1,461 research outputs found
Unraveling the 0ÎœÎČÎČ decay mechanisms
We discuss the possibilities of distinguishing among different mechanisms of neutrinoless double beta decay arising in the effective field theory framework. Following the review and detailed investigation of the particular ways of discrimination, we conclude that the 32 different low-energy effective operators can be split into multiple groups that are in principle distinguishable from each other by measurements of the phase-space observables and by comparison of the decay rates obtained using different isotopes. This would require not only a substantial experimental precision but necessarily also a considerable improvement of the current theoretical knowledge of the underlying nuclear physics. Specifically, the limiting aspect in our approach turns out to be the currently unknown or uncertain values of low-energy constants. Besides the study adopting the effective field theory language we also look into several typical UV models
Ion acceleration processes at reforming collisionless shocks
The identification of pre-acceleration mechanisms for cosmic ray ions in
supernova remnant shocks is an important problem in astrophysics. Recent
particle-in-cell (PIC) shock simulations have shown that inclusion of the full
electron kinetics yields non-time-stationary solutions, in contrast to previous
hybrid (kinetic ions, fluid electrons) simulations. Here, by running a PIC code
at high phase space resolution, ion acceleration mechanisms associated with the
time dependence of a supercritical collisionless perpendicular shock are
examined. In particular the components of
are analysed along trajectories for ions that reach both high and low energies.
Selection mechanisms for the ions that reach high energies are also examined.
In contrast to quasi-stationary shock solutions, the suprathermal protons are
selected from the background population on the basis of the time at which they
arrive at the shock, and thus are generated in bursts.Comment: 12 Pages, 7 Figures, To be published in Phys. Plasma
Magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma protoshock
The prompt emissions of gamma-ray bursts are seeded by radiating
ultrarelativistic electrons. Internal shocks propagating through a jet launched
by a stellar implosion, are expected to amplify the magnetic field & accelerate
electrons. We explore the effects of density asymmetry & a quasi-parallel
magnetic field on the collision of plasma clouds. A 2D relativistic PIC
simulation models the collision of two plasma clouds, in the presence of a
quasi-parallel magnetic field. The cloud density ratio is 10. The densities of
ions & electrons & the temperature of 131 keV are equal in each cloud. The mass
ratio is 250. The peak Lorentz factor of the electrons is determined, along
with the orientation & strength of the magnetic field at the cloud collision
boundary. The magnetic field component orthogonal to the initial plasma flow
direction is amplified to values that exceed those expected from shock
compression by over an order of magnitude. The forming shock is
quasi-perpendicular due to this amplification, caused by a current sheet which
develops in response to the differing deflection of the incoming upstream
electrons & ions. The electron deflection implies a charge separation of the
upstream electrons & ions; the resulting electric field drags the electrons
through the magnetic field, whereupon they acquire a relativistic mass
comparable to the ions. We demonstrate how a magnetic field structure
resembling the cross section of a flux tube grows in the current sheet of the
shock transition layer. Plasma filamentation develops, as well as signatures of
orthogonal magnetic field striping. Localized magnetic bubbles form. Energy
equipartition between the ion, electron & magnetic energy is obtained at the
shock transition layer. The electronic radiation can provide a seed photon
population that can be energized by secondary processes (e.g. inverse Compton).Comment: 12 pages, 15 Figures, accepted to A&
ION ACCELERATION IN FRONT OF THE EARTHâS PARALLEL BOW SHOCK: NEW RESULTS AND OPEN QUESTIONS
Plasmoid-Induced-Reconnection and Fractal Reconnection
As a key to undertanding the basic mechanism for fast reconnection in solar
flares, plasmoid-induced-reconnection and fractal reconnection are proposed and
examined. We first briefly summarize recent solar observations that give us
hints on the role of plasmoid (flux rope) ejections in flare energy release. We
then discuss the plasmoid-induced-reconnection model, which is an extention of
the classical two-ribbon-flare model which we refer to as the CSHKP model. An
essential ingredient of the new model is the formation and ejection of a
plasmoid which play an essential role in the storage of magnetic energy (by
inhibiting reconnection) and the induction of a strong inflow into reconnection
region. Using a simple analytical model, we show that the plasmoid ejection and
acceleration are closely coupled with the reconnection process, leading to a
nonlinear instability for the whole dynamics that determines the macroscopic
reconnection rate uniquely. Next we show that the current sheet tends to have a
fractal structure via the following process path: tearing, sheet thinning,
Sweet- Parker sheet, secondary tearing, further sheet thinning... These
processes occur repeatedly at smaller scales until a microscopic plasma scale
(either the ion Larmor radius or the ion inertial length) is reached where
anomalous resistivity or collisionless reconnection can occur. The current
sheet eventually has a fractal structure with many plasmoids (magnetic islands)
of different sizes. When these plasmoids are ejected out of the current sheets,
fast reconnection occurs at various different scales in a highly time dependent
manner. Finally, a scenario is presented for fast reconnection in the solar
corona on the basis of above plasmoid-induced-reconnection in a fractal current
sheet.Comment: 9 pages, 11 figures, with using eps.sty; Earth, Planets and Space in
press; ps-file is also available at
http://stesun8.stelab.nagoya-u.ac.jp/~tanuma/study/shibata2001
In Vitro and In Vivo Germ Line Potential of Stem Cells Derived from Newborn Mouse Skin
We previously reported that fetal porcine skin-derived stem cells were capable of differentiation into oocyte-like cells (OLCs). Here we report that newborn mice skin-derived stem cells are also capable of differentiating into early OLCs. Using stem cells from mice that are transgenic for Oct4 germline distal enhancer-GFP, germ cells resulting from their differentiation are expected to be GFP+. After differentiation, some GFP+ OLCs reached 40â45 ”M and expressed oocyte markers. Flow cytometric analysis revealed that âŒ0.3% of the freshly isolated skin cells were GFP+. The GFP-positive cells increased to âŒ7% after differentiation, suggesting that the GFP+ cells could be of in vivo origin, but are more likely induced upon being cultured in vitro. To study the in vivo germ cell potential of skin-derived cells, they were aggregated with newborn ovarian cells, and transplanted under the kidney capsule of ovariectomized mice. GFP+ oocytes were identified within a subpopulation of follicles in the resulting growth. Our finding that early oocytes can be differentiated from mice skin-derived cells in defined medium may offer a new in vitro model to study germ cell formation and oogenesis
Histone arginine methylation regulates pluripotency in the early mouse embryo
It has been generally accepted that the mammalian embryo starts its development with all cells identical, and only when inside and outside cells form do differences between cells first emerge. However, recent findings show that cells in the mouse embryo can differ in their developmental fate and potency as early as the four-cell stage1,2,3,4. These differences depend on the orientation and order of the cleavage divisions that generated them2,5. Because epigenetic marks are suggested to be involved in sustaining pluripotency6,7, we considered that such developmental properties might be achieved through epigenetic mechanisms. Here we show that modification of histone H3, through the methylation of specific arginine residues, is correlated with cell fate and potency. Levels of H3 methylation at specific arginine residues are maximal in four-cell blastomeres that will contribute to the inner cell mass (ICM) and polar trophectoderm and undertake full development when combined together in chimaeras. Arginine methylation of H3 is minimal in cells whose progeny contributes more to the mural trophectoderm and that show compromised development when combined in chimaeras. This suggests that higher levels of H3 arginine methylation predispose blastomeres to contribute to the pluripotent cells of the ICM. We confirm this prediction by overexpressing the H3-specific arginine methyltransferase CARM1 in individual blastomeres and show that this directs their progeny to the ICM and results in a dramatic upregulation of Nanog and Sox2. Thus, our results identify specific histone modifications as the earliest known epigenetic marker contributing to development of ICM and show that manipulation of epigenetic information influences cell fate determination
Search for chargino-neutralino production with mass splittings near the electroweak scale in three-lepton final states in âs=13âTeV pp collisions with the ATLAS detector
A search for supersymmetry through the pair production of electroweakinos with mass splittings near the electroweak scale and decaying via on-shell W and Z bosons is presented for a three-lepton final state. The analyzed proton-proton collision data taken at a center-of-mass energy of âs=13ââTeV were collected between 2015 and 2018 by the ATLAS experiment at the Large Hadron Collider, corresponding to an integrated luminosity of 139ââfbâ1. A search, emulating the recursive jigsaw reconstruction technique with easily reproducible laboratory-frame variables, is performed. The two excesses observed in the 2015â2016 data recursive jigsaw analysis in the low-mass three-lepton phase space are reproduced. Results with the full data set are in agreement with the Standard Model expectations. They are interpreted to set exclusion limits at the 95% confidence level on simplified models of chargino-neutralino pair production for masses up to 345 GeV
Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at â s = 8 TeV with the ATLAS detector
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fbâ1 of â s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente
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