surrounding the heliosphere extends less than 0.1 pc in the upwind direction, terminating at an unknown distance, indicating that the outer boundary conditions of the heliosphere could change during the lifetime of the IBEX satellite. Using reasonable values for future outer heliosphere boundary conditions, ENA fluxes are predicted for one possible source of ENAs coming from outside of the heliopause. The ENA-production simulations use three-dimensional MHD plasma models of the heliosphere that include a kinetic description of neutrals and a Lorentzian distribution for ions. Based on this ENA-production model, it is then shown that the sensitivities of the IBEX 1.1 keV skymaps are sufficient to detect the variations in ENA fluxes that are expected to accompany the solar transition into the next upwind cloud. Approximately 20% of the IBEX 1.1 keV pixels appear capable of detecting the predicted model differences at the 3σ level, with these pixels concentrated in the Ribbon region. Regardless of the detailed ENA production model, the success of the modeled B centerdot R ~ 0 directions in reproducing the Ribbon locus, together with our results, indicates that the Ribbon phenomenon traces the variations in the heliosphere distortion caused by the relative pressures of the interstellar magnetic and gaseous components.United States. National Aeronautics and Space Administration (NASA IBEX mission, Explorer Program, grant NNX09AG63G
Inhibition of CNS remyelination by the presence of semaphorin 3A
Failure of oligodendrocyteprecursor cell (OPC)differentiationhasbeen recognized asthe leading causeforthefailure ofmyelin regenerationin
diseases such as multiple sclerosis (MS). One explanation for the failure of OPC differentiation in MS is the presence of inhibitory molecules in
demyelinatedlesions.Sofaronly afewinhibitory substrateshavebeenidentifiedinMSlesions.Semaphorin3A (Sema3A), a secretedmemberof
the semaphorin family, can act as repulsive guidance cue for neuronal and glial cells in the CNS. Recent studies suggest that Sema3A is also
expressedin activeMSlesions.However,theimplicationof Sema3A expressioninMSlesions remainsunclear asOPCs are commonlypresentin
chronic demyelinated lesions. In the present study we identify Sema3A as a potent, selective, and reversible inhibitor of OPC differentiation in
vitro. Furthermore, we show that administration of Sema3A into demyelinating lesions in the rat CNS results in a failure of remyelination. Our
results imply an important role for Sema3A in the differentiation block occurring in MS lesions
- …